biology lesson 5

Sexually Transmitted Diseases

Abstinence is the best protection against the spread of sexually transmitted diseases (STDs). For those who are sexually active, a latex condom offers some protection. Among STDs caused by viruses, treatment is available for AIDS and genital herpes, but these conditions are not curable. Only STDs caused by bacteria (e.g., chlamydia, gonorrhea, and syphilis) are curable with antibiotics.

Nutrition and Health

Many serious disorders in Americans are linked to a diet that results in excess body fat. In the United States, the number of people who are overweight or obese has reached epidemic proportions. Nearly two-thirds of adult Americans have too much body fat. Excess body fat increases the risk of type 2 diabetes, cardiovascular disease, and certain cancers. These conditions are among the leading causes of disability and death in the United States. Therefore, it is important for us all to stay within the recommended weight for our height.

Digestive System

One of the basic characteristics of life (see Section 1.1) is the ability to acquire nutrients for the energy to conduct the activities of living, such as responding to stimuli. In animals, the digestive system consists of the organs involved in the following processes: 1. Ingesting food 2. Breaking down food into smaller molecules that can be transported 3. Absorbing nutrient molecules 4. Eliminating indigestible materials The digestive system interacts with the other organ systems of the body (Fig. 25.1) to maintain homeostasis by providing the body's cells with the nutrients they need to continue functioning.

Fertilization

1 The sperm makes its way through the adhering follicle cells. 2 Acrosomal enzymes digest a portion of the zona pellucida. 3 The sperm binds to and fuses with the egg plasma membrane. 4 The sperm nucleus enters the cytoplasm of the egg. 5 The sperm and egg nuclei fuse to produce a zygote.

STDs Caused by Viruses

Acquired immunodeficiency syndrome (AIDS) is caused by a retrovirus called human immunodeficiency virus (HIV). HIV attacks the type of lymphocyte known as helper T cells (Figure 29.10). Helper T cells (see Section 26.3) stimulate the activities of B lymphocytes, which produce antibodies. After an HIV infection sets in, helper T cells begin to decline in number, and the person becomes debilitated and more susceptible to other types of infection. AIDS has three stages of infection, called categories A, B, and C. During the category A stage, which in untreated individuals typically lasts about a year, the individual is an asymptomatic carrier. He or she may exhibit no symptoms but can pass on the infection. Immediately after infection and before the blood test is positive, a large number of infectious viruses are present in the blood and can be passed on to another person. Even after the blood test is positive, the person remains well as long as the body produces sufficient helper T cells to keep the count higher than 500 per mm³. With a combination therapy of several drugs, AIDS patients can remain in this stage indefinitely. During the category B stage, which may last 6-8 years, the lymph nodes swell and the person may experience weight loss, night sweats, fatigue, fever, and diarrhea. Infections such as thrush (white sores on the tongue and in the mouth) and herpes recur. Finally, the person may progress to category C, which is full-blown AIDS characterized by nervous disorders and the development of an opportunistic disease, such as an unusual type of pneumonia or skin cancer. Opportunistic diseases are those that occur only in individuals who have little or no capability of fighting an infection. Without intensive medical treatment, the AIDS patient dies about 7-9 years after infection. An estimated 20 million Americans are currently infected with the human papillomavirus (HPV). There are over 100 types of HPV. Most cause warts, and about 30 types cause genital warts, which are sexually transmitted. Genital warts may appear as flat or raised warts on the penis and/or foreskin of males, as well Page 569as on the vulva, vagina, and/or cervix of females. Note that if warts are only on the cervix, there may be no outward signs or symptoms of the disease. Newborns can also be infected with HPV during passage through the birth canal. About 10 types of HPV can cause cervical cancer, the second leading cause of cancer death in women in the United States (approximately 500,000 deaths per year). These HPV types produce a viral protein that inactivates a host protein called p53, which normally acts as a "brake" on cell division (see Section 8.3). Once p53 has been inactivated in a particular cell, that cell is more prone to the uncontrolled cell division characteristic of cancer. Early detection of cervical cancer is possible by means of a Pap test, in which a few cells are removed from the region of the cervix for microscopic examination. If the cells are cancerous, a hysterectomy (removal of the uterus) may be recommended. In males, HPV can cause cancers of the penis, anus, and other areas. Several studies have indicated that HPV now causes as many cancers of the mouth and throat in U.S. males as does tobacco, perhaps due to an increase in oral sex as well as a decline in tobacco use. Currently, there is no cure for an HPV infection, but the warts can be treated effectively by surgery, freezing, application of an acid, or laser burning. However, even after treatment, the virus can sometimes be transmitted. Therefore, once someone has been diagnosed with genital warts, abstinence or the use of a condom is recommended to help prevent transmission of the virus. In June 2006, the U.S. Food and Drug Administration licensed Gardasil, an HPV vaccine that is effective against the four most common types of HPV found in the United States, including the two types that cause about 70% of cervical cancers. Because the vaccine doesn't protect those who are already infected, ideally children should be vaccinated before they become sexually active. In 2009, the U.S. Food and Drug Administration approved Gardasil for use in males. The Centers for Disease Control and Prevention recommends that 11- to 12-year-old girls and boys receive three doses of the vaccine. Nonpregnant females between ages 13 and 26, and males from age 13 to 21, can also be vaccinated if they did not receive any or all of the three recommended doses when they were younger. Older individuals should speak with their doctor to find out if getting vaccinated is right for them. Genital herpes is characterized by painful blisters on the genitals. Once the blisters rupture, they leave painful ulcers, which may take as long as 3 weeks or as little as 5 days to heal. The blisters may be accompanied by fever, pain on urination, swollen lymph nodes in the groin, and in women, a copious discharge. After the ulcers heal, the disease is only latent, and blisters can recur, although usually at less frequent intervals and with milder symptoms. Fever, stress, sunlight, and menstruation are associated with the recurrence of symptoms. Hepatitis is an infection of the liver and can lead to liver failure, liver cancer, and death. Several types of hepatitis exist, some of which can be transmitted sexually. Each type of hepatitis and the virus that causes it are designated by a letter. Hepatitis A is usually acquired from sewage-contaminated drinking water, but this infection can also be sexually transmitted through oral/anal contact. Hepatitis B, which is spread in the same manner as AIDS, is even more infectious. Fortunately, a vaccine is available for hepatitis B. Hepatitis C is spread when a person comes in contact with the blood of an infected person.

Immunizations

After you have had an infection, you may be immune to it. Good examples of diseases against which you can acquire immunity are the childhood diseases measles and mumps. Unfortunately, few sexually transmitted diseases stimulate lasting immunity; for example, a person can get gonorrhea over and over again. If lasting immunity is possible, a vaccine for the disease likely exists or can be developed. Vaccines are substances that usually do not cause illness, even though the immune system responds to them. Traditionally, vaccines are the pathogens themselves, or their products, that have been treated so that they are no longer virulent (able to cause disease). Today, it is possible to genetically engineer bacteria to mass-produce a protein from pathogens, and this protein can be used as a vaccine. This method has produced a vaccine against hepatitis B, a virus-induced disease, and it is being used to prepare a vaccine against malaria, a protozoan-induced disease. Immunization promotes active immunity. After a vaccine is given, it is possible to follow an active immune response by determining the amount of antibody present in a sample of plasma; this is called the antibody titer. After the first exposure to a vaccine, a primary response occurs. For a period of several days, no antibodies are present; then their concentration rises slowly, levels off, and gradually declines as the antibodies bind to the antigen or simply break down (Fig. 26.9). After a second exposure to the vaccine, a secondary response is expected. The concentration then rises rapidly to a level much greater than before; then it slowly declines. The second exposure is called a "booster" because it boosts the antibody concentration to a high level. The high antibody concentration is expected to help prevent disease symptoms if the individual is exposed to the disease-causing antigen. Active immunity is dependent on the presence of memory B cells and possibly memory T cells that are capable of responding to lower doses of antigen. Active immunity is usually long-lasting, but a booster may be required after a certain number of years. Although the body usually makes its own antibodies, it is possible to give an individual prepared antibodies (immunoglobulins) to combat a disease. Because these antibodies are not produced by the individual's plasma cells, this is called passive immunity, and it is temporary. For example, newborns are passively immune to some diseases, because antibodies have crossed the placenta from the mother's blood. These antibodies soon disappear, however, so that within a few months infants become more susceptible to infections. Breast-feeding prolongs the natural passive immunity an infant receives from the mother, because antibodies are present in the mother's milk. Even though passive immunity does not last, it is sometimes used to prevent illness in a patient who has been unexpectedly exposed to an infectious disease. Usually, the patient receives an injection of gamma globulin, a portion of blood that contains antibodies, preferably taken from an individual who has recovered from the illness. In the past, horses were immunized and gamma globulin was taken from them to provide the needed antibodies against such diseases as diphtheria, botulism, and tetanus. Unfortunately, patients who received these antibodies became ill about 50% of the time, because the serum contained proteins that the individual's immune system recognized as foreign. This condition was called serum sickness. 26.4

Genetic Engineering of Plants

At least 10,000 years ago, humans began to leave the hunting and gathering lifestyle and settled down to farm. Plants were selected and hybridized to obtain the most desirable crops for food and textiles. Following simple Mendelian genetics, hybridization is the crossing of different varieties of plants to produce plants with desirable traits. Farming in this manner was suitable for feeding small communities of people. Today, there are many more people who need to be fed. Providing enough food and textiles rests on scientists' ability to modify plants through genetic engineering. The invention of large-scale DNA sequencing and advances in modern molecular genetics have increased the understanding on how genes work and how they can be incorporated into plant DNA for very precise additions of desirable traits. A plant that has had its DNA altered in some way is said to be transgenic, or genetically modified (GM); more generally, any organism that has been altered in this way is a genetically modified organism (GMO). Figure 21.25 shows two ways genetic engineering is accomplished. The parent plant lacks a desirable trait; a gene from another organism is studied, isolated, and used to transform the parent plant. A common transformation technique relies on a natural genetic engineer, Agrobacterium tumefaciens. This bacterium infects plant cells and then inserts DNA from its plasmids into the plant's chromosomes. Another technique uses a particle gun that shoots DNA into plant cells for transformation. The transformed plant is then isolated for its new trait, and plantlets are generated. Depending on the purpose, the GM plant can then be cloned through tissue culture or can be involved in sexual reproduction to produce seeds. GM plants are often modified with genes that improve agricultural, food-quality, and medicinal traits (Table 21.2). A notable GM plant is Bt corn. In the past, corn plants were attacked by a butterfly larva called a corn borer. Growers sprayed their fields with insecticides that were costly and had a negative impact on the environment. The Bt gene, isolated from the soil bacterium Bacillus thuringiensis, produces proteins that kill corn borers. The Bt gene was inserted into corn chromosomes, resulting in corn plants resistant to the larvae. Currently, 65% of the total U.S. corn crop is Bt corn. Other GM crops, such as Bt potato and Bt cotton, also have this gene (Fig. 21.26). One of the recent successes of GM crops is the development of golden rice. This rice has been genetically modified with daffodil and bacterial genes to produce beta-carotene (provitamin A). The World Health Organization (WHO) estimates that vitamin A deficiency affects between 140 and 250 million preschool children worldwide. The deficiency is especially severe in developing countries where the major staple food is rice. Provitamin A in the diet can be converted by enzymes in the body to vitamin A, alleviating the deficiency. Environmental concerns about GM plants have focused on possible cross-hybridization with wild plants, the possibility of creating herbicide-resistant "super weeds," and the effects of GM pollen on pollinators. The biggest social concern is about the human and livestock consumption of GM plants, and laws have been proposed to place labels on all foods made with GM crops (Fig. 21.27). Currently, new GM crops under development must undergo rigorous analysis and testing before being approved in the United States. After citing a study that reviewed 25 years of GM crop research, the American Association for the Advancement of Science (AAAS) released a statement in 2012 stating that genetic engineering, resulting in crop improvement, is safe and that food labeling would only falsely alarm consumers.

Cardiovascular Disorders

Cardiovascular disease (CVD) is the leading cause of untimely death in Western countries. In the United States, it is estimated that about 31% of the population suffers from hypertension, which is high blood pressure. Normal blood pressure is 120/80 mm Hg. The top number is called systolic blood pressure because it is due to the contraction of the ventricles, and the bottom number is called diastolic blood pressure because it is measured during the relaxation of the ventricles. Hypertension occurs when blood pressure readings are higher than these numbers—say, 160/100. Hypertension is sometimes called a silent killer because it may not be detected until a stroke or heart attack occurs. Heredity and lifestyle contribute to hypertension. For example, hypertension is often seen in individuals who have atherosclerosis, an accumulation of soft masses of fatty materials, particularly cholesterol, beneath the inner linings of arteries (Fig. 23.17). Such deposits, called plaque, tend to protrude into the lumen of the vessel, interfering with the flow of blood. Atherosclerosis begins in early adulthood and develops progressively through middle age, but symptoms may not appear until an individual is 50 or older. To prevent its onset and development, the American Heart Association and other organizations recommend a diet low in saturated fat and rich in fruits and vegetables. Smoking, alcohol or other drug abuse, obesity, and lack of exercise contribute to the risk of atherosclerosis. Plaque can cause a clot to form on the irregular arterial wall. As long as the clot remains stationary, it is called a thrombus, but if it dislodges and moves along with the blood, it is called an embolus. If thromboembolism is not treated, serious health problems can result. A cerebrovascular accident, also called a stroke,often occurs when a small cranial arteriole bursts or is blocked by an embolus. Lack of oxygen causes a portion of the brain to die, and paralysis or death can result. A person is sometimes forewarned of a stroke by a feeling of numbness in the hands or face, difficulty speaking, or temporary blindness in one eye. If a coronary artery becomes completely blocked due to thromboembolism, a heart attack can result. The coronary arteries take O2-rich blood from the aorta to capillaries in the wall of the heart, and the cardiac veins return O2-poor blood from the capillaries to the right ventricle. If the coronary arteries narrow due to cardiovascular disease, the individual may first suffer from angina pectoris, chest pain that is often accompanied by a radiating pain in the left arm. When a coronary artery is completely blocked, a portion of the heart muscle dies due to a lack of oxygen. This is known as a heart attack. Two surgical procedures are frequently performed to correct a blockage or facilitate blood flow (Figure 23.18). In a coronary bypass operation, a portion of a blood vessel from another part of the body is sutured from the aorta to the coronary artery, past the point of obstruction (Fig. 23.18a). Now blood flows normally again from the aorta to the wall of the heart. In balloon angioplasty, a plastic tube is threaded through an artery to the blockage, and a balloon attached to the end of the tube is inflated to break through the blockage. A stent is often used to keep the vessel open (Fig. 23.18b).

The germ layers.

Each germ layer is responsible for the development of specific structures in the body.

Lymphatic Organs

Each of the lymphatic organs has a particular function in immunity, and each is rich in lymphocytes, one of the types of white blood cells.

Human Embryonic Development

Embryonic development encompasses all the events that occur from the time of fertilization until an animal—in this case, a human—is fully formed. Humans and other mammals have developmental stages similar to those of all animals but with some marked differences, chiefly due to the presence of extraembryonic membranes (membranes outside the embryo). Developing mammalian embryos (and fetuses in placental mammals, such as humans), such as those of reptiles and birds, depend on these membranes to protect and nourish Page 571them. Figure 29.11 shows what these membranes are and what they do in a mammal.

Epithelial Tissue Protects

Epithelial tissue, also called epithelium, forms the external coverings and internal linings of many organs and covers the entire surface of the body (Fig. 22.3). Therefore, for a substance to enter or exit the body—for example, in the digestive tract, the lungs, or the genital tract—it must cross an epithelial tissue. Epithelial cells adhere to one another, but an epithelium is generally only one cell layer thick. This enables an epithelium to serve a protective function, as substances have to pass through epithelial cells in order to reach a tissue beneath them. Notice in Figure 22.3 that the epithelial cells differ in shape. Cuboidal epithelium, which lines the kidney tubules and the lumen (cavity) of a portion of the kidney, contains cube-shaped cells that are roughly the same height as width. Columnar epithelium has cells resembling rectangular pillars or columns, with nuclei usually located near the bottom of each cell. Columnar epithelium lines portions of the lumen of the digestive tract. In addition to the shape difference, epithelial cells can be classified by the number of layers these cells make in tissues. A layer that is only one cell thick is referred to as simple. Multiple layers of cells are called stratified. Pseudostratified epithelium is a special classification in which the tissue appears to have multiple layers of cells but actually has only one. This is normally found in columnar cells, where the nuclei of the cells, instead of all being near the bottom of each cell, are in various locations in each cell, giving the appearance of multiple layers (see Fig. 22.3d). Pseudostratified epithelium lines the trachea (windpipe), where mucus secreted by some of its cells traps foreign particles and the upward motion of cilia on other cells carries the mucus to the back of the throat, where it may be either swallowed or expelled. Smoking can cause a change in mucus production and secretion and inhibit ciliary action, resulting in an inflammatory condition called chronic bronchitis. Squamous epithelium, such as that lining blood vessels and areas of gas exchange in the lungs, is composed of thin, flattened cells. The outer region of the skin, called the epidermis, is stratified squamous epithelium in which the cells have been reinforced by keratin, a protein that provides strength and waterproofing (Fig. 22.4). The stratified structure of this epithelium allows skin to protect the body from injury, drying out, and possible pathogen (virus and bacterium) invasion. One or more types of epithelial cells are the primary components of glands, which produce and secrete products (mainly hormones). For example, each mucus-secreting goblet cell in the lining of the digestive tract is a single-celled gland that produces mucus that protects the digestive tract from acidic gastric juices (see Fig. 22.3c). In the pancreas, special cells form glands that secrete the hormones responsible for maintaining blood glucose levels. Epithelial tissue cells can go through mitosis frequently and quickly, which is why it is found in places that get a lot of wear and tear. This feature is particularly useful along the digestive tract, where rough food particles and enzymes can damage the lining. Swallowing a potato chip and having a sharp edge scrape down the esophagus is a typical injury that can heal quickly due to the epithelial cell lining of the digestive tract. The liver, which is composed of cells of epithelial origin, can regenerate whole portions of itself that have been removed due to injury or surgery. But there is a price to pay for the ability of epithelial tissue to divide constantly—it is more likely than other tissue types to become cancerous. Cancers of epithelial tissue within the digestive tract, lungs, and breast are called carcinomas.

For viviparous mammals, what are typical gestation periods?

Gestation periods, the times of development in the uterus from conception to birth, vary greatly between species. The average gestation periods for some common mammals are as follows (listed in days of gestation): hamster, 16; mouse, 21; squirrel, 44; pig, 114; grizzly bear, 220; human, 270; giraffe, 425; killer whale, 500; Indian elephant, 624.

Stems

If you carry a bouquet of daisies in one hand and lean against a tree with the other hand, you will be touching two different kinds of stems. The daisy has a nonwoody stem, and the tree trunk is a woody stem.

Sexual Reproduction

In sexual reproduction, animals usually produce gametes in specialized organs called gonads. The male gonads are called testes, which produce sperm. The female gonads, or ovaries, produce eggs. Eggs or sperm are derived from germ cells that become specialized for this purpose during early development. During sexual reproduction, the egg of one parent is usually fertilized by the sperm of another, and a zygote (fertilized egg) results. Even among earthworms, which are hermaphrodites (each worm has both male and female sex organs), cross-fertilization frequently occurs. In some animals, the gonads may change function due to environmental conditions. For example, in coral reef fishes called wrasses, a male has a harem of several females. If the male dies, the largest female becomes a male. Reproduction in the slipper snail involves forming a stack of individuals. The individual that is currently at the top of the pile is the male, and all beneath are females. Many aquatic animals practice external fertilization, meaning the egg and sperm join in the water outside the body (Fig. 29.2). Following fertilization, many aquatic animals have a larval stage, an immature form capable of feeding. Over time, the larva develops into a new adult. Internal fertilization involves copulation, or sexual union, to facilitate the reception of sperm by a female. Some aquatic animals have copulatory organs. Lobsters and crayfish have modified swimmerets. In terrestrial animals, males typically have a penis for depositing sperm into the vagina of females (Fig. 29.3). However, this is not the case for all terrestrial animals. For example, birds lack a penis or vagina. Instead they have a cloaca, a chamber that receives products from the digestive, urinary, and reproductive tracts. A male transfers sperm to a female after placing his cloacal opening against hers. ollowing internal fertilization, animals either produce eggs or give birth to live offspring. Egg-laying animals are oviparous, producing eggs that will hatch after ejection from the body. Reptiles, particularly birds, are oviparous animals that provide their eggs with plentiful yolk, a rich nutrient material. Complete development takes place within a shelled egg. Special membranes, called extraembryonic membranes, serve the needs of the embryo and prevent it from drying out. The shelled egg frees these animals from the need to reproduce in the water—a significant adaptation to the terrestrial environment (see Section 19.5). Birds, in particular, tend their eggs, and newly hatched birds usually have to be fed before they are able to fly away and seek food for themselves. Complex hormones and neural regulation are involved in the reproductive behavior of parental birds. Other animals follow a different course after internal fertilization. They do not deposit and tend their eggs; instead, these animals are ovoviviparous, meaning that the eggs are retained in the body until they hatch, releasing fully developed offspring that have a way of life like that of the parent. Oysters, which are molluscs, retain their eggs in the mantle cavity, and male sea horses, which are vertebrates, have a brood pouch in which the eggs develop. Garter snakes, water snakes, and pit vipers retain their eggs in their bodies until they hatch, thus giving birth to living young. Finally, most mammals are viviparous, meaning that they produce living young. After offspring are born, the mother supplies the nutrients needed for further growth. Viviparity represents the ultimate in caring for the offspring. Some mammals, such as the duckbill platypus and the spiny anteater (monotremes), lay eggs. After hatching, the offspring are nourished by the mother. In contrast, marsupial offspring are born in a very immature state; they finish their development in a pouch, where they are supplied milk from the mother. Placental mammals, including humans, provide nourishment during development via the placenta (see Section 29.3) and after birth by mammary glands. The evolution of the placenta allowed the developing offspring to exchange materials with the mother internally.

Hearing

Most invertebrates cannot hear. Some arthropods, including insects, do have sound receptors but they are quite simple. In insects, the ear consists of a pair of air pockets, each enclosed by a membrane, called the tympanic membrane, that passes sound vibrations to sensory receptors. The human ear has a tympanic membrane also, but it is between the outer ear and middle ear (Fig. 28.3a). The outer ear collects sound waves that cause the tympanic membrane to move back and forth (vibrate) ever so slightly. Three tiny bones in the middle ear (the ossicles) amplify the sound about 20 times as it moves from one to the other. The last of the ossicles (the stapes) strikes the membrane of the oval window, causing it to vibrate; in this way, the pressure is passed to a fluid within the hearing portion of the inner ear called the cochlea (Fig. 28.3b). The term cochlea means "snail shell." Specifically, the sensory receptors of hearing are located in the cochlear canal of the cochlea. The sensory receptors for hearing are hair cells whose stereocilia are embedded in a gelatinous membrane. Collectively, they are called the spiral organ, or organ of Corti (Fig. 28.3c,d). a. The outer ear collects sound waves, the middle ear amplifies sound waves, and the cochlea contains the sensory receptors for hearing. b. The inner ear contains the cochlea, as well as the semicircular canals and the vestibule. c. The sensory receptors for hearing are hair cells within the spiral organ. d. We hear when pressure waves within the canals of the cochlea cause the hair cells to vibrate and their stereocilia to bend. The outer ear and middle ear, which collect and amplify sound waves, are filled with air. The auditory tube relieves pressure in the middle ear. But the inner ear is filled with fluid; therefore, fluid pressure waves actually stimulate the spiral organ. When the stapes strikes the membrane of the oval window, pressure waves cause the hair cells to move up and down, and the stereocilia of the hair cells embedded in the gelatinous membrane bend. The hair cells of the spiral organ synapse with the cochlear nerve, and when their stereocilia bend, nerve impulses begin in the cochlear nerve and travel to the brain stem. When these impulses reach the auditory areas of the cerebral cortex, they are interpreted as sound. Each part of the spiral organ is sensitive to different wave frequencies, which correspond to the pitch of a sound. Near the tip, the spiral organ responds to low pitches, such as the sound of a tuba; near the base, it responds to higher pitches, such as a bell or whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the brain. The pitch sensation we experience depends on which region of the spiral organ vibrates and which area of the brain is stimulated. Volume is a function of the amplitude of sound waves. Loud noises cause the spiral organ to vibrate to a greater extent. The brain interprets the resulting increased stimulation as volume. It is believed that the brain interprets the tone of a sound based on the distribution of the stimulated hair cells. Hearing Loss Especially when we are young, the middle ear is subject to infections that can lead to hearing impairment. It is quite common for youngsters to have "tubes" put into the tympanic membrane to allow the middle ear to drain, in an effort to prevent this type of hearing loss. The mobility of ossicles decreases with age, and if bone grows over the stapes, the only remedy is implantation of an artificial stapes that can move. Deafness due to middle ear damage is called conduction deafness. Deafness due to spiral organ damage is called nerve deafness. In today's society, noise pollution is common, and even city traffic can be loud enough to damage the stereocilia of hair cells (Fig. 28.4). It stands to reason, then, that frequently attending rock concerts, constantly playing a stereo loudly, or using earphones at high volume can also damage hearing. The first hint of danger can be temporary hearing loss, a "full" feeling in the ears, muffled hearing, or tinnitus (ringing in the ears). If exposure to noise is unavoidable, noise-reduction earmuffs are available, as are earplugs made from compressible, spongelike material. These earplugs are not the same as those worn for swimming, and they should not be worn interchangeably. Finally, you should be aware that some medicines may damage the ability to hear. Anyone taking anticancer drugs, such as cisplatin, and certain antibiotics, such as streptomycin, should be especially careful to protect the ears from loud noises.

The Brain

Our discussion will center on these parts of the brain: the cerebrum, the diencephalon, the cerebellum, and the brain stem (Fig. 27.8b). Cerebrum The cerebrum communicates with and coordinates the activities of the other parts of the brain. The cerebrum has two halves, and each half has a number of lobes. Most of the cerebrum is white matter, where the long axons of interneurons are taking nerve impulses to and from the cerebrum. The highly convoluted outer layer of gray matter that covers the cerebrum is called the cerebral cortex. The cerebral cortex contains over a billion cell bodies, and it is the region of the cerebrum that interprets sensation, initiates voluntary movement, and carries out higher thought processes. Investigators have found that each part of the cerebrum has specific functions (Fig. 27.9). To take an example, the primary sensory area located in the parietal lobe receives information from the skin, skeletal muscles, and joints. Each part of the body has its own receiving area in this region. The primary motor area, on the other hand, is in the frontal lobe just before the small cleft that divides the frontal lobe from the parietal lobe. Voluntary commands to skeletal muscles arise in the primary motor area, and the muscles in each part of the body are controlled by a certain section of the primary motor area. The lobes of the cerebral cortex have a number of specialized centers to receive information from the sensory receptors for sight, hearing, and smell. The lobes also have association areas, where integration occurs. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan actions. Integration in this area accounts for our most cherished human abilities: to think critically and to formulate appropriate behaviors. Diencephalon Beneath the cerebrum is the diencephalon, which contains the hypothalamus and the thalamus (see Fig. 27.8b). The hypothalamus is an integrating center that helps maintain homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland, thereby serving as a link between the nervous and endocrine systems. The thalamus is on the receiving end for all sensory input except smell. Information from the eyes, ears, and skin arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum; it participates in motor functions and higher mental processes, such as memory and emotions. The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in melatonin because it is released at night when we are sleeping. Supplements of melatonin have been effectively used to treat seasonal affective disorder, jet lag, and some sleep disorders. Cerebellum The cerebellum has two portions joined by a narrow median strip. Each portion is primarily composed of white matter, which in longitudinal section has a treelike pattern (see Fig. 27.8b). Overlying the white matter is a thin layer of gray matter, which forms a series of complex folds. The cerebellum receives sensory input from the eyes, ears, joints, and skeletal muscles about the present position of body parts, and it receives motor output from the cerebral cortex that specifies where these parts should be located. After integrating this information, the cerebellum sends motor impulses by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all of the muscles work together to produce smooth, coordinated voluntary movements. The cerebellum helps us learn new motor skills, such as playing the piano or hitting a baseball. Brain Stem The brain stem, which contains the midbrain, the pons, and the medulla oblongata, connects the rest of the brain to the spinal cord (see Fig. 27.8b). It contains tracts that ascend or descend between the spinal cord and higher brain centers. The midbrain contains important visual and auditory reflex centers, and it coordinates responses such as the startle reflex. This occurs when you automatically turn your head in response to a sudden, loud noise, trying to see its source. The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. In addition, the medulla oblongata helps control various internal organs. The pons links the medulla oblongata with the midbrain, and it is vital for the control of breathing.

Small Intestine

Processing of food in the human digestive tract is more complicated than one might think. Food is chewed in the mouth and worked on by the enzyme salivary amylase, which digests starch to maltose. In addition, the digestion of proteins begins in the stomach as pepsin digests these molecules to peptides. At this point, the contents of the digestive tract are called chyme. Chyme passes from the stomach to the small intestine, a long, coiled tube that has two functions: (1) digestion of all the molecules in chyme, including polymers of carbohydrates, proteins, nucleic acids, and fats, and (2) absorption of the nutrient molecules into the body. The first part of the small intestine is called the duodenum. Two important accessory glands—the pancreas, located behind the stomach, and the liver—send secretions to the duodenum by way of ducts (Fig. 25.8). The liver produces bile, which is stored in the gallbladder. Bile looks green because it contains pigments that are the products of hemoglobin breakdown. This green color is familiar to anyone who has observed how bruised tissue changes color. Hemoglobin within the bruised area breaks down into the same types of pigments found in bile. Bile also contains bile salts, which break up fat into fat droplets by a process called emulsification. Fat droplets mix with water and have more surface area where digestion by enzymes can occur. The pancreas produces pancreatic juice, which contains sodium bicarbonate (NaHCO3) and digestive enzymes. Sodium bicarbonate neutralizes chyme and makes the pH of the small intestine slightly basic. The higher pH helps prevent autodigestion of the intestinal lining by pepsin and is the optimal pH for the action of pancreatic enzymes. Pancreatic amylase digests starch to maltose; trypsin digests proteins to peptides; lipase digests fat droplets to glycerol and fatty acids; and nuclease digests nucleic acids to nucleotides. Still more digestive enzymes are present in the small intestine. The wall of the small intestine contains fingerlike projections called villi (Fig. 25.9). The epithelial cells of the villi produce intestinal enzymes, which remain attached to them. These enzymes complete the digestion of peptides and sugars. Peptides, which result from the first step in protein digestion, are digested by peptidase to amino acids. Maltose, which results from the first step in starch digestion, is digested by maltase to glucose. Other disaccharides, each of which is acted upon by a specific enzyme, are digested to simple sugars as well. Finally, these small nutrient molecules are absorbed into cells throughout the body from the bloodstream. Our cells use these molecules as a source of energy and as building blocks to make their own macromolecules. Absorption by Villi The wall of the small intestine is adapted to absorbing nutrient molecules because it has an extensive surface area—approximately that of a tennis court! First, the mucous membrane layer of the small intestine has circular folds that give it an almost corrugated appearance (Fig. 25.9a). Second, on the surface of these circular folds are the villi (Fig. 25.9b). Finally, the cells on the surface of the villi have minute projections called microvilli (Fig. 25.9c). If the human small intestine were simply a smooth tube, it would have to be 500 to 600 meters long to have a comparable surface area for absorption. Carnivores have a much shorter digestive tract than herbivores because meat is easier to process than plant material (Fig. 25.10). The villi of the small intestine absorb small nutrient molecules into the body. Each villus contains an extensive network of blood capillaries and a lymphatic capillary called a lacteal. As discussed in Section 23.2, the lymphatic system is an adjunct to the cardiovascular system—its vessels carry fluid, called lymph, to the cardiovascular veins. Sugars and amino acids enter the blood capillaries of a villus and are carried to the liver by way of the hepatic portal system. In contrast, glycerol and fatty acids (digested from fats) enter the epithelial cells of the villi and, within them, are joined and packaged as lipoprotein droplets, which enter a lacteal. Absorption continues until almost all nutrient molecules have been absorbed. Absorption occurs by diffusion, as well as by active transport, which requires an expenditure of cellular energy. Lymphatic vessels transport lymph to cardiovascular veins. Eventually, the bloodstream carries the nutrients absorbed by the digestive system to all the cells of the body.

Proprioceptors

Proprioceptors help the body maintain equilibrium and posture, despite the force of gravity always acting on the skeleton and muscles. A muscle spindle consists of sensory nerve endings wrapped around a few muscle cells within a connective tissue sheath. Golgi tendons and other sensory receptors are located in the joints. The rapidity of nerve impulses from proprioceptors is proportional to the stretching of the organs they occupy. A motor response results in the contraction of muscle fibers adjoining the proprioceptor.

Control of Reproduction

Table 29.1 lists various means of birth control and gives their rates of effectiveness, assuming consistent and correct usage. Figure 29.9 illustrates some birth control devices. Abstinence—that is, not engaging in sexual intercourse, is very reliable and has the added advantage of avoiding sexually transmitted diseases. An oral contraceptive, commonly called the birth control pill, often contains a combination of estrogen and progesterone and is taken on a daily basis. These hormones effectively shut down the pituitary production of both FSH and LH, so that no follicle in the ovary begins to develop; because ovulation does not occur, pregnancy cannot take place. Because of possible side effects, including headaches, blurred vision, chest or abdominal pain, and swollen legs, women taking birth control pills should see a physician regularly. Contraceptive implants use a synthetic progesterone to prevent ovulation by disrupting the ovarian cycle. The older version of the implant consists of six match-sized, time-release capsules that are surgically implanted under the skin of a woman's upper arm. The newest version consists of a single capsule that remains effective for about 3 years. Contraceptive injections are available as progesterone only or as a combination of estrogen and progesterone. The length of time between injections can vary from a few weeks to several months. Interest in barrier methods of birth control, such as condoms, has increased, because they offer some protection against sexually transmitted diseases. A female condom consists of a large, polyurethane tube with a flexible ring that fits onto the cervix. The open end of the tube has a ring that covers the external genitals. A male condom is most often a latex sheath that fits over the erect penis. The ejaculate is trapped inside the sheath and thus does not enter the vagina. When a condum is used in conjunction with a spermicide, the protection is better than with the condom alone. The diaphragm is a soft, latex cup with a flexible rim that lodges behind the pubic bone and fits over the cervix. Each woman must be properly fitted by a physician, and the diaphragm can be inserted into the vagina no more than 2 hours before sexual relations. Also, it must be used with spermicidal jelly or cream and should be left in place for at least 6 hours after sexual relations. The cervical cap is a mini-diaphragm. An intrauterine device (IUD) is a small piece of molded plastic that is inserted into the uterus by a physician or another qualified health-care practitioner. IUDs are believed to alter the environment of the uterus and uterine tubes, so that fertilization probably will not occur—but if it should occur, implantation cannot take place. Contraceptive vaccines are currently being researched. For example, a vaccine intended to immunize women against human chorionic gonadotropin (hCG), a hormone necessary to maintain the implantation of the embryo, has been successful in a limited clinical trial. Since hCG is not normally present in the body, no autoimmune reaction is expected, but the immunization does wear off with time. Other researchers believe that it would also be possible to develop a safe antisperm vaccine for women. Emergency contraception includes approaches to birth control that can prevent pregnancy after unprotected intercourse. The expression "morning-after pill" is a misnomer in that a woman can use these medications up to several days after unprotected intercourse. The first FDA-approved medication produced for emergency contraception was a kit called Preven. Preven includes four synthetic progesterone pills; two are taken up to 72 hours after unprotected intercourse, and two more are taken 12 hours later. The hormone upsets the normal uterine cycle, making it difficult for an embryo to implant in the endometrium. One study estimated that Preven was 85% effective in preventing unintended pregnancies. The Preven kit also includes a pregnancy test; women are instructed to take the test first before using the hormone, because the medication is not effective on an established pregnancy. In 2006 the FDA approved another drug, called Plan B One-Step, which is up to 89% effective in preventing pregnancy if taken within 72 hours after unprotected sex. It is available without a prescription to women age 17 and older. In August 2010, ulipristal acetate (also known as "ella") was also approved for emergency contraception. It can be taken up to 5 days after unprotected sex, and studies indicate it is somewhat more effective than Plan B One-Step. Unlike Plan B One-Step, however, a prescription is required. Mifepristone, also known as RU-486 or the "abortion pill," can cause the loss of an implanted embryo by blocking the progesterone receptors of endometrial cells. This causes the endometrium to slough off, carrying the embryo with it. When taken in conjunction with a prostaglandin to induce uterine contractions, RU-486 is 95% effective at inducing an abortion up to the 49th day of gestation. Because of its mechanism of action, the use of RU-486 is more controversial compared to other medications, and while it is currently available in the United States for early medical abortion, it is not approved for emergency contraception.

Male Reproductive System

The human male reproductive system includes the testes (sing., testis), the epididymis (pl., epididymides), the vas deferens (pl., vasa deferentia), and the urethra (Fig. 29.4a). The urethra in males is a part of both the urinary system and the reproductive system. The paired testes, which produce sperm, are suspended within the scrotum. The testes begin their development inside the abdominal cavity, but they descend into the scrotum as embryonic development proceeds. If the testes do not descend soon after birth, and the male does not receive hormone therapy or undergo surgery to place the testes in the scrotum, sterility results. Sterility is the inability to produce offspring. This type of sterility occurs because normal sperm production is inhibited at body temperature; a slightly cooler temperature is required. Sperm produced by the testes mature within the epididymis, a coiled tubule lying just outside each testis. Maturation seems to be required for the sperm to swim to the egg. Once the sperm have matured, they are propelled into the vas deferens by muscular contractions. The vasa deferentia are severed or blocked in a surgical form of birth control called a vasectomy (see Table 29.1). Sperm are stored in both the epididymis and the vas deferens. When a male becomes sexually aroused, sperm enter first the ejaculatory duct and then the urethra, part of which is located within the penis. The penis is a cylindrical organ that hangs in front of the scrotum. Three cylindrical columns of spongy, erectile tissue containing distensible blood spaces extend through the shaft of the penis (Fig. 29.4b). During sexual arousal, nervous reflexes cause an increase in arterial blood flow to the penis. This increased blood flow fills the blood spaces in the erectile tissue, and the penis, which is normally limp (flaccid), stiffens and increases in size. These changes are called an erection. If the penis fails to become erect, the condition is called erectile dysfunction (ED). Drugs such as Viagra, Levitra, and Cialis work by increasing blood flow to the penis, so that when a man is sexually excited, he can achieve and keep an erection. Semen (seminal fluid) is a thick, whitish fluid that contains sperm and secretions from three glands: the seminal vesicles, prostate gland, and bulbourethral glands. The seminal vesicles lie at the base of the bladder. Each joins a vas deferens to form an ejaculatory duct that enters the urethra. As sperm pass from the vas deferens into the ejaculatory duct, these vesicles secrete a thick, viscous fluid containing nutrients for use by the sperm. Just below the bladder is the prostate gland, which secretes a milky, alkaline fluid believed to activate or increase the motility of the sperm and neutralize the acidity of the urethra, which is due to urine. In older men, the prostate gland may become enlarged, thereby constricting the urethra and making urination difficult. Also, prostate cancer is the most common form of cancer in men. Slightly below the prostate gland, one on each side of the urethra, is a pair of small glands called bulbourethral glands, which have mucous secretions with a lubricating effect. Notice from Figure 29.4 that the urethra also carries urine from the bladder during urination. If sexual arousal reaches its peak, ejaculation follows an erection. The first phase of ejaculation is called emission. During emission, the spinal cord sends nerve impulses via appropriate nerve fibers to the epididymides and vasa deferentia. Their muscular walls contract, causing sperm to enter the ejaculatory ducts, whereupon the seminal vesicles, prostate gland, and bulbourethral glands release their secretions. Secretions from the bulbourethral glands occur first and may or may not contain sperm. During the second phase of ejaculation, called expulsion, rhythmic contractions of muscles at the base of the penis and within the urethral wall expel semen in spurts from the opening of the urethra. These contractions are an example of release from muscle tension. An erection lasts for only a limited amount of time. The penis then returns to its normal, flaccid state. Following ejaculation, a male may typically experience a time, called the refractory period, during which stimulation does not bring about an erection. The contractions that expel semen from the penis are a part of male orgasm, the physiological and psychological sensations that occur at the climax of sexual stimulation.

Accessory Organs

The pancreas and the liver are the main accessory organs of digestion, along with the salivary glands and gallbladder.

Other Locations of Lymphoid Tissue

The tonsils, which are located in the pharynx, and the appendix, which is attached to a portion of the large intestine, are lymphatic tissue structures that also belong to the immune system.

Disorders Associated with Obesity

Type 2 diabetes and cardiovascular disease are often seen in people who are obese.

AIDS

Understanding why patients with acquired immunodeficiency syndrome (AIDS) are so sick gives us a whole new level of appreciation for the workings of a healthy immune system. Human immunodeficiency virus (HIV), which causes AIDS, lives in and destroys helper T cells, which promote the activity of all the other cells in the immune system. Initially, the body of the individual infected with HIV is able to maintain an adequate number of T cells, but over time the helper T cells are destroyed faster than they can be produced and the virus gains the upper hand. Without helper T cells, the ravaged immune system can no longer fight off the onslaught of viruses, fungi, and bacteria that the body encounters every day. Without drug therapy, the number of T cells eventually drops from thousands to hundreds as the immune system becomes helpless (Fig. 26.12). More information on the life cycle of the HIV virus is provided in Section 17.1. The symptoms of AIDS begin with weight loss, chronic fever, cough, diarrhea, swollen glands, and shortness of breath and progress to those of rare diseases. Pneumocystis pneumonia, a respiratory disease found in cats, and Kaposi sarcoma, a very rare type of cancer, are often observed in patients with advanced AIDS. Death approaches rapidly and certainly. As of 2014, an estimated 36.9 million people were living with HIV infection. Among the 2.0 million new HIV infections, nearly 11% are in people under the age of 15. Although the number of deaths due to HIV/AIDS is declining, in 2014 the disease still claimed 1.2 million lives, bringing the total number of deaths attributed to HIV/AIDS to over 36 million. As of 2014, at least 0.8% of the adults in the world had an HIV infection. HIV is transmitted by sexual contact with an infected person, including vaginal or rectal intercourse and oral/genital contact. Also, needle sharing among intravenous drug users is a high-risk behavior. Babies born to HIV-infected women may become infected before or during birth, or through breast-feeding after birth. Even though male-to-male sexual contact still accounts for the greatest number of new HIV cases in the United States, heterosexual contact and intravenous drug use account for the greatest percentage of increase in new cases. Advances in treatment have reduced the serious complications of an HIV infection and have prolonged life. The sooner drug therapy begins after infection, the better the chances that HIV will not destroy the immune system. Also, medication must be continued indefinitely. Unfortunately, new strains of the virus have emerged that are resistant to the new drugs used for treatment. The likelihood of transmission from mother to child at birth can be lessened if the mother receives medication prior to birth and the child is delivered by cesarean section. Although there are many difficulties in vaccine development, AIDS vaccine trials are underway. The process can take many years. After a vaccine has been tested in animals, it must pass through three phases of clinical trials before it is marketed or administered to the public. In Phases I and II of a clinical trial, the vaccine is tested from one to two years in a small number of HIV-uninfected volunteers. The most effective vaccines move into Phase III. In Phase III, the vaccine is tested three to four years in thousands of HIV-uninfected people. A phase III trial of a preventative vaccine called RV144 concluded in 2009 in Thailand; though there is some evidence that the vaccine may help reduce HIV infection rates, researchers are still analyzing the data and working on follow-up studies. The success of RV144 has encouraged researchers to believe that a preventative vaccine may be developed in the near future. A program called the HIV Vaccine Trials Network (HVTN) has been created to coordinate and analyze the data from all of the efforts currently under way to develop a vaccine. But the most compelling reason for optimism is the human body's ability to suppress the infection. The immune system is able to decrease the HIV viral load in the body, helping delay the onset of AIDS an average of 10 years in 60% of people who are HIV-infected in the United States. Studies have shown that a small number of people remain HIV-uninfected after repeated exposure to the virus, and a few HIV-infected individuals maintain a healthy immune system for over 15 years. It is these stories of the human body's ability to fight the infection that keep scientists hopeful that there is a way to help the body overcome HIV infection. HIV infection is preventable. Suggestions for preventing this infection are to (1) abstain from sexual intercourse or develop a long-term, monogamous (always the same partner) sexual relationship with a person who is free of HIV; (2) be aware that having relations with an intravenous drug user is risky behavior; (3) avoid anal-rectal intercourse, because the lining of the rectum is thin and infected T cells easily enter the body there; (4) always use a latex condom during sexual intercourse if you do not know that your partner has been free of HIV for the past five years; (5) avoid oral sex, because this can be a means of transmission; and (6) be cautious about the use of alcohol or any other drug that may prevent you from being able to control your behavior.

Urine Formation

Urine formation requires three steps: filtration, reabsorption, and secretion (Fig. 24.14). Filtration Filtration occurs whenever small substances pass through a filter and large substances are left behind. During urine formation, filtration is the movement of small molecules from a blood capillary to the inside of the glomerular capsule as a result of blood pressure. Small molecules, such as water, nutrients, salts, and urea, move to the inside of the capsule. Plasma proteins and blood cells are too large to be part of this filtrate, so they remain in the blood. If the composition of the filtrate were not altered in other parts of the nephron, death from loss of nutrients (starvation) and loss of water (dehydration) would quickly follow. The next step, reabsorption, helps prevent this. Reabsorption of Solutes Reabsorption takes place when water and other substances move from the proximal convoluted tubule into the blood. Nutrients such as glucose and amino acids also return to the blood. This process is selective because some molecules, such as glucose, are both passively and actively reabsorbed. The cells of the proximal convoluted tubule have numerous microvilli, which increase the surface area, and numerous mitochondria, which supply the energy needed for active transport. However, if there is more glucose in the filtrate than there are carriers to handle it, glucose will appear in the urine. Glucose in the urine is a sign of diabetes mellitus, sometimes caused by a lack of the hormone insulin. Sodium ions (Na+) are also actively pumped into the peritubular capillary, and then chloride ions (Cl-) follow passively. Water moves by osmosis from the tubule into the blood. Protein channels, called aquaporins, in the membrane of the epithelial cells lining the proximal convoluted tubule allow for the rapid reabsorption of water. Overall, about 60-70% of salt and water are reabsorbed at the proximal convoluted tubule. Some of the urea, the primary nitrogenous waste product of human metabolism, and other types of nitrogenous wastes excreted by humans are passively reabsorbed, but most of these wastes remain in the filtrate. Secretion Secretion is the transport of substances into the nephron by means other than filtration. For our purposes, secretion may be particularly associated with the distal convoluted tubule. Substances such as uric acid, hydrogen ions, ammonia, and penicillin are eliminated by secretion. The process of secretion helps rid the body of potentially harmful compounds that were not filtered into the capsule. Regulation of Water-Salt Balance and pH All animals have some means of maintaining the water-salt balance and pH of the internal environment. In humans, the long nephron loop allows for the secretion of a hypertonic urine (Fig. 24.14). The ascending limb of the nephron loop pumps salt and urea into the renal medulla, and water follows by osmosis both at the descending limb of the nephron loop and at the collecting duct. As you will see in Section 27.2, several hormones are involved in regulating water-salt reabsorption by the kidneys. Drinking coffee interferes with one of these hormones, and that is why coffee is a diuretic, a substance that causes the production of more urine. The human kidney also assists in the regulation of the pH of the blood. Both the bicarbonate (HCO3-) buffer system of the blood and the regulation of breathing rate help rid the body of CO2 and contribute to maintaining blood pH. As helpful as these mechanisms might be, only the kidneys can excrete a wide range of acidic and basic substances. The kidneys are slower-acting than the buffer/breathing mechanism, but they have a more powerful effect on pH. For the sake of simplicity, we can think of the kidneys as reabsorbing bicarbonate ions and excreting hydrogen ions as needed to maintain the normal pH of the blood (Fig. 24.15). If the blood is acidic, hydrogen ions are excreted and bicarbonate ions are reabsorbed. If the blood is basic, hydrogen ions are not excreted and bicarbonate ions are not reabsorbed. The fact that urine is most often acidic shows that usually an excess of hydrogen ions is produced by the body and excreted. Ammonia (NH3) provides a means for buffering these hydrogen ions in urine: (NH3 + H+ → NH4+). Ammonia (the presence of which is quite obvious in a diaper pail or kitty litter box) is produced in tubule cells by the breakdown of amino acids. Phosphate provides another means of buffering hydrogen ions in urine. Regulation of Water-Salt Balance in Other Animals Insects have a unique excretory system consisting of long, thin tubules called Malpighian tubules attached to the gut (Fig. 24.16). Uric acid, the primary nitrogenous waste product of insects, simply diffuses from the surrounding hemolymph into these tubules, and water follows a salt gradient established by active transport of potassium (K+). Water and other useful substances are reabsorbed at the rectum, but the uric acid leaves the body at the anus. Insects that live in water or eat large quantities of moist food reabsorb little water. But insects in dry environments reabsorb most of the water and excrete a dry, semisolid mass of uric acid. Most animals can regulate the blood levels of both water and salt. For example, freshwater fishes take up salt in the digestive tract and gills and produce large amounts of dilute urine. Saltwater fishes take in salt water by drinking, but then they pump ions out at the gills and produce only small amounts of concentrated urine.

Stomach

The human stomach is a thick-walled, J-shaped organ (Fig. 25.6) on the left side of the abdominal cavity below the liver. The stomach is continuous with the esophagus above and the duodenum of the small intestine below. The cardiac sphincter separates the esophagus from the stomach. A sphincter is a muscle that surrounds a tube and closes or opens it by contracting and relaxing. The stomach is about 25 cm (10 in.) long, regardless of the amount of food it holds, but the diameter varies, depending on how full it is. The stomach receives food from the esophagus, stores food, starts the digestion of proteins, and moves food into the small intestine. The wall of the stomach has deep folds, which disappear as the stomach fills to an approximate capacity of 1 liter. Therefore, humans can periodically eat relatively large meals, freeing the rest of their time for other activities. But the stomach is much more than a mere storage organ. The wall of the stomach contains three muscle layers: one is longitudinal, another is circular, and the third is obliquely arranged. The muscular walls mechanically digest food by contracting vigorously to mix it with digestive juices, which are secreted whenever food enters the stomach. The epithelial lining of the stomach, called a mucosa, has millions of gastric glands. These gastric glands produce gastric juice containing so much hydrochloric acid that the stomach routinely has a pH of about 2. Such strong acidity is usually sufficient to kill any microbes that might be in food. This low pH also promotes the activity of pepsin, a hydrolytic enzyme that acts on proteins to produce peptides. Sometimes, the stomach's strong acidity causes heartburn or even gastric reflux disease when gastric juice backs up into the esophagus. As with the rest of the digestive tract, a thick layer of mucus protects the wall of the stomach from enzymatic action. Ulcers are open sores in the wall caused by the gradual destruction of tissues. Most ulcers are due to an infection by an acid-resistant bacterium, Helicobacter pylori, which is able to attach to the epithelial lining. Wherever the bacterium attaches, the lining stops producing mucus and the area becomes exposed to digestive action, allowing an ulcer to develop. Alcohol and other liquids are absorbed in the stomach, but other nutrients are not. Peristalsis pushes food along in the stomach, as it does in other digestive organs (see Fig. 25.6b). At the base of the stomach is a narrow opening called the pyloric sphincter. When food leaves the stomach, it is a thick, soupy liquid called chyme. Whenever the pyloric sphincter relaxes, a small quantity of chyme squirts through the opening into the small intestine. Ruminants Ruminants, a type of mammal that includes cattle, sheep, goats, deer, and buffalo, are named for a part of their stomach, the rumen (Fig. 25.7). The rumen contains symbiotic bacteria and protozoans that produce enzymes that can digest cellulose, an ability that other mammals lack. After herbivores feed on grass, it goes to the rumen, where it is broken down by the enzymes released by the microbes, and then it becomes small balls of cud. The cud returns to the mouth, where the animal "chews the cud." The cud may return to the rumen for a second go-round before passing through the other chambers of the stomach. The rumen is an adaptation to a diet rich in fiber that may have been promoted by competition among the many types of animals that feed on grass. The last chamber in ruminants is analogous to the human stomach, being the place where proteins are digested to peptides.

Can Proteins Be Harmful?

According to nutritionists, proteins should not supply the bulk of dietary calories. The average American eats about twice as much protein as he or she needs, and some people may be on a diet that encourages the intake of proteins instead of carbohydrates as an energy source. Also, body builders, weight lifters, and other athletes may include amino acid or protein supplements in the diet because they think these supplements will increase muscle mass. However, excess amino acids are not always converted into muscle tissue. When they are used as an energy source, the liver removes the nitrogen portion (in the process called deamination) and uses it to form urea, which is excreted in urine. The water needed for the excretion of urea can cause dehydration when a person is exercising and losing water by sweating. High-protein diets can also increase calcium loss in the urine and encourage the formation of kidney stones. Furthermore, many high-protein foods contain a high amount of fat.

Tissue Rejection

Certain organs, such as the skin, the heart, and the kidneys, could be transplanted easily from one person to another if the body did not attempt to reject them. Rejection occurs because cytotoxic T cells and antibodies bring about the destruction of foreign tissues in the body. When rejection occurs, the immune system is correctly distinguishing between self and nonself. Organ rejection can be controlled by carefully selecting the organ to be transplanted and administering immunosuppressive drugs. It is best if the transplanted organ has the same type of MHC proteins as those of the recipient; otherwise, the transplanted organ is antigenic to the recipient's T cells. Several immunosuppressive drugs act by inhibiting the response of T cells to cytokines. Without cytokines, all types of immune responses are weak.

Cutaneous Receptors and Proprioceptors

Cutaneous receptors and proprioceptors provide sensory input from the skin, muscles, and joints to the primary sensory area of the cerebral cortex. Within the cerebral cortex are areas that represent each part of the body. Sensory receptors in the skin, muscles, and joints send nerve impulses to the CNS. Sensation and perception occur when these reach the primary sensory area of the cerebral cortex. A motor response can be initiated by the spinal cord and cerebellum without the involvement of the cerebrum.

Hormonal Regulation in Females

Estrogen and progesterone are the female sex hormones. Estrogen, in particular, is essential for the normal development and functioning of the female reproductive organs. Estrogen is also largely responsible for the secondary sex characteristics in females, including body hair and fat distribution. In general, females have a more rounded appearance than males because of a greater accumulation of fat beneath their skin. Also, the pelvic girdle aligns so that females have wider hips than males, and the thighs converge at a greater angle toward the knees. Both estrogen and progesterone are required for breast development as well. Menopause, which usually occurs between ages 45 and 55, is the time in a woman's life when the ovarian and menstrual cycles cease. Menopause is not considered complete until menstruation has been absent for a year.

The Digestive Tract

The digestive tract of humans, and most other vertebrates, consists of the mouth, esophagus, stomach, small intestine, and large intestine (Fig. 25.3).

Organ Formation Continues

A human embryo at 5 weeks has little flippers called limb buds (Fig. 29.15). Later, the arms and legs develop from the limb buds, and even the hands and feet become apparent. During the fifth week, the head enlarges and the sense organs become more prominent. The umbilical cord has developed from a bridge of mesoderm called the body stalk, which connects the caudal (tail) end of the embryo with the chorion. A fourth extraembryonic membrane, the allantois, is contained within this stalk, and its blood vessels become the umbilical blood vessels. The head and the tail then lift up as the body stalk moves anteriorly by constriction. Once this process is complete, the umbilical cord, which connects the developing embryo to the placenta, is fully formed. A remarkable change in external appearance occurs during the sixth through eighth weeks of development—the embryo becomes easily recognized as human. Concurrent with brain development, the neck region develops, making the head distinct from the body. The nervous system is developed well enough to permit reflex actions, such as a startle response to touch. At the end of this period, the embryo is about 38 mm (1.5 in) long and weighs no more than an aspirin tablet, even though all its organ systems are established.

Gastrulation

A major event, called gastrulation, turns the inner cell mass into the embryonic disk. Gastrulation is an example of morphogenesis, during which cells move or migrate. In this case, cells migrate to become tissue layers called germ layers. By the time gastrulation is complete, the embryonic disk has become an embryo with three primary germ layers: ectoderm, mesoderm, and endoderm. Gastrulation is complete when the three layers of cells that will develop into adult organs have been produced. The outer layer is the ectoderm; the inner layer is the endoderm; and the third, or middle, layer of cells is called the mesoderm. Ectoderm, mesoderm, and endoderm are called the embryonic germ layers. As shown in Figure 29.14, the organs of an animal's body develop from these three germ layers.

Are genetically modified crops organic?

According to the U.S. Department of Agriculture (USDA), an organic crop is one that is produced without the use of pesticides, irradiation, hormones, antibiotics, or bioengineering. Therefore, a genetically modified crop may not be marketed as an organic crop, since it is a product of artificial technology.

The Senses

All living organisms respond to stimuli. Stimuli are environmental signals that tell us about the external environment or the internal environment. In Chapter 21, you learned that plants often respond to external stimuli, such as light, by changing their growth pattern. An animal's response often results in motion. Complex animals rely on sensory receptors to provide information to the central nervous system (brain and spinal cord), which integrates sensory input before directing a motor response (Fig. 28.1). Section 28.2 will explore how the muscular and skeletal systems are involved in the motor response. Sense organs, as a rule, are specialized to receive one kind of stimulus. The eyes ordinarily respond to light, ears to sound waves, pressure receptors to pressure, and chemoreceptors to chemical molecules. Sensory receptors transform the stimulus into nerve wimpulses that reach a particular section of the cerebral cortex. Those from the eye reach the visual areas, and those from the ears reach the auditory areas. Before sensory receptors initiate nerve signals, they also carry out integration, the summing up of signals. One type of integration is called sensory adaptation, a decrease in response to a stimulus. We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it. When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased. However, the brain, not the sensory receptor, is ultimately responsible for sensation and perception, and each part of the brain interprets impulses in only one way. For example, if by accident the photoreceptors of the eye are stimulated by pressure and not light, the brain causes us to see "stars" or other visual patterns.

Asexual Versus Sexual Reproduction

Animals usually reproduce sexually, but some can reproduce asexually. In asexual reproduction, there is only one parent and the offspring are usually genetically the same as the parents. In sexual reproduction, there are two parents, and the genetic material is shuffled, so the offspring usually have combinations of genes that are not like those of either parent.

The Function of Antibodies

Antibodies are immunoglobulin proteins that are capable of combining with a specific antigen (Fig. 26.6). The antigen-antibody reaction can take several forms, but quite often the reaction produces complexes of antigens combined with antibodies. Such antigen-antibody complexes, sometimes called immune complexes, mark the antigens for destruction (see Fig. 26.5). An antigen-antibody complex may be engulfed by neutrophils or macrophages, or it may activate the complement system (see Section 26.2), making the pathogens more susceptible to phagocytosis.

Skeletal Muscle Structure and Physiology

As we explored in Section 22.1, the three types of muscle—smooth, cardiac, and skeletal—have different structures and functions in the body (see Fig. 22.7). Smooth muscles contain sheets of long, spindle-shaped cells, each with a single nucleus. Cardiac cells are striated (having a striped appearance) and typically possess a single nucleus. Cardiac muscle tissue contains branched chains of cells that interconnect, forming a lattice network. Skeletal muscle cells, called muscle fibers, are also striated. They are quite elongated, and they run the length of a skeletal muscle. Skeletal muscle fibers arise during development when several cells fuse, resulting in one long, multinucleated cell. We will focus our discussion on skeletal muscles, since they represent the major type of muscle in the body and are the ones that are under voluntary control of the nervous system. Figure 28.16 illustrates some of the major skeletal muscles in the body.

What is blood doping?

Blood doping is any method of increasing the normal supply of red blood cells for the purpose of delivering oxygen more efficiently, reducing fatigue, and giving an athlete a competitive edge. To accomplish blood doping, athletes can inject themselves with EPO some months before the competition. These injections will increase the number of red blood cells in their blood. Several weeks later, units of their blood are removed and centrifuged to concentrate the red blood cells. The concentrated cells are reinfused shortly before the athletic event. Blood doping is a dangerous, illegal practice. Several cyclists died in the 1990s from heart failure, probably due to blood that was too thick with cells for the heart to pump.

Blood: A Transport Medium

Blood is a form of fluid connective tissue that has a number of important roles in the body. These include: 1. Transport of substances to and from the capillaries, where exchange with the interstitial fluid takes place 2. Defense of the body against invasion by pathogens (e.g., disease-causing viruses and bacteria) 3. Assisting in homeostasis by regulating body temperature and pH In humans, blood has two main portions: the liquid portion, called plasma, and the formed elements, consisting of various cells and platelets. These portions can be separated by spinning blood in a centrifuge (Fig. 23.14).

STDs Caused by Bacteria

Chlamydia is a bacterial infection of the lower reproductive tract that is usually mild or asymptomatic, especially in women. About 8-21 days after infection, men may experience a mild burning sensation upon urination and a mucoid discharge. Women may have a vaginal discharge, along with the symptoms of a urinary tract infection. Chlamydia also causes cervical ulcerations, which increase the risk of acquiring AIDS. If the infection is misdiagnosed or if a woman does not seek medical help, there is a risk of the infection spreading from the cervix to the uterine tubes, and pelvic inflammatory disease (PID)results. This very painful condition can result in blockage of the uterine tubes, with the possibility of sterility or infertility. Gonorrhea is easier to diagnose in males than in females because males are more likely to experience painful urination and a thick, greenish-yellow urethral discharge. In males and females, a latent infection leads to PID, which affects the vasa deferentia or uterine tubes. As the inflamed tubes heal, they may become partially or completely blocked by scar tissue, resulting in sterility or infertility. Syphilis has three stages, which are typically separated by latent periods. During the final stage, syphilis may affect the cardiovascular system and/or nervous system. An infected person may become mentally retarded or blind, walk with a shuffle, or show signs of insanity. Gummas, which are large, destructive ulcers, may develop on the skin or in the internal organs. Syphilitic bacteria can cross the placenta, causing birth defects or stillbirth. Syphilis is easily diagnosed with a blood test.

Where does most of the sodium in the diet come from?

Contrary to popular belief, the majority of the sodium in your diet does not come from the salt you put on your food when you are eating. Instead, most dietary sodium (over three-quarters!) comes from processed foods and condiments. Sodium is used in these items both to preserve the food or condiment and to make it taste better. But sometimes the amount of sodium is phenomenal. A single teaspoon of soy sauce contains almost 1,000 mg of sodium, and a half-cup of prepared tomato sauce typically has over 400 mg of sodium. Websites such as nutritiondata.com can help you track your daily sodium intake.

Diabetes Mellitus

Diabetes mellitus is a fairly common hormonal disease in which the cells of the body do not take up and/or metabolize glucose. Therefore, the cells are in need of glucose, even though there is plenty in the blood. As the blood glucose level rises, water and glucose are excreted in the urine. The loss of water in this way causes the diabetic person to be extremely thirsty. There are two types of diabetes mellitus. In type 1 diabetes (insulin-dependent diabetes), the pancreas is not producing insulin. The condition is believed to be brought on by exposure to an environmental agent, most likely a virus, whose presence causes cytotoxic T cells to destroy the pancreatic islets. The cells turn to the breakdown of protein and fat for energy. The metabolism of fat leads to acidosis (acid blood), which can eventually cause coma and death. As a result, the individual must have daily insulin injections. These injections control the diabetic symptoms but can still cause inconveniences, since either taking too much insulin or failing to eat regularly can bring on the symptoms of hypoglycemia (low blood sugar). These symptoms include perspiration, pale skin, shallow breathing, and anxiety. Because the brain requires a constant supply of glucose, unconsciousness can result. The treatment is quite simple: Immediate ingestion of a sugar cube or fruit juice can very quickly counteract hypoglycemia. Of the 29.1 million people who now have diabetes in the United States, most have type 2 diabetes (non-insulin-dependent diabetes). This type of diabetes mellitus usually occurs in people of any age who are obese and inactive (see Section 25.5). The pancreas produces insulin, but the liver and muscle cells do not respond to it in the usual manner. These cells are said to be insulin-resistant. If type 2 diabetes is untreated, the results can be as serious as those of type 1. People with diabetes of either type are prone to blindness, kidney disease, and circulatory disorders. It is usually possible to prevent or at least control type 2 diabetes by adhering to a low-fat and low-sugar diet and exercising regularly.

Ovarian and menstrual cycles.

During the follicular phase of the ovarian cycle (pink), FSH released by the anterior pituitary promotes the maturation of follicles in the ovary. Ovarian follicles produce increasing levels of estrogen, which cause the endometrium to thicken during the proliferative phase of the menstrual cycle (bottom). After ovulation and during the luteal phase of the ovarian cycle (yellow), LH promotes the development of a corpus luteum. This structure produces increasing levels of progesterone, which cause the endometrium to become secretory. Menstruation begins when progesterone production declines to a low level.

Nephrons

Each nephron is made up of several parts (Fig. 24.14). The blind, or closed, end of a nephron is pushed in on itself to form the glomerular capsule. Filtration of the blood occurs at this portion of the nephron. The inner layer of the capsule is composed of specialized cells that allow the easy passage of molecules. Leading from the capsule is a portion of the nephron called the proximal convoluted tubule, which is lined by cells with many mitochondria and tightly packed microvilli. Then comes the nephron loop, with a descending limb and an ascending limb. This is followed by the distal convoluted tubule. Several distal tubules enter one collecting duct. A collecting duct delivers urine to the renal pelvis. The nephron loop and the collecting duct give the pyramids of the renal medulla their striped appearance (see Fig. 24.13a). Each nephron has its own blood supply, and various exchanges occur between parts of the nephron and a blood capillary as urine forms.

Can Lipids Be Harmful?

Elevated blood cholesterol levels are associated with an increased risk of cardiovascular disease, the number one killer of Americans. A diet rich in cholesterol and saturated fats increases the risk of cardiovascular disease (see Section 25.5). Statistical studies suggest that trans fats are even more harmful than saturated fats. Trans fats are formed when unsaturated oils are hydrogenated to produce solid fats, found largely in processed foods. Trans fatty acids may reduce the function of the plasma membrane receptors that clear cholesterol from the bloodstream. Trans fatty acids are found in commercially packaged foods, such as cookies and crackers; in commercially fried foods, such as french fries; in packaged snacks, such as microwave popcorn; and in vegetable shortening and some margarines. Table 25.3 tells you how to reduce harmful lipids in the diet. To Reduce Dietary Saturated Fat: - Choose poultry, fish, or dry beans and peas as a protein source. - Remove skin from poultry and trim fat from red meats before cooking; place on a rack, so that fat drains off. - Broil, boil, or bake rather than frying. - Limit your intake of butter, cream, trans fats, shortening, and tropical oils (coconut and palm oils).* - To season vegetables, use herbs and spices instead of butter, margarine, or sauces. Use lemon juice instead of salad dressing. - Drink skim milk instead of whole milk, and use skim milk in cooking and baking. To Reduce Dietary Cholesterol: -Eat white fish and poultry in preference to cheese, egg yolks, liver, and certain shellfish (shrimp and lobster). -Substitute egg whites for egg yolks in both cooking and eating. -Include soluble fiber in the diet. Oat bran, oatmeal, beans, corn, and fruits, such as apples, citrus fruits, and cranberries, are high in soluble fiber. *Although coconut and palm oils are from plant sources, they are mostly saturated fat.

Adrenal Medulla

Epinephrine (adrenaline) and norepinephrine (noradrenaline) produced by the adrenal medulla rapidly bring about all the body changes that occur when an individual reacts to an emergency situation. In so doing, these two hormones complement the actions of the sympathetic autonomic system. The effects of these hormones are short-term. In contrast, the hormones produced by the adrenal cortex provide a long-term response to stress.

Fertilization

Fertilization", which results in a zygote, requires that the sperm and the secondary oocyte interact (Fig. 29.12). The plasma membrane of the secondary oocyte is surrounded by an extracellular material termed the zona pellucida. In turn, the zona pellucida is surrounded by a few layers of adhering follicle cells. During fertilization, a sperm moves past the leftover follicle cells, and acrosomal enzymes released by exocytosis digest a route through the zona pellucida. A sperm binds and fuses to the secondary oocyte's plasma membrane, and the sperm's nucleus enters. Only then does the secondary oocyte complete meiosis II and become an egg. Finally, the haploid egg and sperm nuclei fuse. Only one sperm should fertilize an egg, or the zygote will have too many chromosomes and the resulting zygote will not be viable. Changes in the zona pellucida usually prevent the binding and penetration of additional sperm (called polyspermy).

Photoreceptors of the Eye

Figure 28.9 illustrates the structure of the photoreceptors in the human eye, which are called rods and cones. Both types of photoreceptors contain a visual pigment similar to that found in all types of eyes throughout the animal kingdom. The visual pigment in rods is a deep-purple pigment called rhodopsin. Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, which is a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal, leading to a cascade of reactions that ends in the generation of nerve impulses. Rods are very sensitive to light and therefore are suited to night vision. Because carrots are rich in vitamin A, it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina; therefore, they also provide us with peripheral vision and the perception of motion. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing opsin, which sets in motion a cascade of reactions that ends in nerve impulses. The cones, on the other hand, are located primarily in a part of the retina called the fovea. Cones are activated by bright light; they allow us to detect the fine detail and color of an object. Color vision depends on three different kinds of cones, which contain pigments called B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but a slight difference in the opsin structure of each accounts for their specific absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color. Retina The retina has three layers of cells, and light has to penetrate through the first two layers to reach the photoreceptors (Fig. 28.10). The intermediate cells of the middle layer process and relay visual information from the photoreceptors to the ganglion cells that have axons forming the optic nerve. The relative sensitivity of cones versus rods is mirrored by how directly these two kinds of photoreceptors connect to ganglion cells. Information from several hundred rods may converge on a single ganglion cell, while cones show very little convergence. As signals pass through the layers of the retina, integration occurs. Integration improves the overall contrast and quality of the information sent to the brain, which uses the information to form an image of the object. No rods and cones occur where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a very small dot to the right of center on a piece of paper. Close your left eye; then use your right hand to move the paper slowly toward your right eye while you look straight ahead. The dot will disappear at one point—this point is your blind spot.

Similar Receptors in Other Animals

Gravitational equilibrium organs, called statocysts, are found in several types of invertebrates, including cnidarians, molluscs, and crustaceans. These organs give information only about the position of the head; they are not involved in the sensation of movement (Fig. 28.6a). When the head stops moving, a small particle called a statolith stimulates the cilia of the closest hair cells, and these cilia generate impulses, indicating the position of the head. The lateral line system of fishes uses sense organs similar to those in the human inner ear (Fig. 28.6b). In bony fishes, the system consists of sense organs located within a canal that has openings to the outside. As you might expect, the sense organ is a collection of hair cells with cilia embedded in a gelatinous membrane. Water currents and pressure waves from nearby objects cause the membrane and the cilia of the hair cells to bend. Thereafter, the hair cells initiate nerve impulses that go to the brain. Fishes use these data not for hearing or balance but to locate other fish, including predators, prey, and mates.

Gills of Fish

In contrast to the lungs of terrestrial vertebrates, fish and other aquatic animals use gills as their respiratory organ (Fig. 24.9). In fish, water is drawn into the mouth and out from the pharynx across the gills. The flow of blood in gill capillaries is opposite the flow of water across the gills; therefore, the blood is always exposed to water having a higher oxygen content. In the end, about 80-90% of the dissolved oxygen in the water is absorbed.

Human Reproduction

In human males and females, the reproductive system consists of two components: (1) the gonads, either testes or ovaries, which produce gametes and sex hormones, and (2) accessory organs that conduct gametes and, in the female, house the embryo/fetus.

Urinary System

In vertebrate animals, such as humans, the urinary system plays an important role in homeostasis. This role is primarily conducted by the kidneys (Fig. 24.11), the organs that are primarily responsible for the following functions of the urinary system: 1. Excretion of nitrogenous wastes, such as urea and uric acid 2. Maintenance of the water-salt balance of the blood 3. Maintenance of the acid-base balance of the blood In humans, the kidneys are bean-shaped, reddish-brown organs, each about the size of a fist. They are located on each side of the vertebral column, just below the diaphragm, where they are partially protected by the lower rib cage. Urine made by the kidneys is conducted from the body by the other organs in the urinary system. Each kidney is connected to a ureter, a tube that takes urine from the kidney to the urinary bladder, where it is stored until it is voided from the body through the single urethra (Fig. 24.12). In males, the urethra passes through the penis; in females, it opens in front of the opening of the vagina. In females, there is no connection between the genital (reproductive) and urinary systems, but there is a connection in males, since the urethra also carries sperm during ejaculation. In amphibians, birds, reptiles, and some fishes, the bladder empties into the cloaca, a common chamber and outlet for the digestive, urinary, and genital tracts.

Control of Breathing

Increased concentrations of hydrogen ions (H+) and carbon dioxide (CO2) in the blood are the primary stimuli that increase the breathing rate in humans. The chemical content of the blood is monitored by chemoreceptors called aortic bodies and carotid bodies, specialized structures in the walls of the aorta, and common carotid arteries. These receptors are very sensitive to changes in H+ and CO2 concentrations, but they are only minimally sensitive to lower oxygen (O2) concentrations. The need to breathe comes from a buildup of CO2 in the bloodstream, not necessarily from a lack of oxygen. Information from the chemoreceptors goes to the breathing center in the brain, which increases the breathing rate when concentrations of hydrogen ions and carbon dioxide rise (Fig. 24.7). The breathing center is also directly sensitive to the chemical content of the blood, including its oxygen content.

What is LASIK surgery?

LASIK, which stands for laser in-situ keratomileusis, is a quick and painless procedure that involves the use of a laser to permanently change the shape of the cornea. During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea and allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser will remove a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. Improvements to vision begin as soon as the day after the surgery but typically take two to three months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person's vision was before the surgery.

Lymph Nodes

Lymph nodes are small, ovoid structures along lymphatic vessels (Fig. 26.1b). Lymph nodes filter lymph and keep it free of pathogens and antigens. Lymph is filtered as it flows through a lymph node because the node's many sinuses (open spaces) are lined by macrophages—large, phagocytic cells that engulf and then devour as many as a hundred pathogens and still survive (see Fig. 26.4a). Lymph nodes are also instrumental in fighting infections and cancer, because they contain many lymphocytes. Some lymph nodes are located near the surface of the body and are named for their location. For example, inguinal nodes are in the groin, and axillary nodes are in the armpits. Physicians often feel for the presence of swollen, tender lymph nodes in the neck as evidence that the body is fighting an infection. This method is a noninvasive, preliminary way to help them make a diagnosis.

Drug Abuse

Many drugs that affect the nervous system act by interfering with or promoting the action of neurotransmitters. A drug can either enhance or block the release of a neurotransmitter, mimic the action of a neurotransmitter or block the receptor for it, or interfere with the removal of a neurotransmitter from a synaptic cleft. Stimulants are drugs that increase the activity of the CNS, and depressants decrease its activity. Increasingly, researchers are coming to believe that dopamine, a neurotransmitter in the brain, is responsible for mood. Many of the new medications developed to counter drug dependence and mental illness affect the release, reception, or breakdown of dopamine. Drug abuse is apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect. A drug abuser often takes more of the drug than was intended. Drug abusers are apt to display a psychological and/or physical dependence on the drug. With physical dependence, formerly called "addiction," more of the drug is needed to get the same effect, and withdrawal symptoms occur when the user stops taking the drug.

Problems with Kidney Function

Many types of illnesses, especially diabetes, hypertension, and inherited conditions, cause progressive renal disease and renal failure. Urinary tract infections, an enlarged prostate gland, pH imbalances, or simply an intake of too much calcium can lead to kidney stones. Kidney stones form in the renal pelvis and usually pass unnoticed in the urine flow. If they grow to several centimeters and block the renal pelvis or ureter, back pressure builds up and destroys nephrons. One of the first signs of nephron damage is the presence of albumin, white blood cells, or even red blood cells in the urine, as detected by a urinalysis. If damage is so extensive that more than two-thirds of the nephrons are inoperative, urea and other waste substances accumulate in the blood. Although nitrogenous waste in the blood is a threat to homeostasis, the retention of water and salts is of even greater concern. Edema, fluid accumulation in the body tissues, may occur. Imbalance in the ionic composition of body fluids can lead to loss of consciousness and even heart failure.

Body Mass Index

Medical researchers use the body mass index (BMI) to determine if a person is overweight or obese. On the whole, our height is determined genetically, while our weight is influenced by other factors as well, particularly diet and lifestyle. BMI is a number that reflects the relationship between a person's weight and height. Here's how to calculate your BMI: BMI = weight x height^2 For example, a woman whose height is 63 inches and whose weight is 133 pounds has a BMI of 23.56. underweight BMI = less than 18.5 heathy BMI = 18.5-24.9 overweight BMI = 25.0-29.9 obese BMI = 30.0-39.9 morbidly obese BMI = 40.0 or more

Methamphetamine

Methamphetamine is a synthetic drug made by adding a methyl group to amphetamine. Over 9 million people in the United States have used methamphetamine at least once in their lifetime; teenagers and young adults represent approximately one-fourth of these. The addition of the methyl group is fairly simple, so methamphetamine is often produced from amphetamine in makeshift home laboratories. It is available as a powder (speed) or as crystals ("crystal meth," "glass," or "ice"). The crystals are smoked, and the effects are almost instantaneous and nearly as quick as when methamphetamine is snorted. When the drug is smoked, the effects last 4-8 hours. Methamphetamine has a structure similar to that of dopamine, and its stimulatory effect mimics that of cocaine. It reverses fatigue, maintains wakefulness, and temporarily elevates the mood of the user. After the initial rush, there is typically a state of high agitation that, in some individuals, leads to violent behavior. Chronic use can lead to what is called an amphetamine psychosis, resulting in paranoia; auditory and visual hallucinations; self-absorption; irritability; and aggressive, erratic behavior. Drug tolerance, dependence, and addiction are common. Hyperthermia, convulsions, and death can occur.

How many muscles are in the human body?

Most experts agree that there are over 600 muscles in the human body. The exact number varies, since some experts lump muscles together under one name, while others split them apart. The smallest muscle is the stapedius, a 1.27-millimeter-long muscle in the middle ear. The longest muscle is the sartorius, which starts at the hip and extends to the knee. The biggest muscle (in terms of mass) is the gluteus maximus, the muscle that makes up the majority of each buttock.

The Bottom Line

Most nutritionists agree that a healthy diet: - Has a moderate total fat intake and is low in saturated fats, trans fats, and cholesterol (see Table 25.3 for help in achieving this goal) - Is rich in whole-grain products, vegetables, and legumes (e.g., beans and peas) as sources of complex carbohydrates and fiber - Is low in refined carbohydrates, such as starches and sugars (see Table 25.2 for help in achieving this goal) - Is low in salt and sodium content (see Table 25.5 for help in achieving this goal) - Contains only adequate amounts of protein, largely from poultry, fish, and plants - Includes only moderate amounts of alcohol - Contains adequate amounts of minerals and vitamins but avoids questionable food additives and supplements

Multiple Sclerosis (MS)

Multiple sclerosis, or MS, is a disease that affects a major control system of the body, the nervous system. The first symptoms of MS tend to be weakness or tingling in the arms and legs, fatigue, a loss of coordination, and blurred vision. As the disease progresses, the individual may experience problems with speech and vision, tremors that make coordinated movement difficult, and numbness in the extremities. We now know that MS is an inflammatory disease that affects the myelin sheaths, which wrap parts of some nerve cells like insulation around an electrical cord. As these sheaths deteriorate, the nerves no longer conduct impulses normally. For unknown reasons, MS often attacks the optic nerves first, before spreading to other areas of the brain. Most researchers think MS results from a misdirected attack on myelin by the body's immune system, although other factors may be involved. Almost 400,000 people in the United States have MS, and it is the most common disease of the nervous system in young adults. Typically, the first symptoms occur between the ages of 20 and 40. The disease is not contagious, is rarely fatal, and does not appear to be inherited, although some studies suggest a genetic component associated with susceptibility to MS. Like many diseases that affect the nervous system, there is no cure, so affected individuals must deal with their condition for the rest of their lives. The good news is that the disease is not severe in almost 45% of cases, and its symptoms can be controlled with medication.

Gluten and Celiac Disease

On any trip to the grocery store, you may notice a wide abundance of foods that are labeled "gluten-free," and almost every restaurant has a gluten-free section of the menu. So, what exactly is gluten? Contrary to what most people believe, gluten is not a carbohydrate. Instead it is a type of protein that is commonly found in wheat, rye, and barley. For most of us, gluten is processed by the digestive system in the same manner as any other protein, meaning that it is broken down into amino acids and absorbed into the circulatory system. However, for about one out of every 100 individuals, the body misidentifies gluten as a foreign pathogen. The result is celiac disease, a disorder of the lining of the small intestine. In celiac disease, an example of an autoimmune response, the body's reaction to gluten causes inflammation and loss of the intestinal linings. The damage to the intestines causes malnutrition and other health problems. Individuals with celiac disease must eliminate gluten from their diet. Interestingly, a gluten-free diet does not seem to have health benefits for individuals who do not suffer from celiac disease. In this chapter, you will learn about the function of the digestive system as well as the basic principles of nutrients and nutrition.

How do antihistamines work?

Once histamine is released from mast cells, it binds to receptors on other body cells. This signals the nearby capillaries to become more "leaky," allowing fluid to leave the capillaries and enter the tissue. This excess fluid is the cause of the familiar symptoms of a runny nose and watery eyes. Antihistamines work by blocking the receptors on the cells, so that histamine can no longer bind. For allergy relief, antihistamines are most effective when taken before exposure to the allergen.

ABO Blood Type

One of the best ways to understand the role of antibodies is by examining human blood types. You are familiar with the concept of specific antibodies through blood types (A, B, AB, or O). These letters stand for antigens on your red blood cells. If you have type O blood, you do not have either antigen A or antigen B on your red cells. Some blood types have antibodies in the plasma (Table 26.3). As an example, type O blood has both anti-A and anti-B antibodies in the plasma. You cannot give a person with type O blood a transfusion from an individual with type A blood. If you do, the antibodies in the recipient's plasma will react to type A red blood cells, and agglutination will occur. Agglutination, the clumping of red blood cells, causes blood to stop circulating and red blood cells to burst. On the other hand, you can give type O blood to a person with any blood type, because type O red blood cells bear neither A nor B antigens. It is possible to determine who can give blood to whom based on the ABO system. However, other red blood cell antigens, in addition to A and B, are used in typing blood. Therefore, it is best to put the donor's blood on a slide with the recipient's blood to observe whether the two types match (no agglutination occurs) to determine whether blood can be safely transfused from one person to another.

Water Transport in Xylem

People have long wondered how plants, especially very tall trees, lift water from the roots to the leaves against gravity. The cohesion-tension model is an explanation of how water and minerals travel upward in xylem cells. To understand this proposed mechanism, one must start in the soil. Recall the concept of osmosis. In plants, water will move from an area of high concentration in the soil to an area of low concentration in the root hairs (Fig. 20.18). All of the water entering the roots creates root pressure. Root pressure is helpful for the upward movement of water but is not nearly enough to get it all the way up to the leaves. Transpiration is the loss of water to the environment, mainly through evaporation from leaf stomata. This is the phenomenon that explains how water can completely resist gravity and travel upward. Focusing on the top of the tree (Fig. 20.18), notice the water molecules escaping from the spongy mesophyll and into the air through the stomata. The key is that it is not just one water molecule escaping but a chain of water molecules. Think about drinking water from a straw. Drinking exerts pressure on the straw and a chain of water molecules is drawn upward. Water molecules are polar and "stick" together with hydrogen bonds. Water's ability to stay linked in a chain is called cohesion, and its ability to stick to the inside of a straw is adhesion (see Fig. 2.10). In plants, evaporation of water at the leaves provides the tension that pulls on a chain of water molecules. Transpiration produces a constant tugging or pulling of the water column from the top due to evaporation. Cohesion of water molecules and their adhesion to the inside of xylem vessels facilitate this process. As transpiration occurs, the water column is pulled upward—first within the leaf, then from the stem, and finally from the roots. The total amount of water a plant loses through transpiration over a long period of time is surprisingly large. At least 90% of the water taken up by roots is eventually lost at the leaves. A single corn plant loses between 135 and 200 liters of water through transpiration during a growing season.

Hemodialysis and Kidney Transplants

Patients with renal failure can undergo hemodialysis, utilizing either an artificial kidney machine or continuous ambulatory peritoneal dialysis (CAPD). Dialysisis defined as the diffusion of dissolved molecules through a semipermeable membrane with pore sizes that allow only small molecules to pass through. In an artificial kidney machine, the patient's blood is passed through a membranous tube, which is in contact with a dialysis solution, or dialysate (Fig. 24.17). Substances more concentrated in the blood diffuse into the dialysate, and substances more concentrated in the dialysate diffuse into the blood. The dialysate is continuously replaced to maintain favorable concentration gradients. In this way, the artificial kidney can be utilized either to extract substances from the blood, including waste products or toxic chemicals and drugs, or to add substances to the blood—for example, bicarbonate ions (HCO3-) if the blood is acidic. In the course of a three- to six-hour hemodialysis procedure, from 50 to 250 grams of urea can be removed from a patient, which greatly exceeds the amount excreted by our kidneys within the same time frame. Therefore, a patient needs to undergo treatment only about twice a week. CAPD is so named because the peritoneum, the epithelium that lines the abdominal cavity, is the dialysis membrane. A fresh amount of dialysate is introduced directly into the abdominal cavity from a bag that is temporarily attached to a permanently implanted plastic tube. The dialysate flows into the peritoneal cavity by gravity. Waste and salt molecules pass from the blood vessels in the abdominal wall into the dialysate before the fluid is collected 4 or 8 hours later. The solution is drained into a bag from the abdominal cavity by gravity, and then it is discarded. One advantage of CAPD over an artificial kidney machine is that the individual can go about his or her normal activities during CAPD. Patients with renal failure may undergo a transplant operation, receiving a functioning kidney from a donor. A person needs only one functioning kidney; however, the possibility of organ rejection exists, as it does with all organ transplants. Receiving a kidney from a close relative has the highest chance of success. The current 1-year survival rate is 98%, and the 5-year survival rate is more than 91%. In the future, it may be possible to use either kidneys from pigs or kidneys created in the laboratory for transplant operations.

Plant Responses

Plant responses are strongly influenced by such environmental stimuli as light, day length, gravity, and touch. A plant's ability to respond to environmental signals enhances the survival of the plant in that environment. Plant responses to environmental signals can be rapid, as when stomata open in the presence of light, or they can take some time, as when a plant flowers in season. Despite their variety, plant responses to environmental signals are most often exhibited in growth and sometimes changes in plant tissues, brought about at least in part by certain hormones.

What is medical marijuana used for?

Researchers continue to examine the potential medical benefits of THC, the active compound in marijuana. Initial studies have indicated that THC acts as an analgesic, or pain reducer. Medical marijuana is sometimes prescribed for patients in extreme pain, such as those in the later stages of AIDS or those who have spinal cord injuries. However, it is important to note that it is the active compound in marijuana that has potential benefits. Smoking marijuana presents more health problems than the use of cigarettes. Marijuana smoke contains 50-70% more carcinogens than cigarette smoke. In addition, in the hour following marijuana use, there is almost a fivefold increase in the risk of a heart attack.

Barriers to Entry

Skin and the mucous membranes lining the respiratory, digestive, reproductive, and urinary tracts serve as mechanical barriers to entry by pathogens. Oil gland secretions contain chemicals that weaken or kill certain bacteria on the skin (Fig. 26.2). The upper respiratory tract is lined by ciliated cells that sweep mucus and trapped particles up into the throat, where they can be swallowed or expectorated (spit out). The stomach has an acidic pH, which inhibits the growth of or kills many types of bacteria. The various bacteria that normally reside in the large intestine and other areas, such as the vagina, prevent pathogens from taking up residence.

Cutaneous Receptors

Skin, the outermost covering of our body, contains numerous sensory receptors, called cutaneous receptors, that help us respond to changes in our environment, be aware of dangers, and communicate with others. The sensory receptors in skin are for touch, pressure, pain, and temperature. Skin has two layers, the epidermis and the dermis (Fig. 28.12). The epidermis is packed with cells that become keratinized as they rise to the surface. Among these cells are free nerve endings responsive to cold or to warmth. Cold receptors are far more numerous than warmth receptors, but there are no known structural differences between the two. Also in the epidermis are pain receptors (also called nociceptors) sensitive to extremes in temperature or pressure and to chemicals released by damaged tissue. Sometimes, the stimulation of internal receptors is felt as pain in the skin. This is called referred pain. For example, pain from the heart may be felt in the left shoulder and arm. This effect most likely is produced when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons that are also receiving impulses from the skin. Numerous receptors are in the skin. Free nerve endings (yellow) in the epidermis detect pain, heat, and cold. Various touch and pressure receptors (red) are in the dermis. The dermis of the skin contains sensory receptors for pressure and touch (Fig. 28.12). The Pacinian corpuscles are onion-shaped pressure receptors that lie deep inside the dermis. Several other cutaneous receptors detect touch. A free nerve ending, called a root hair plexus, winds around the base of a hair follicle and produces nerve impulses if the hair is touched. Touch receptors are concentrated in parts of the body essential for sexual stimulation: the fingertips, palms, lips, tongue, nipples, penis, and clitoris.

The Human Skeleton

The 206 bones of the human skeleton are arranged into an axial skeleton and an appendicular skeleton. Axial Skeleton The bones of the axial skeleton are those that are in the midline of the body (blue shading in Fig. 28.14). The skull consists of the cranium, which protects the brain, and the facial bones. The most prominent of the facial bones are the lower and upper jaws, the cheekbones, and the nasal bones. The vertebral column (spine) extends from the skull to the sacrum (tailbone). It consists of a series of vertebrae separated by pads of fibrocartilage called the intervertebral discs. On occasion, discs can slip or even rupture. A damaged disc pressing against the spinal cord or spinal nerves causes pain, and removal of the disc may be required. The rib cage, composed of the ribs and sternum (breastbone), demonstrates the dual function of the skeleton, providing protection and flexibility at the same time. In the United States, automobile accidents are the most common cause of sternum fractures; people with such injuries are typically examined for signs of heart damage. The rib cage protects the heart and lungs but moves when we breathe. Appendicular Skeleton The appendicular skeleton contains the bones of two girdles and their attached limbs (unshaded bones in Fig. 28.14). The pectoral girdle(shoulder) and upper limbs are specialized for flexibility; the pelvic girdle (hip) and lower limbs are specialized for strength. The pelvic girdle also protects the internal organs. A clavicle (collarbone) and a scapula (shoulder blade) make up the shoulder girdle. The shoulder pads worn by football players are designed to cushion direct blows that could cause clavicle fractures. The humerus of the arm articulates only with the scapula; the joint is stabilized by tendons and ligaments that form a rotator cuff. Vigorous circular movements of the arm can lead to rotator cuff injuries. Two bones (the radius and ulna) contribute to the easy twisting motion of the forearm. The flexible hand contains bones of the wrist, palm, and five fingers. The pelvic girdle contains two massive coxal bones, which form a bowl, called the pelvis, and articulate with the longest and strongest bones of the body, the femurs (thighbones). The strength of the femurs allows these massive bones to support the weight of the upper half of the body. The kneecap protects the knee. The tibia of the leg is the shinbone. The fibula is the more slender bone in the leg. Each foot contains bones of the ankle, instep, and five toes.

Nonspecific Defenses and Innate Immunity

The body has an innate immunity composed of the various types of nonspecific defenses—our first line of defense against most types of infections. The nonspecific defenses are the barriers to entry, the inflammatory response, the complement system, and natural killer cells.

The Action of Hormones

The cells that can respond to a hormone have receptor proteins that bind to the hormone. Hormones cause these cells to undergo a metabolic change. The type of change is dependent on the chemical structure of the hormone. Steroid hormones are lipids, and they can pass through the plasma membrane. The hormone-receptor complex then binds to DNA, and gene expression follows—for example, a protein (such as an enzyme) is made by the cell. The enzyme goes on to produce a change in the target cell's function. Since steroid hormones can pass through the membrane of every cell in the body, they are typically used to control changes that need to occur at an organismal level. For example, the steroid hormone testosterone is a sex hormone that is primarily involved in directing the development of male sexual characteristics. Peptide hormones comprise peptides, proteins, glycoproteins, and modified amino acids. Peptide hormones can't pass through the plasma membrane, so they bind to a receptor protein in the plasma membrane (Fig. 27.15b). The peptide hormone is called the "first messenger," because a signal transduction pathway leads to a second molecule—that is, the "second messenger"—that changes the metabolism of the cell. The second messenger sets in motion an enzyme pathway, which is sometimes called an enzyme cascade because each enzyme in turn activates another. Because enzymes work over and over, every step in an enzyme cascade leads to more reactions—the binding of a single peptide hormone molecule can result in as much as a thousandfold response. Since peptide hormones interact with receptors, they may be specific with regard to the types of cells they target. For example, insulin, a peptide hormone, has a different effect on muscle cells than on adipose tissue cells due to the larger number of insulin receptors on the surfaces of muscle cells.

Marijuana

The dried flowers, leaves, and stems of the Indian hemp plant, Cannabis sativa, contain and are covered by a resin that is rich in 9-tetrahydrocannabinol (THC). The names cannabis and marijuana apply to either the plant or THC. Usually, marijuana is smoked in a cigarette form called a "joint." Recently, researchers have found that marijuana binds to a receptor for anandamide, a neurotransmitter that seems to create a feeling of peaceful contentment. The occasional marijuana user experiences a mild euphoria, along with alterations in vision and judgment, which result in distortions of space and time. Motor incoordination, including the inability to speak coherently, is experienced. Heavy use can result in hallucinations, anxiety, depression, rapid flow of ideas, body image distortions, paranoid reactions, and similar psychotic symptoms. Craving and difficulty in stopping usage can occur as a result of regular use.

Epidermal Tissue

The entire body of a plant is covered by an epidermis, a layer of closely packed cells that act as a barrier, similar to skin. The walls of epidermal cells that are exposed to air are covered with a waxy cuticle to minimize water loss. The cuticle also protects against bacteria and other organisms that might cause disease. Epidermal cells can be modified (changed) into other types of cells. Root hairs are long, slender projections of epidermal cells that increase the surface area of the root for absorption of water and minerals (Fig. 20.2a). In leaves, the epidermis often contains stomata (sing., stoma). A stoma is a small opening surrounded by two guard cells (Fig. 20.2b). When the stomata are open, gas exchange and water loss occur. Trichomes are another type of epidermal cell that make plant leaves and stems feel prickly or hairy and discourage insects from eating the plant. In the trunk of a tree, the epidermis is replaced by cork, which is a part of bark (Fig. 20.2d). New cork cells are made by meristem tissue called cork cambium. As the new cork cells mature, they increase slightly in volume, and their walls become encrusted with suberin, a lipid material, so that they are waterproof and chemically inert. These nonliving cells protect the plant and make it resistant to attack by fungi, bacteria, and animals.

Chemical Senses

The fundamental functions of sensory receptors include helping animals stay safe, find food, and find mates. Chemoreceptors give us the ability to detect chemicals in the environment, which is believed to be our most primitive sense. Chemoreceptors occur almost universally in animals. For example, they are present all over the bodies of planarians (flatworms) but found in higher concentrations on the auricles at the sides of the head. Male moths have receptors for a sex attractant on their antennae. The receptors on the antennae of the male silkworm moth are so sensitive that only 40 out of 40,000 receptor proteins need to be activated in order for the male to respond to a chemical released by the female. Other insects, such as the housefly, have chemoreceptors largely on their feet—a fly tastes with its feet instead of its mouth. In mammals, the receptors for taste are located in the mouth, and the receptors for smell are in the nose.

Hearing and Balance

The human ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these consist of hair cells with long microvilli called stereocilia. These microvilli, unlike those of taste cells, are sensitive to mechanical stimulation. Therefore, these belong to a class of receptors called mechanoreceptors. The similarity of the sensory receptors for balance and hearing and their presence in the same organ suggest an evolutionary relationship between them. In fact, the sense organs of the mammalian ear may have evolved from a type of sense organ in fishes.

Sensory Input and Motor Output

The integumentary system (Fig. 22.12a) consists of the skin and its accessory structures. The sensory receptors in the skin, and in organs such as the eyes and ears, respond to specific external stimuli and communicate with the brain and spinal cord by way of nerve fibers. These messages may cause the brain to respond to a stimulus. The skeletal system (Fig. 22.12b) and the muscular system (Fig. 22.12c) enable the body and its parts to move as a result of motor output. The skeleton, as a whole, serves as a place of attachment for the skeletal muscles. Contraction of the muscles in the muscular system accounts for the movement of body parts. These three systems protect and support the body. The skeletal system, consisting of the bones of the skeleton, protects body parts. For example, the skull forms a protective encasement for the brain, as does the rib cage for the heart and lungs. The skin serves as a barrier between the outside world and the body's tissues.

Why is the female gender sometimes referred to as the default sex?

The term default sex has to do with the presence or absence of the Y chromosome in the fetus. On the Y chromosome is a gene called sex determining region Y (SRY), which produces a protein that causes Sertoli cells in the testes to produce Müllerian inhibiting substance (MIS). This causes Leydig cells in the testes to produce testosterone, which signals the development of the male sex organs (vas deferens, epididymis, penis, and so on). Without the SRYgene and this hormone cascade, female structures (uterus, fallopian tubes, ovaries, and so on) will begin to form.

Nutrition

The vigilance of your immune system, the strength of your muscles and bones, the ease with which your blood circulates—all aspects of your body's functioning—depend on proper nutrition. A nutrient is a substance in food that performs a physiological function in the body. Nutrients provide us with energy, promote growth and development by supplying the material for cellular structures, and regulate cellular metabolism. They are also involved in homeostasis. For example, nutrients help maintain the fluid balance and proper pH of blood. Your body can make up for a nutrient deficiency to a degree, but eventually signs and symptoms of a deficiency disorder will appear. As an example, vitamin C is needed to synthesize and maintain collagen, the protein that holds tissues together. When the body lacks vitamin C, collagen weakens and capillaries break easily. Gums may bleed, especially when the teeth are brushed, or tiny bruises may form under the skin when it is gently pressed. In other words, early signs of vitamin C deficiency are gums that bleed and skin that bruises easily. By learning about nutrition, you can improve your diet and increase the likelihood of enjoying a longer, more active, and more productive life. Conversely, poor diet and lack of physical activity are responsible for seven of the leading ten causes of death in the United States annually. Such lifestyle factors may soon overtake smoking as the major cause of preventable death. We all can benefit from learning what constitutes a poor diet versus a healthy diet, which will allow us to choose foods that supply all the nutrients in proper balance.

Maintaining a Healthy Weight

To maintain weight at an appropriate level, the daily kcal intake (from eating) should not exceed the daily kcal output (metabolism + physical activity + processing food). For many Americans, this ratio is out of sync; they take in more calories than they need. The extra energy is converted to fat stored in adipose tissue, and they become overweight. To lose weight, an overweight person needs to lower the kcal intake and increase the kcal output in the form of physical activity. Only then does the body metabolize its stored fat for energy needs, allowing the person to lose weight. Figure 25.21 illustrates how body weight changes in relation to kcal intake and kcal output. Dieting Fad weight-reduction diets—high-protein, low-carb, high-fiber, and even cabbage soup diets—come and go. During the first few weeks of a fad diet, overweight people often lose weight rapidly, because they consume fewer calories than usual, and excess body fat is metabolized for energy needs. In most cases, however, dieters become bored with eating the same foods and avoiding their favorite foods, which may be high in fat and sugars. When most dieters go off their diets, they regain the weight they lost, and they often feel frustrated and angry at themselves for failing to maintain the weight loss. There are no quick and easy solutions for losing weight. The typical fad diet is nutritionally unbalanced and difficult to follow over the long term. Weight loss and weight maintenance require permanent lifestyle changes, such as increasing the level of physical activity and reducing portion sizes. Behavior modification allows an overweight person to lose weight safely, generally at a reasonable rate of about ½ to 2 pounds per week. Once body weight is under control, it needs to be maintained by continuing to eat sensibly.

Comparison of Vertebrate Circulatory Pathways

Two types of circulatory pathways are seen among vertebrate animals. In fishes, blood follows a one-circuit (single-loop) pathway through the body. The heart has a single atrium and a single ventricle (Fig. 23.4a). The pumping action of the ventricle sends blood under pressure to the gills, where gas exchange occurs. After passing through the gills, blood is under reduced pressure and flow. However, this single circulatory loop has advantages in that the gill capillaries receive oxygen-poor (O2-poor) blood and the systemic capillaries receive O2-rich blood. As a result of evolutionary adaptations to life on land, the other vertebrates have a two-circuit (double-loop) circulatory pathway. The heart pumps blood to the tissues through the systemic circuit, and it pumps blood to the lungs through the pulmonary circuit. This double pumping action is seen in terrestrial animals that utilize lungs to breathe air. In amphibians, the heart has two atria, but only a single ventricle (Fig. 23.4b), and some mixing of O2-rich and O2-poor blood does occur. The same holds true for most reptiles, except that the ventricle has a partial septum, so this mixing is reduced. The hearts of some reptiles (for example, crocodilians and birds) and mammals are divided into right and left halves (Fig. 23.4c). The right ventricle pumps blood to the lungs, and the left ventricle, which is larger than the right ventricle, pumps blood to the rest of the body. This arrangement provides adequate blood pressure for both the pulmonary and systemic circuits.

Vitamins

Vitamins are organic compounds (other than carbohydrates, fats, and proteins, including amino acids) that regulate various metabolic activities. Vitamins are classified as either water-soluble (Table 25.6) or fat-soluble (Table 25.7). Generally, water soluble vitamins are readily removed in the urine, and most must be replenished daily. Fat-soluble vitamins are stored in adipose tissue and persist longer in the body. GENERAL GUIDELINES - Consume less than 10 percent of calories per day from sugar. - Consume less than 10 percent of calories per day from saturated fats. - Consume less than 2,300 milligrams (mg) per day of sodium. - Alcohol should be consumed in moderation: a maximum of one drink per day for women and two drinks per day for men (and only by adults of legal drinking age). Although many people think vitamins can enhance health dramatically, prevent aging, and cure diseases such as arthritis and cancer, there is no scientific evidence that vitamins are "wonder drugs." However, vitamins C, E, and A have been shown to defend the body against free radicals, and therefore they are termed antioxidants. These vitamins are especially abundant in fruits and vegetables, so it is suggested that we eat about 4½ cups of fruits and vegetables per day. To achieve this goal, we should consume salad greens, raw or cooked vegetables, dried fruit, and fruit juice in addition to traditional apples and oranges and other fresh foods. Vitamin deficiencies can lead to disorders, and even death, in humans. Although many foods in the United States are now enriched, or fortified, with vitamins, some individuals, especially the elderly, young children, alcoholics, and people with low incomes, are still at risk for vitamin deficiencies, generally as a result of poor food choices. For example, skin cells normally contain a precursor cholesterol molecule that is converted to vitamin D after UV exposure. But a vitamin D deficiency leads to a condition called rickets, in which defective mineralization of the skeleton causes bowing of the legs. Most milk today is fortified with vitamin D, which helps prevent the occurrence of rickets. Another example is vitamin C deficiency, the effects of which are illustrated in Figure 25.14. If a diet involves high alcohol consumption, then vitamin deficiencies may occur even if the intake of vitamins is adequate. Deficiencies occur because alcohol interferes with the absorption of certain vitamins, such as vitamin B12, folacin, and vitamin A, and it increases the excretion of other vitamins, such as vitamin C.

What is gestational diabetes, and what causes it?

Women who were not diabetic prior to pregnancy but have high blood glucose levels during pregnancy have gestational diabetes. Gestational diabetes affects a small percentage of pregnant women. This form of diabetes is caused by insulin resistance—body insulin concentration is normal, but the cells fail to respond normally. Gestational diabetes and insulin resistance generally develop later in the pregnancy. Carefully planned meals and exercise often control this form of diabetes, but insulin injections may be necessary. If the woman is not treated, additional glucose crosses the placenta, causing high blood glucose in the fetus. The extra energy in the fetus is stored as fat, resulting in macrosomia, or a "fat" baby. Delivery of a very large baby can be dangerous for both the infant and the mother. Cesarean section is often necessary. Complications after birth are common for these babies. Further, there is a greater risk that the child will become obese and develop type 2 diabetes mellitus later in life. Usually, gestational diabetes goes away after the birth of the child. However, once a woman has experienced gestational diabetes, she has a greater chance of developing it again during future pregnancies. These women also tend to develop type 2 diabetes later in life.

What can you do to reduce your chances of contracting an STD?

1. Abstain from sexual intercourse or develop a long-term, monogamous (having intercourse with only one person) relationship with a person who is free of STDs. 2. Refrain from having multiple sex partners or a relationship with a person who does have multiple sex partners. 3. Be aware that having relations with an intravenous drug user is risky, because the behavior of this group puts them at risk for AIDS and hepatitis B. 4. Avoid anal intercourse, because HIV has easy access through the lining of the rectum. 5. Always use a latex condom if your partner has not been free of STDs for the past 5 years. 6. Avoid oral sex, because this may be a means of transmitting AIDS and other STDs. 7. Stop, if possible, the habit of injecting drugs; if you cannot stop, at least always use a sterile needle.

Making Sense of Nutrition Labels

A "Nutrition Facts" panel, shown in Figure 25.19, provides specific dietary information about the product and general information about the nutrients the product contains. Serving Size and Calories The serving size is based on the typical serving size for the product. If you are comparing Calories (kcal)1 and other data about products of the same type, you want to be sure the serving size is the same for each product. The total number of Calories is based on the serving size. Obviously, if you eat twice the serving size, you have taken in twice the number of Calories. The new food labels (Fig. 25.19b) are designed to provide a better indication of a realistic serving size for most people and to increase the emphasis on the total Calories per serving. % Daily Value The % daily value (the percentage of the total amount needed in a 2,000-Calorie diet) is calculated by comparing the specific information about this product with the information given at the bottom of the panel. For example, the product in Figure 25.19a has a fat content of 13 g, and the total daily recommended amount is less than 65 g, so 13/65 = 20%. The % daily values are not applicable for people who require more or less than 2,000 Calories (kcal) per day. A % daily value for protein is generally not given because determining such a value would require expensive testing of the protein quality of the product by the manufacturer. Also, notice there is a % daily value for carbohydrates but not sugars, because there is no recommended daily value for sugar. How to Use the Panel If the serving sizes are the same, you can use "Nutrition Facts" panels to compare two products of the same type. For example, if you wanted to reduce your Caloric intake and increase your fiber and vitamin C intakes, comparing the panels from two different food products would allow you to see which one is lowest in Calories and highest in fiber and vitamin C.

Skeletal Muscles Move Bones at Joints

Joints are classified as immovable, such as those of the cranium; slightly movable, such as those between the vertebrae; and freely movable (synovial joints), such as those in the knee and hip. In synovial joints, ligaments bind the two bones together, providing strength and support and forming a capsule containing lubricating synovial fluid. All of our movements, from those of graceful and agile ballet dancers to those of aggressive and skillful football players, occur because muscles are attached to bones by tendons that span movable joints. Because muscles shorten when they contract, they have to work in antagonistic pairs. If one muscle of an antagonistic pair flexes the joint and raises the limb, the other extends the joint and straightens the limb. Figure 28.19 illustrates this principle with regard to the movement of the forearm at the elbow joint. Figure 28.20b illustrates the anatomy of a freely movable synovial joint. Note that, in addition to a cavity filled with synovial fluid, a synovial joint may include additional structures—namely, menisci and bursae. Menisci (sing., meniscus), are C-shaped pieces of hyaline cartilage between the bones. These give added stability and act as shock absorbers. Fluid-filled sacs called bursae (sing., bursa) ease friction between bare areas of bone and overlapping muscles or between skin and tendons. The ball-and-socket joints at the hips and shoulders are synovial joints that allow movement in all directions, even rotational movement (Fig. 28.20c). The elbow and knee joints are synovial joints called hinge joints because, like a hinged door, they largely permit movement in one direction only (Fig. 28.20d). Joint Disorders Sprains occur when ligaments and tendons are overstretched at a joint. For example, a sprained ankle can result if you turn your ankle too far. Overuse of a joint may cause inflammation of a bursa, called bursitis. Tennis elbow is a form of bursitis. A common knee injury is a torn meniscus. Because fragments of menisci can interfere with joint movements, most physicians believe they should be removed. Today, arthroscopic surgery is used to remove cartilage fragments or to repair ligaments or cartilage. A small instrument bearing a tiny lens and light source is inserted into a joint, as are the surgical instruments. Fluid is then added to distend the joint and allow visualization of its structure. Usually, the surgery is displayed on a monitor, so that the whole operating team can see the operation. Arthroscopy is much less traumatic than surgically opening the knee with long incisions. The benefits of arthroscopy are small incisions, faster healing, a more rapid recovery, and less scarring. Because arthroscopic surgical procedures are often performed on an outpatient basis, the patient is able to return home on the same day. Rheumatoid arthritis (see Section 26.5) is not as common as osteoarthritis, which is the deterioration of an overworked joint. Constant compression and abrasion continually damage articular cartilage, and eventually it softens, cracks, and in some areas wears away entirely. As the disease progresses, the exposed bone thickens and forms spurs that cause the bone ends to enlarge and restrict joint movement. Weight loss can ease arthritis. Taking off 3 pounds can reduce the load on a hip or knee joint by 9 to 15 pounds. A sensible exercise program helps build up muscles, which stabilize joints. Low-impact activities, such as biking and swimming, are best. Today, the replacement of damaged joints with a prosthesis (artificial substitute) is often possible. Some people have found glucosamine-chondroitin supplements beneficial as an alternative to joint replacement. Glucosamine, an amino sugar, is thought to promote the formation and repair of cartilage. Chondroitin, a carbohydrate, is a cartilage component that is thought to promote water retention and elasticity and to inhibit enzymes that break down cartilage. Both compounds are naturally produced by the body. Exercise A sensible exercise program has many benefits. Exercise improves muscular strength, muscular endurance, and flexibility. It improves cardiorespiratory endurance and may lower blood cholesterol levels. People who exercise are less likely to develop various types of cancer. Exercise promotes the activity of osteoblasts; therefore, it helps prevent osteoporosis. It helps prevent weight gain, not only because of increased activity but also because, as muscle mass increases, the body is less likely to accumulate fat. Exercise even relieves depression and enhances mood.

Opening and Closing of Stomata

A plant cell that is full of water will bulge (see Fig. 5.13). Turgor pressure is the force of the water creating this bulge. Stomata open and close due to changes in turgor pressure within guard cells (Fig. 20.19). When water enters the guard cells, turgor pressure increases, and the unique "banana" shape of the guard cells causes them to bow out and expose the pore (stoma); when water leaves the guard cells, turgor pressure decreases, and the pore is once again covered. For transpiration to occur, the stomata must stay open. But when a plant is under stress and about to wilt from lack of water, the stomata close. Now the plant is unable to take up carbon dioxide from the air, and photosynthesis ceases.

Open Circulatory Systems

A circulatory system consists of a heart and associated vessels. The role of the heart is to keep the fluid (blood) moving within the vessels. Circulatory systems may be classified as being either open or closed. The grasshopper is an example of an invertebrate animal that has an open circulatory system (Fig. 23.2), meaning that the fluid is not always confined to the vessels. A tubular heart pumps a fluid called hemolymph through a network of channels and cavities in the body. Collectively known as the hemocoel, these cavities, or sinuses, contain the animal's organs. Eventually, hemolymph (a combination of blood and interstitial fluid) drains back to the heart. When the heart contracts, openings called ostia (sing., ostium) are closed; when the heart relaxes, the hemolymph is sucked back into the heart by way of the ostia. The hemolymph of a grasshopper is colorless, because it does not contain hemoglobin or any other respiratory pigment that combines with and carries oxygen. Oxygen is taken to cells, and carbon dioxide is removed from them by way of air tubes, called tracheae, which are found throughout the body. The tracheae provide efficient transport and delivery of respiratory gases while restricting water loss.

The Testes

A longitudinal section of a testis shows that it is composed of compartments, called lobules, each of which contains one to three tightly coiled seminiferous tubules (Fig. 29.5a,b). A microscopic cross section of a seminiferous tubule reveals that it is packed with cells undergoing spermatogenesis, a process that involves reducing the chromosome number from diploid (2n) to haploid (n). Also present are Sertoli cells, which support, nourish, and regulate the production of sperm. A sperm (Fig. 29.5c) has three distinct parts: a head, a middle piece, and a tail. The head contains a nucleus and is capped by a membrane-bound acrosome that contains digestive enzymes, so that the sperm can penetrate the outer membrane of an egg. The tail is a flagellum that allows sperm to swim toward the egg, and the middle piece contains energy-producing mitochondria. The ejaculated semen of a normal human male contains 40 million sperm per milliliter, ensuring an adequate number for fertilization to take place. Fewer than 100 sperm ever reach the vicinity of the egg, however, and only one sperm normally enters an egg.

The Biology of Performing

A performance of Taylor Swift is a complex production, and not only from the perspective of the people with her on the stage. Whether she is singing or dancing, she is demonstrating how the human body is able to perform very complicated feats. Her nervous system must coordinate a variety of tasks, including monitoring her breathing rate and remembering the words and tempo to the song. This is influenced by sensory input from her eyes and ears. At the same time, her brain is instructing her muscles how to move onstage, as well as working to maintain her balance while interacting with the other dancers onstage. Of course, in biological terms, the same observations can be made about humans engaging in almost any other type of physical activity, such as playing sports, building a house, performing surgery, or driving a car. Even while you are sitting quietly, perhaps reading a textbook, your body systems are involved in a flurry of activity. Your muscles and bones work to keep your body upright. Your respiratory and circulatory systems provide oxygen to your tissues while transporting wastes for elimination. Your digestive system is providing the nutrients needed to power all of these activities while your immune system is keeping a watch out for harmful microbes. Each of these activities is being coordinated by the nervous and endocrine systems. Even when you are asleep, these body systems are at work. In this chapter, we will explore the basic organization of the human body and see how the body maintains a stable internal environment in the face of changing external conditions.

Introducing the Nutrients

A person's diet is his or her typical food choices. Several factors, including cultural and ethnic backgrounds, financial situations, environmental conditions, and psychological states, influence what we eat. A balanced diet supplies all the nutrients in the proper proportions necessary for a healthy, functioning body. There are six classes of nutrients: carbohydrates, lipids, proteins, minerals, vitamins, and water. An essential nutrient must be supplied by the diet because the body is not able to produce it, or at least not in sufficient quantity to meet the body's needs. For example, amino acids are needed for protein synthesis, and the body is unable to produce several of them. Therefore, there are essential amino acids that are needed in the diet. Most vitamins, including vitamin C, and all of the minerals are considered to be essential nutrients. We can also describe nutrients based on the quantities that are needed daily by our bodies. Carbohydrates, lipids, and proteins are called macronutrientsbecause the body requires relatively large quantities of them. Micronutrients, such as vitamins and minerals, are needed in small quantities only. Macronutrients, not micronutrients, supply our energy needs. Although advertisements often imply that people can boost their energy levels by taking vitamin or mineral supplements, the body does not metabolize these nutrients for energy. Water does not provide energy, either. Therefore, foods with high water content, such as fruits and vegetables, are usually lower in energy content than foods with less water and more macronutrient content. Nearly every food is a mixture of nutrients. A slice of bread, for example, is about 50% carbohydrates, 35% water, 10% proteins, and 4% lipids. Vitamins and minerals make up less than 1% of the bread's nutrient content (Fig. 25.15). No single, naturally occurring food contains enough essential nutrients to meet all of our nutrient needs. Contrary to popular belief, "bad" foods, or "junk" foods, do have nutritional value. If a food contains water, sugar, or fat, it has nutritional value. However, such foods as sugar-sweetened soft drinks, cookies, and pastries have high amounts of fat and/or sugar in relation to their vitamin and mineral content. Therefore, these foods are more appropriately called empty-calorie foods, rather than junk foods. Diets that contain too many empty-calorie foods will assuredly lack enough vitamins and minerals.

Tropisms

A plant's growth response toward or away from a directional stimulus is called a tropism. Differential growth causes one side of an organ to elongate faster than the other, and the result is a curving toward or away from the stimulus. Growth toward a stimulus is called a positive tropism, and growth away from a stimulus is called a negative tropism. The following tropisms were each named for the stimulus that causes the response: 1. Phototropism: growth in response to a light stimulus 2. Gravitropism: growth in response to gravity 3. Thigmotropism: growth in response to touch Phototropism is the growth of plants toward a source of light. If the light is coming to the plant from a single direction, auxin migrates to the shady side of the plant and cell elongation causes the stem and leaves to bend toward the sunlight (Fig. 21.7a). Thigmotropism is a response to touch from another plant, an animal, rocks, or the wind. Climbing vines such as English ivy use touch contact with rocks, tree trunks, or other supports for growth (Fig. 21.7b). This adaptation for thigmotropism supports leaf growth toward sunlight rather than investing energy in building supportive tissues of the stem. Gravitropism is the growth response of plants to Earth's gravity. When a seed germinates, the embryonic shoot exhibits negative gravitropism by growing upward against gravity. Increased auxin concentration on the lower side of the young stem causes the cells in that area to grow more than the cells on the upper side, resulting in growth upward. The embryonic root exhibits positive gravitropism by growing with gravity downward into the soil (Fig. 21.8a). Root cells know which way is down because of the presence of an organelle called an amyloplast. Imagine placing a few marbles in a tennis ball. No matter how you move the ball, the marbles will always settle to the bottom. The same holds true for amyloplasts, which settle at the bottom of endodermal root cells and signal downward growth (Fig. 21.8b).

Reproducing on land.

Animals that reproduce on land need to protect their gametes and embryos from drying out. Here, the male passes sperm to the female by way of a penis, and the developing embryo/fetus will remain in the female's body until it is capable of living independently.

How is labor induced if a woman's pregnancy extends past her due date?

After the woman is given medication to prepare the birth canal for delivery, pitocin (a synthetic version of oxytocin) is used to induce labor. During labor, it may also be given to increase the strength of the contractions. Stronger contractions speed the labor process, if necessary (for example, if the woman's uterus is contracting poorly or if the health of the mother or child is at risk during delivery). Pitocin is routinely used following delivery to minimize postpartum bleeding by ensuring that strong uterine contractions continue. Administration of pitocin must be monitored carefully, because it may cause excessive uterine contractions. Should this occur, the uterus could tear itself. Further, a reduced blood supply to the fetus, caused by very strong contractions, may be fatal to the baby. Although it reduces the duration of labor, induction with pitocin can be very painful for the mother. Whenever possible, gentler and more natural methods should be used to induce labor and/or strengthen contractions.

What health benefits are associated with drinking green tea?

All tea is derived from a plant native to China and India called Camellia sinensis. Green tea is made by steaming the leaves of this plant. Typically, green tea has less caffeine than black teas, which are made by fermenting the tea leaves. Research studies on the health benefits of green tea have shown that, overall, it has a very positive effect on the body. Antioxidants in green tea (called flavonoids) inhibit the growth of certain forms of cancer, prevent plaque buildup in the blood vessels, and may help improve blood cholesterol levels. However, most nutritionists still warn against taking green tea supplements. Instead, substitute a cup of tea for soda in your daily diet.

Allergies

Allergies are hypersensitivities to substances in the environment, such as pollen, food, or animal hair, that ordinarily would not cause an immune reaction. The response to these antigens, called allergens, usually includes some unpleasant symptoms (Fig. 26.10). An allergic response is regulated by cytokines secreted by both T cells and macrophages. Immediate allergic responses are caused by receptors attached to the plasma membranes of mast cells in the tissues. When an allergen attaches to receptors on mast cells, they release histamine and other substances that bring about the symptoms. An immediate allergic response can occur within seconds of contact with the antigen. The symptoms can vary, but a dramatic example, anaphylactic shock, is a severe reaction characterized by a sudden and life-threatening drop in blood pressure. Allergy shots, injections of the allergen in question, sometimes prevent the onset of an allergic response. It has been suggested that injections of the allergen may cause the body to build up large quantities of antibodies released by plasma cells, and these combine with allergens received from the environment before they have a chance to reach the receptors located in the membranes of mast cells. Delayed allergic responses are probably initiated by memory T cells at the site of allergen contact in the body. A classic example of a delayed allergic response is the skin test for tuberculosis (TB). When the test result is positive, the tissue where the antigen was injected becomes red and hardened. This shows that the person has been previously exposed to tubercle bacillus, the cause of TB. Contact dermatitis, which occurs when a person is allergic to poison ivy, jewelry, cosmetics, and so forth, is also an example of a delayed allergic response.

The Search for a Vaccine Against Zika

Although Zika virus was first reported in Africa in 1952, the virus did not make an appearance in the Western hemisphere until 2015, when cases occurred in Brazil. The most common way the virus is transmitted between people is via an infected Aedes mosquito. However, it can be sexually transmitted from infected males to females. In a relatively short period of time, the virus has spread throughout South and Central America, and there have already been cases of infected travelers returning to the United States. For most people, infection with the Zika virus produces mild symptoms, such as fevers, rashes, or joint pain. Some individuals do not experience any symptoms at all, and thus may not know that they have been infected. However, in a small number of cases, pregnant females who have been infected with the Zika virus have given birth to children with microcephaly. Microcephaly is a birth defect that causes the head and brain of an infant to be much smaller than normal. This can cause a number of developmental problems, including seizures, intellectual disabilities, and vision problems. Since there is no cure for microcephaly, researchers have been actively looking at ways to develop a vaccine against the virus. In order to create a vaccine, researchers must identify the parts of a virus that will cause our immune system to react as if it has been infected and build up antibodies to the actual virus. Later, if an individual is exposed to the virus, these antibodies are used by the body to provide immunity. In this chapter, we will explore how our immune system protects us, not only from viruses such as Zika, but from a wide variety of pathogens.

Forms of Asexual Reproduction

An example of asexual reproduction occurs in the hydra, a type of cnidarian (see Section 19.2). Hydras can reproduce by budding. A new individual arises as an outgrowth (bud) of the parent (Fig. 29.1). Many flatworms can constrict into two halves; each half regenerates to become a new individual. Fragmentation, followed by regeneration, is also seen among sponges, echinoderms, and corals. Chopping up a sea star does not kill it; instead, each fragment can grow into another animal if a portion of the oral disk is retained with the cut fragment. Parthenogenesis is a modification of sexual reproduction in which an unfertilized egg develops into a complete individual. In honeybees, the queen bee can fertilize eggs or allow eggs to pass unfertilized as she lays them. The fertilized eggs become diploid females called workers, and the unfertilized eggs become haploid males called drones. Parthenogenesis is also observed in some fish (including sharks) and in a number of amphibian and reptile species.

The Ovaries

An oogonium, an undifferentiated germ cell, in the ovary gives rise to an oocyte surrounded by epithelium (Fig. 29.7). This is called a primary follicle. An ovary contains many primary follicles, each containing an oocyte. At birth, a female has as many as 2 million primary follicles, but the number has been reduced to 300,000-400,000 by the time of puberty. Only a small number of primary follicles (about 400) ever mature and produce a secondary oocyte. When mature, the follicle balloons out on the surface of the ovary and bursts, releasing the secondary oocyte surrounded by follicle cells. The release of a secondary oocyte from a mature follicle is termed ovulation. Oogenesis is completed when and if the secondary oocyte is fertilized by a sperm. A follicle that has lost its oocyte develops into a corpus luteum and stays inside the ovary. If fertilization and pregnancy do not occur, the corpus luteum begins to degenerate after about 10 days. The ovarian cycle is controlled by the gonadotropic hormones FSH and LH from the anterior pituitary gland (Fig. 29.8). During the first half, or follicular phase, of the cycle (pink in Fig. 29.8), FSH promotes the development of follicles that secrete estrogen. As the blood level of estrogen rises, it exerts feedback control over FSH secretion, and ovulation occurs. Ovulation marks the end of the follicular phase. During the second half, or luteal phase, of the ovarian cycle (yellow in Fig. 29.8), LH promotes the development of a corpus luteum, which secretes primarily progesterone. As the blood level of progesterone rises, it exerts feedback control over LH secretion, so that the corpus luteum begins to degenerate if fertilization does not occur. As the luteal phase comes to an end, menstruation occurs. Notice that the female sex hormones estrogen and progesterone affect the endometrium of the uterus, causing the series of events known as the menstrual cycle (Fig. 29.8, bottom). The 28-day (on average) menstrual cycle in the nonpregnant female is divided as follows: During days 1-5, female sex hormones are at a low level in the body, causing the endometrium to disintegrate and its blood vessels to rupture. A flow of blood, mucus, and degenerating endometrium, known as the menses, passes out of the vagina during menstruation, also known as the menstrual period. During days 6-13, increased production of estrogen by ovarian follicles causes the endometrium to thicken and become vascular and glandular. This is called the proliferative phase of the menstrual cycle. Ovulation usually occurs on day 14 of the 28-day cycle. During days 15-28, increased production of progesterone by the corpus luteum causes the endometrium to double in thickness and the uterine glands to mature, producing a thick, mucoid secretion. This is called the secretory phase of the menstrual cycle. The endometrium now is prepared to receive the developing embryo. But if fertilization does not occur and no embryo embeds itself, the corpus luteum degenerates, and the low level of sex hormones in the female body causes the endometrium to break down. Menses begins, marking day 1 of the next cycle. Even while menstruation is occurring, the anterior pituitary begins to increase its production of FSH, and new follicles begin to mature.

Type 2 Diabetes

As discussed in Section 27.2, diabetes comes in two forms, type 1 and type 2. When a person has type 1 diabetes, the pancreas does not produce insulin, and the patient has to have daily insulin injections. In contrast to type 1 diabetes, children and more often adults with type 2 diabetes are usually obese and display impaired insulin production and insulin resistance. Normally, the presence of insulin causes the cells of the body to take up and metabolize glucose. In a person with insulin resistance, the body's cells fail to take up glucose even when insulin is present. Therefore, the blood glucose value exceeds the normal level, and glucose appears in the urine. Type 2 diabetes is increasing rapidly in most industrialized countries of the world. Because type 2 diabetes is most often seen in people who are obese, dietary factors are generally believed to contribute to its development. Further, a healthy diet, increased physical activity, and weight loss have been seen to improve insulin's ability to function properly in type 2 diabetics. How might diet contribute to the occurrence of type 2 diabetes? Simple sugars in foods, such as candy and ice cream, immediately enter the bloodstream, as do sugars from the digestion of starch in white bread and potatoes. When the blood glucose level rises rapidly, the pancreas produces an overload of insulin to bring the level under control. Chronically high insulin levels apparently lead to insulin resistance, increased fat deposition, and a high level of fatty acids in the blood. Over the years, the body's cells become insulin resistant, and thus type 2 diabetes can occur. In addition, high fatty acid levels can lead to increased risk for cardiovascular disease. It is well worth the effort to control type 2 diabetes, because all diabetics, whether type 1 or type 2, are at risk for blindness, kidney disease, and cardiovascular disease.

How does aspirin work?

Aspirin is made of a chemical called acetylsalicylic acid (ASA). When tissue is damaged, it produces large amounts of a type of fatty acid called prostaglandin. Prostaglandin acts as a signal to the pain receptors that tissue damage has occurred, which the brain interprets as pain. Prostaglandins are manufactured in cells by an enzyme called COX. ASA reduces the capabilities of this enzyme, lowering the amount of prostaglandin produced and the perception of pain.

Assisted Reproductive Technologies

Assisted reproductive technologies (ART) are techniques used to increase the chances of pregnancy. Often, sperm and/or eggs are retrieved from the testes and ovaries, and fertilization takes place in a clinical or laboratory setting. Artificial Insemination by Donor (AID) During artificial insemination, harvested sperm are placed in the vagina by a physician. Sometimes a woman is artificially inseminated by her partner's sperm. This technique is especially helpful if the partner has a low sperm count, because the sperm can be collected over a period of time and concentrated, so that the sperm count is sufficient to result in fertilization. Often, however, a woman is inseminated by sperm acquired from a donor who is a complete stranger to her. In Vitro Fertilization (IVF) and Intracytoplasmic Sperm Injection (ICSI) During IVF and ICSI, conception occurs outside the body in a laboratory. Ultrasound machines can spot follicles in the ovaries that hold immature eggs; therefore, the latest method is to forgo the administration of fertility drugs and retrieve immature eggs by using a needle. In IVF, the immature eggs are then brought to maturity in glassware before concentrated sperm are added. In ICSI, a single sperm is injected into an egg, usually because a male has severe infertility problems. After about 2-4 days, embryos are ready to be transferred to the uterus of the woman, who is now in the secretory phase of her menstrual cycle. If desired, embryos can be tested for a genetic disease, and only those found to be free of disease will be used. If implantation is successful and development is normal, pregnancy continues to term. If several embryos are produced and transferred at a time, multiple births are common. When excess embryos are left over, these may be frozen for transfer later, or they may be donated to other couples or used in research. Gamete Intrafallopian Transfer (GIFT) The term gamete refers to a sex cell, either a sperm or an egg. Gamete intrafallopian transfer was devised to overcome the low success rate (15-20%) of in vitro fertilization. The method is similar to IVF, except the eggs and sperm are placed in the uterine tubes immediately after they have been brought together. GIFT has the advantage of being a one-step procedure for the woman—the eggs are removed and reintroduced at the same time. A variation on this procedure is to fertilize the eggs in the laboratory and then place the zygotes in the uterine tubes.

How do medical examiners use rigor mortis to estimate time of death?

Body temperature and the presence or absence of rigor mortis allow the time of death to be estimated. For example, the body of someone who has been dead for 3 hours or less will still be warm (close to body temperature, 98.6°F, or 37°C), and rigor mortis will be absent. After approximately 3 hours, the body will be significantly cooler than normal, and rigor mortis will begin to develop. The corpse of an individual dead at least 8 hours will be in full rigor mortis, and the temperature of the body will be the same as the surroundings. Forensic pathologists know that a person has been dead for more than 24 hours if the body temperature is the same as the environment and there is no longer a trace of rigor mortis.

100-Meter Dash World Record Holder

At the 2012 Summer Olympics, Usain Bolt set a new Olympic record of 9.63 seconds in the 100-meter dash. However, this was not his personal best. In Berlin in 2009, Bolt set a world record of 9.58 seconds for the 100-meter dash. When Bolt is running at these incredible speeds, his brain is coordinating sensory input from his eyes, ears, and internal sensors (the sensory input) to maximize the efforts of his skeletal and muscular systems (the motor output). Like Usain Bolt, we all perform these functions daily, when we are walking to class, exercising, or even working on the computer. The senses, skeletal system, and muscular system all contribute to homeostasis. Our senses provide us with information about the external environment. Aside from giving our bodies shape and protecting our internal organs, the skeleton serves as a storage area for inorganic calcium and produces blood cells. The skeleton also protects internal organs while supporting the body against the pull of gravity. While contributing to body movement, the skeletal muscles give off heat, which warms the body. In this chapter, we will explore how the senses provide information to the brain, as well as how the skeletal and muscular systems are involved in movement and support.

Is a fever always bad?

At the first sign of a fever, most people reach for over-the-counter (OTC) medicines to bring it under control. However, in many cases, using OTC medicines does more harm than good. Medical professionals now widely regard a low fever as a beneficial aspect of the immune system. When your body runs a fever, the elevated temperature increases your metabolic rate and slows down bacterial and viral reproduction. A low fever also promotes the release of chemicals, called interferons, that prevent the infection from spreading. Of course, a high fever or one that lasts for several days should immediately be brought to the attention of your physician.

Why does your stomach "growl" when you are hungry?

Borborygmi is the medical term for the "growl" sound in your stomach. It is produced when the stomach walls squeeze together in an attempt to mix digestive juices and gases for digestion. If your stomach is empty, the result is the sound of these juices bouncing off the walls of the hollow stomach. The "hunger center" of the brain will send a message to your stomach to begin the process of digestion, sometimes initiating borborygmi and signaling the need to eat.

Placenta.

Blood vessels within the umbilical cord lead to the placenta, where exchange takes place between fetal blood and maternal blood.

The Motor Systems

Both the muscular system and the skeletal system of animals are involved in the nervous system's motor response to some form of stimuli. In many ways, the functions of the muscular and skeletal systems overlap, which is why some refer to these systems as the musculoskeletal system. In humans, the musculoskeletal system performs the following functions: Both skeletal muscles and bones support the body and make the movement of body parts possible. Both skeletal muscles and bones protect internal organs. Skeletal muscles pad the bones that protect the heart and lungs, the brain, and the spinal cord. Both muscles and bones aid the functioning of other systems. Without the movement of the rib cage, breathing would not occur. As an aid to digestion, the jaws have sockets for teeth; skeletal muscles move the jaws, so that food can be chewed, and smooth muscle moves food along the digestive tract (see Section 25.1). Red bone marrow supplies the red blood cells that carry oxygen, and the pumping of cardiac muscle in the wall of the heart moves the blood to the tissues, where exchanges with tissue fluid occur. In addition to these shared functions, the muscles and bones have individual functions: Skeletal muscle contraction assists the movement of blood in the veins and lymphatic vessels (see Section 23.2). Without the return of lymph to the cardiovascular system and blood to the heart, circulation could not continue. Skeletal muscles help maintain a constant body temperature (see Section 22.3). Skeletal muscle contraction causes ATP to break down, releasing heat that is distributed about the body. Bones store fat and calcium. Fat is stored in yellow bone marrow (see Fig. 28.15), and the extracellular matrix of bone contains calcium. Calcium ions play a major role in muscle contraction and nerve conduction.

Carbohydrates

Carbohydrates are present in food as sugars, starch, and fiber. Fruits, vegetables, milk, and honey are natural sources of sugars. Glucose and fructose are monosaccharide sugars, and lactose (milk sugar) and sucrose (table sugar) are disaccharides. After absorption into the body, all sugars are converted to glucose for transport in the blood. Glucose is the preferred direct energy source in cells. Plants store glucose as starch, and animals store glucose as glycogen. High-starch foods are beans, peas, cereal grains, and potatoes. Starch is digested to glucose in the digestive tract, and any excess glucose is stored as glycogen. Although other animals likewise store glucose as glycogen in liver or muscle tissue (meat), it has broken down by the time an animal is eaten for food. Except for honey and milk, which contain sugars, animal sources of food do not contain carbohydrates.

Cocaine

Cocaine is an alkaloid derived from the shrub Erythroxylon coca. It is sold in powder form and as crack, a more potent extract. Because cocaine prevents the synaptic uptake of dopamine, the neurotransmitter remains in the synapse for a prolonged period of time and continues to stimulate the postsynaptic cell. As a result, the user experiences a "rush" sensation. The epinephrine-like effects of dopamine account for the state of arousal that lasts for several minutes after the rush experience. A cocaine binge can go on for days, after which the individual suffers a crash. During the binge period, the user is hyperactive and has little desire for food or sleep but has an increased sex drive. During the crash period, the user is fatigued, depressed, and irritable; has memory and concentration problems; and displays no interest in sex. Cocaine causes extreme physical dependence. With continued cocaine use, the postsynaptic cells become increasingly desensitized to dopamine. The user, therefore, experiences the withdrawal symptoms arising from physical dependence and an intense craving for cocaine. These are indications that the person is highly dependent on the drug. Overdosing on cocaine can cause seizures and cardiac and respiratory arrest. It is possible that long-term cocaine abuse causes brain damage (Fig. 27.7). Babies born to cocaine addicts suffer withdrawal symptoms and may have neurological and developmental problems.

Infertility

Control of reproduction does not only mean preventing pregnancy. It also involves methods of treating infertility. Infertility is generally defined as the inability to produce offspring; a medical definition is the failure of a couple to achieve pregnancy after 1 year of regular, unprotected intercourse. It is estimated that one in six couples in America face problems with infertility. The cause of infertility can be attributed to the male (40%), the female (40%), or both (20%). The most frequent cause of infertility in males is low sperm count and/or a large proportion of abnormal sperm, which can be due to environmental influences. Physicians advise that a sedentary lifestyle coupled with smoking and alcohol consumption can lead to male infertility. When males spend most of the day driving or sitting in front of a computer or television, the testes' temperature remains too high for adequate sperm production. In females, body weight is an important fertility factor. Only if a woman is of normal weight do fat cells produce a hormone, called leptin, that stimulates the hypothalamus to release GnRH, so that follicle formation begins in the ovaries. If a woman is overweight, the ovaries may contain many small, ineffective follicles, and ovulation does not occur. About 10% of women of childbearing age have an endocrine condition that may be due to an inability to utilize insulin properly. The condition is called polycystic ovary syndrome (PCOS), because the ovaries contain many cysts (small, fluid-filled sacs) but no functioning follicles, and ovulation does not occur. An impaired ability to respond to insulin is implicated because many women with PCOS eventually develop type 2 diabetes. Other causes of infertility in females are blocked uterine tubes due to pelvic inflammatory disease (see "Sexually Transmitted Diseases," later in this section) and endometriosis, the presence of uterine tissue outside the uterus, particularly in the uterine tubes and on the abdominal organs. Sometimes the causes of infertility can be corrected by medical intervention, so that couples can have children. It is also possible to give females fertility drugs, gonadotropic hormones that stimulate the ovaries and bring about ovulation. Such hormone treatments have been known to cause multiple ovulations and multiple births. When reproduction does not occur in the usual manner, many couples adopt a child. Others try one of the assisted reproductive technologies discussed next.

Functions of Cytotoxic T Cells and Helper T Cells

Cytotoxic T cells specialize in cell-to-cell combat. They have storage vacuoles containing proteins called perforins or enzymes called granzymes. After a cytotoxic T cell binds to a virus-infected or cancer cell presenting the antigen it has learned to recognize, it releases perforin molecules, which perforate the target cell's plasma membrane, forming a pore. The cytotoxic T cell then delivers a supply of granzymes into the pore, and these cause the cell to undergo apoptosis. Once cytotoxic T cells have released their perforins and granzymes, they move on to the next target cell. Cytotoxic T cells are responsible for a cellular response to virus-infected and cancer cells Helper T cells specialize in regulating immunity by secreting cytokines that, in particular, stimulate B cells and cytotoxic T cells. Similar to B cells, cloned T cells include memory T cells that live for many years and can jump-start an immune response to an antigen that was dealt with before. Because HIV, the virus that causes AIDS, infects helper T cells and other cells of the immune system, it inactivates the immune response and makes HIV-infected individuals susceptible to opportunistic infections. Infected macrophages serve as reservoirs for the HIV virus. AIDS is discussed in Section 26.5.

Updating Dietary Guidelines

Dietary guidelines are typically revised by the U.S. government every 5 years to reflect changes in nutrition science. The latest guidelines were released in 2015 by the Departments of Agriculture and Health and Human Services. The overall purposes of these guidelines were to: 1. Promote health 2. Prevent chronic long-term disease 3. Assist people in reaching and maintaining a healthy weight The new guidelines focus less on prescribing quantitative levels for nutrients and more on establishing healthy eating patterns. A healthy eating pattern includes the following foods: 1. A variety of vegetables, including leafy vegetables, beans, red and yellow vegetables, and starches Fruits 2. Grains, at least half of which should be whole grains 3. Fat-free or low-fat dairy products (including soy) 4. Proteins in the form of seafood, lean meats, poultry, eggs, legumes, nuts, and soy products 5 .Oils To establish these healthy eating patterns, specific recommendations were made to limit certain nutrients that are recognized as raising health concerns. These are outlined in Table 25.8.

Proteins

Dietary proteins are digested to amino acids, which cells use to synthesize hundreds of cellular proteins. Of the 20 different amino acids, 9 are essential amino acids that must be present in the diet. Children will not grow if their diets lack the essential amino acids. Eggs, milk products, meat, poultry, and most other foods derived from animals contain all 9 essential amino acids and are "complete," or "high-quality," protein sources. Foods derived from plants generally do not have as much protein per serving as those derived from animals, and each type of plant food generally lacks one or more of the essential amino acids. Therefore, most plant foods are "incomplete," or "low-quality," protein sources. Vegetarians, however, do not have to consume animal sources of protein. To meet their protein needs, total vegetarians (vegans) can eat grains, beans, and nuts in various combinations. Also, tofu, soymilk, and other foods made from processed soybeans are complete protein sources. A balanced vegetarian diet is quite possible with a little planning.

Dietary Supplements

Dietary supplements are nutrients and plant products (such as herbal teas) that are used to enhance health. The U.S. government does not require dietary supplements to undergo the same safety and effectiveness testing that new prescription drugs must complete before they are approved. Therefore, many herbal products have not been tested scientifically to determine their benefits. Although people often think herbal products are safe because they are "natural," many plants, including lobelia, comfrey, and kava kava, can be poisonous. Dietary supplements that contain nutrients can also cause harm. Most fat-soluble vitamins are stored in the body and can accumulate to toxic levels, particularly vitamins A and D. Although excesses of water-soluble vitamins can be excreted, cases involving toxic amounts of vitamin B6, thiamine, and vitamin C have been reported. Minerals can be harmful, even deadly, when ingested in amounts that exceed the body's needs. Healthy people can take a daily supplement that contains recommended amounts of vitamins and minerals. Some people have metabolic diseases or physical conditions that interfere with their ability to absorb or metabolize certain nutrients. These individuals may need to add certain nutrient supplements to their diet. However, people should not take high doses of dietary supplements without checking with their physician.

Digestive Enzymes

Does your mouth water when you smell food cooking? Even the thought of food can sometimes cause the nervous system to order the secretion of digestive juices. The secretion of these juices is also under the influence of several peptide hormones (whose structure consists of a small sequence of amino acids). When you eat a meal rich in protein, the stomach wall produces a peptide hormone that enters the bloodstream and doubles back to cause the stomach to produce more gastric juices. When protein and fat are present in the small intestine, another peptide hormone made in the intestinal wall stimulates the secretion of bile and pancreatic juices. In this way, the organs of digestion regulate their own needs. The various digestive enzymes present in the digestive juices help break down carbohydrates, proteins, nucleic acids, and fats, the major components of food. Starch is a carbohydrate, and its digestion begins in the mouth. Saliva from the salivary glands has a neutral pH and contains salivary amylase, the first enzyme to act on starch. salivary amylase- starch + H2O = maltose Maltose, a disaccharide, cannot be absorbed by the intestine; additional digestive action in the small intestine converts maltose to glucose, which can be absorbed. Protein digestion begins in the stomach. Gastric juice secreted by gastric glands has a very low pH—about 2—because it contains hydrochloric acid (HCl). Pepsin, which is also present in gastric juice, acts on a protein molecule to produce peptides. Pepsin- protein + H2O = peptides Peptides are usually too large to be absorbed by the intestinal lining, but later they are broken down to amino acids in the small intestine. Starch, proteins, fats, and nucleic acids are all enzymatically broken down in the small intestine (Fig. 25.13). Pancreatic juice, which enters the duodenum, has a basic pH because it contains sodium bicarbonate (NaHCO3). One pancreatic enzyme, pancreatic amylase, digests starch Another pancreatic enzyme, trypsin, digests proteins Maltase and peptidases, enzymes produced by the small intestine, complete the digestion of starch to glucose and proteins to amino acids, respectively. Glucose and amino acids are small molecules that are absorbed into the cells of the villi and enter the blood (Fig. 25.13a,b). Maltose, a disaccharide that results from the first step in starch digestion, is digested to glucose by maltase. maltase- maltose + H2O = glucose + glucose Other disaccharides have their own enzyme and are digested in the small intestine. The absence of any one of these enzymes can cause illness. Peptides, which result from the first step in protein digestion, are digested to amino acids by peptidases. peptidases- peptides + H2O = amino acids Lipase, a third pancreatic enzyme, digests fat molecules in fat droplets that have been emulsified by bile salts. lipase- fat droplets + H2O = glycerol + 3 fatty acids Specifically, the end products of lipase digestion are monoglycerides (whose structure consists of glycerol and one fatty acid) and fatty acids. These products enter the cells of the villi, where they are rejoined and packaged as lipoprotein droplets, called chylomicrons. Chylomicrons enter the lacteals

The Synapse

Each axon has many axon terminals. In the CNS, a terminal of one neuron, known as the presynaptic cell, lies very close to the dendrite (or cell body) of another neuron, the postsynaptic cell. This region of close proximity is called a synapse. In the PNS, when the postsynaptic cell is a muscle cell, the region is called a neuromuscular junction. A small gap exists at a synapse, and this gap is called the synaptic cleft. While the synaptic cleft is very narrow, the nerve impulses are not able to cross it directly. Instead, transmission across a synaptic cleft is carried out by chemical signals called neurotransmitters, which are stored in synaptic vesicles. When nerve impulses traveling along an axon reach an axon terminal, synaptic vesicles release a neurotransmitter into the synaptic cleft. Neurotransmitter molecules diffuse across the cleft and bind to a specific receptor protein on the postsynaptic cell (Fig. 27.6). Depending on the type of neurotransmitter and/or the type of receptor, the response of the postsynaptic cell can be toward excitation or toward inhibition. Several dozen different neurotransmitters have been identified, a few of the more widely used neurotransmitters are acetylcholine (ACh), norepinephrine, serotonin, and gamma-aminobutyric acid (GABA). Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, the neurotransmitter is removed from the cleft. In some synapses, the postsynaptic cell produces enzymes that rapidly inactivate the neurotransmitter. For example, the enzyme acetylcholinesterase (AChE) breaks down acetylcholine. In other synapses, the presynaptic cell rapidly reabsorbs the neurotransmitter. For example, norepinephrine is reabsorbed by the axon terminal. The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of the postsynaptic cell. A single neuron has many dendrites plus the cell body, and both can have synapses with many other neurons; 1,000 to 10,000 synapses per single neuron are not uncommon. Therefore, a neuron is on the receiving end of many signals. An excitatory neurotransmitter produces a potential change that drives the neuron closer to an action potential, and an inhibitory neurotransmitter produces a potential change that drives the neuron farther from an action potential. Neurons integrate these incoming signals. Integration is the summing up of both excitatory and inhibitory signals. If a neuron receives many excitatory signals (either at different synapses or at a rapid rate from one synapse), chances are its axon will transmit a nerve impulse. On the other hand, if a neuron receives both inhibitory and excitatory signals, the integration of these signals may prohibit the axon from firing.

STDs Caused by Other Organisms

Females very often have vaginitis, or infection of the vagina, caused by either the flagellated protozoan Trichomonas vaginalis or the yeast Candida albicans.Trichomoniasis is most often acquired through sexual intercourse, and the asymptomatic male is usually the reservoir of infection. Candida albicans, however, is an organism normally found in the vagina; its growth simply increases beyond normal under certain circumstances. For example, women taking birth control pills are sometimes prone to yeast infections. Also, the legitimate and indiscriminate use of antibiotics for infections elsewhere in the body can alter the normal balance of organisms in the vagina, so that a yeast infection flares up.

Fetal Development and Birth

Fetal development encompasses the third to the ninth months (Fig. 29.17). Fetal development is marked by an extreme increase in size. The weight changes significantly, increasing from less than 28 grams to approximately 3 kilograms. During this time, too, the fetus grows to about 50 centimeters in length. The genitalia appear in the third month, so it is possible to tell if the fetus is male or female. Soon after the third month, hair, eyebrows, and eyelashes add finishing touches to the face and head. In the same way, fingernails and toenails complete the hands and feet. Later, during the fifth through seventh months, a fine, downy hair (lanugo) covers the limbs and trunk, only to disappear later. The fetus looks very old because the skin is growing so fast that it wrinkles. A waxy, almost cheeselike substance (called vernix caseosa) protects the wrinkly skin from the watery amniotic fluid. The fetus at first only flexes its limbs and nods its head, but later it can move its limbs vigorously to avoid discomfort. The mother feels these movements from about the fourth month on. The other systems of the body also begin to function. As early as 10 weeks, the fetal heartbeat can be heard through a stethoscope. A fetus born at 22 weeks has a chance of surviving, although the lungs are still immature and often cannot capture oxygen adequately. Weight gain during the last couple of months increases the likelihood of survival.

Fiber

Fiber includes various nondigestible carbohydrates derived from plants. Foods rich in fiber include beans, peas, nuts, fruits, and vegetables. Whole-grain products are also a good source of fiber and are therefore more nutritious than food products made from refined grains. During the refinement of grains, fiber and vitamins and minerals are removed, so primarily starch remains. A slice of bread made from whole-wheat flour, for example, contains 3 grams (g) of fiber; a slice of bread made from refined wheat flour contains less than a gram of fiber. Technically, fiber is not a nutrient for humans because it cannot be digested to small molecules that enter the bloodstream. Insoluble fiber, however, adds bulk to fecal material, which stimulates movements of the large intestine, preventing constipation. Soluble fiber combines with bile acids and cholesterol in the small intestine and prevents them from being absorbed. In this way, high-fiber diets may protect against heart disease. The typical American consumes only about 15 g of fiber each day; the recommended daily intake is 25 g for women and 38 g for men. To increase your fiber intake, eat whole-grain foods, snack on fresh fruits and raw vegetables, and include nuts and beans in your diet

Pharmaceutical Products

Genetic engineering has resulted in a new wave of research into the development of plant-made pharmaceuticals. Plants can produce antigens, antibodies, hormones, and therapeutic proteins. The advantages of using plants to produce pharmaceuticals include decreased cost, increased amount of protein produced, and decreased risk of contamination with animal and human pathogens. One type of antibody made by tobacco plants is being developed to combat tooth decay. Already approved for veterinary use, a plant-made antibody against certain forms of the cancer lymphoma is being developed for humans. In addition to producing antibodies, plants excel at creating "hard-to-make" proteins, such as anticoagulants, growth hormones, blood substitutes, collagen replacements, and antimicrobial agents, to name a few.

What is hormone replacement therapy?

Hormone replacement therapy (HRT) is often begun after menopause. The fluctuation of hormone levels during menopause can cause symptoms such as hot flashes, mood swings, trouble sleeping, increased abdominal fat, and thinning hair, which HRT can alleviate. The use of HRT has its pros and cons; there is evidence that using HRT after menopause can help prevent bone loss, decrease the risk of colorectal cancer, and decrease certain types of heart disease. Studies also show that some types of HRT in certain patients can increase incidences of stroke and blood clots. Women on HRT should be evaluated every six months by their physician.

Balance

Humans have two senses of balance (equilibrium): rotational and gravitational. We are able to detect the rotational (angular) movement of the head as well as the straight-line movement of the head with respect to gravity. Rotational equilibrium involves the semicircular canals (see Fig. 28.3b). In the base of each canal, hair cells have stereocilia embedded within a gelatinous membrane (Fig. 28.5a). Because there are three semicircular canals, each responds to head movement in a different plane of space. As fluid within a semicircular canal flows over and displaces the gelatinous membrane, the stereocilia of the hair cells bend, and the pattern of impulses carried to the central nervous system (CNS) changes. These data, usually supplemented by vision, tell the brain how the head is moving. Vertigo is dizziness and a sensation of rotation. It is possible to bring on a feeling of vertigo by spinning rapidly and stopping suddenly. Now the person feels like the room is spinning because of sudden stimulation of stereocilia in the semicircular canals. Gravitational equilibrium refers to the position of the head in relation to gravity. It depends on the utricle and saccule, two membranous sacs located in the inner ear (see Fig. 28.3b). Both of these sacs also contain hair cells with stereocilia in a gelatinous membrane (Fig. 28.5b). Calcium carbonate (CaCO3) granules, called otoliths, rest on this membrane. When the head moves forward or back, up or down, the otoliths are displaced and the membrane moves, bending the stereocilia of the hair cells. This movement alters the frequency of nerve impulses to the CNS. These data, usually supplemented by vision, tell the brain the direction of the movement of the head.

Human Kidney

If a kidney is sectioned longitudinally, three major parts can be distinguished (Fig. 24.13a). The renal cortex, the outer region of a kidney, has a somewhat granular appearance. The renal medulla consists of the cone-shaped renal pyramids, which lie inside the renal cortex. The innermost part of the kidney is a hollow chamber called the renal pelvis. Urine collects in the renal pelvis and then is carried to the bladder by a ureter. Microscopically, the cortex and medulla of each kidney are composed of about 1 million tiny tubules called nephrons (Fig. 24.13b). The nephrons of a kidney produce urine.

Abscisic Acid

If environmental conditions are not favorable, a plant needs to protect itself. Abscisic acid is sometimes called the stress hormone because it initiates and maintains seed and bud dormancy and brings about the closure of stomata. Dormancy has begun when a plant stops growing and prepares for adverse conditions (even though conditions at the time may be favorable for growth). For example, it is believed that abscisic acid moves from leaves to vegetative buds in the fall, and thereafter these buds are converted to winter buds. A winter bud is covered by thick, hardened scales (Fig. 21.5a). A reduction in the level of abscisic acid and an increase in the level of gibberellins are believed to break seed and bud dormancy. Seeds will then germinate, and buds will develop into leaves. Abscisic acid brings about the closing of stomata when a plant is under water stress (Fig. 21.5b). Abscisic acid causes potassium ions (K+) to leave guard cells. Thereafter, the guard cells lose water, and the stoma closes.

Can Carbohydrates Be Harmful?

If you or someone you know has lost weight by following the Atkins or Paleo diet, you may think "carbs" are unhealthy and should be avoided. According to nutritionists, however, carbohydrates should supply a large portion of your energy needs. Evidence suggests that Americans are not eating the right kind of carbohydrates. In some countries, the traditional diet is 60-70% high-fiber carbohydrates, and these people have a low incidence of the diseases that plague Americans. Obesity is associated with type 2 diabetes and cardiovascular disease, as discussed in Section 25.5. Some nutritionists hypothesize that the high intake of foods rich in refined carbohydrates and fructose sweeteners processed from cornstarch may be responsible for the prevalence of obesity in the United States. These are empty-calorie foods that provide sugars but no vitamins or minerals. Table 25.2 tells you how to reduce dietary sugars in your diet. Other nutritionists point out that consuming too much energy from any source contributes to body fat, which increases a person's risk of obesity and associated illnesses. Because many foods, such as donuts, cakes, pies, and cookies, are high in both refined carbohydrates and fat, it is difficult to determine which dietary component is responsible for the current epidemic of obesity among Americans.

Sexual Reproduction in Flowering Plants

In Section 18.1, we discussed the two multicellular stages that alternate in the plant life cycle, which is called an alternation of generations. In this life cycle, a diploid (2n) sporophyte alternates with a haploid (n) gametophyte: 1. The sporophyte (2n) produces haploid spores by meiosis. The spores develop into gametophytes. 2. The gametophytes (n) produce gametes. Upon fertilization, the cycle returns to the 2n sporophyte.

Why do tests for cancer often take biopsies of the lymph nodes?

In a biopsy, a physician uses a small needle to take a sample of a tissue for additional examination. For individuals with cancer, the doctor often takes a biopsy of the lymph nodes surrounding the original tumor. The reason for this is to see if the cancer has begun to spread, or metastasize, to other tissues. Since the lymphatic system filters the fluids returning from the tissues, metastasizing cancer cells can often be first detected in the lymph nodes closest to the tumor.

Red Bone Marrow

In a child, most bones have red bone marrow, and in an adult, it is still present in the bones of the skull, the sternum (breastbone), the ribs, the clavicle, the pelvic bones, the vertebral column, and the ends of the humerus and femur nearest their attachment to the body. Red bone marrow produces all types of blood cells, but in this chapter we are interested in those cells that are directly associated with the immune system (see Table 26.1). Macrophages Phagocytize pathogens; inflammatory response and specific immunity Mast cells Release histamine, which promotes blood flow to injured tissues; inflammatory response Neutrophils Phagocytize pathogens; inflammatory response Natural killer cells Kill virus-infected and tumor cells by cell-to-cell contact B lymphocytes Involved in the process of specific immunity by producing plasma cells and memory B cells Plasma cells Produce specific antibodies Memory cells Long-lived cells that may produce new B and T cells in the future T lymphocytes Regulate immune response; produce cytotoxic T cells and helper T cells Cytotoxic T cells Kill virus-infected and cancer cells Helper T cells Coordinate the adaptive immune responses Lymphocytes differentiate into either B lymphocytes (B cells) or T lymphocytes (T cells), which are discussed at length in Section 26.3. Bone marrow is not only the source of B lymphocytes but also the place where B lymphocytes mature. T lymphocytes mature in the thymus. As we will see, the B lymphocytes produce antibodies, while the T lymphocytes kill antigen-bearing cells outright.

The Body's Organization

In Section 4.4, you studied the general structure and function of a plant cell and an animal cell. Here we will take that knowledge of animal cell structure and see how millions of individual cells of different types come together to make an organism. Cells of the same structural and functional type occur within a tissue. An organ contains different types of tissues, each performing a function to aid in the overall action of the organ. In other words, the structure and function of an organ are dependent on the tissues it contains. That is why it is sometimes said that tissues, not organs, are the structural and functional units of the body. An organ system contains multiple organs that work together to perform a specific physiological function within the organism. Let's look at an example (Fig. 22.1). In humans, the functions of the urinary system are to filter wastes out of the blood, produce urine from those waste products, and permanently remove the urine from the body. The urinary system is composed of several individual organs—the kidneys, ureters, bladder, and urethra—each playing a particular role in the overall process. The kidneys are composed of several different tissue types; one type in particular, the epithelial tissue, contains cells that function in filtration. These cells can filter blood and remove the waste products (forming urine) into another organ, the ureters. The tissues in the ureters form tubelike structures that allow urine to pass from the kidneys into the bladder. The bladder, also composed of many different types of tissues, has one tissue made of cells that allow the entire organ to distend, or expand, when full of urine. And finally, the urethra, like the ureters, is composed of tissues that form a cylindrical structure, allowing urine to flow from the bladder out of the body. The overall functions of the urinary system—to produce, store, and rid the body of metabolic wastes—are dependent on the cells that make up the tissues, the tissues that make up the organs, and the organs that make up the organ system. The structure of the cells, the tissues, and the organs they compose also directly aid function. A common saying in biology is "structure equals function," meaning that the structure of an organ (and hence the tissues and cells that compose it) dictates its function. For example, the small intestine functions in the absorption of nutrients from the digestive tract. The larger the surface area of each cell, the more absorption can occur. Some intestinal cells have areas covered in microvilli (small, fingerlike projections that are extensions of the plasma membrane) that increase the surface area of the cell, thus increasing areas where absorption can occur, without increasing the overall size of the cell itself. A skeletal muscle cell has an internal arrangement of contractile fibers that can slide past each other when the cell contracts, instead of having an arrangement of fibers that would curl or twist in order for a contraction to occur. This cellular structure enables the entire muscle tissue to contract, without wear and tear on the fibers that might increase the possibility of damage. From the many different types of animal cells, biologists have been able to categorize tissues into just four major types: 1. Epithelial tissue (epithelium) covers body surfaces and lines body cavities. 2. Connective tissue binds and supports body parts. 3. Muscular tissue moves the body and its parts. 4. Nervous tissue receives stimuli and conducts nerve impulses. Except for nervous tissue, each type of tissue is subdivided into even more types (Fig. 22.2). This chapter looks at the structure and function of each of these tissue types, as well as the organs and organ systems where they are used.

Asexual Reproduction and Genetic Engineering in Plants

In asexual reproduction, there is only one parent and all the offspring are clones—genetically identical individuals. Clones are desirable for plant sellers, because the plants will look and behave exactly like the parent.

How do drugs that regulate depression and anxiety work?

In general, pharmaceutical drugs that regulate behavior work by regulating the amount of certain neurotransmitters in the synapses. For example, drugs such as Xanax and Valium increase the levels of gamma-aminobutyric acid. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as Prozac, Paxil, and Cymbalta allow norepinephrine and/or serotonin to accumulate at the synapses, usually by blocking their reabsorption. Increasing the levels of these neurotransmitters means that the postsynaptic cells receive a more constant chemical message, which explains the effectiveness of the antidepressant drugs.

Taste and Smell

In humans, taste buds, located primarily on the tongue, contain taste receptor cells, and the nose contains olfactory receptor cells (Fig. 28.2a). Receptor proteins for chemicals are located on the microvilli of taste receptor cells and on the cilia of olfactory receptor cells (Fig. 28.2b). When molecules bind to these receptor proteins, nerve impulses are generated in sensory nerve fibers that go to the brain. When they reach the appropriate cortical areas, they are interpreted as taste and smell, respectively. There are at least five primary types of tastes: bitter, sour, salty, sweet, and umami (Japanese for "savory" or "delicious"). Foods rich in certain amino acids, such as the common seasoning monosodium glutamate (MSG), as well as certain flavors of cheese, beef broth, and some seafood, produce the taste of umami. Taste buds for each primary taste are located throughout the tongue but may be concentrated in particular regions. A particular food can stimulate more than one of these types of taste buds. In this way, the response of taste buds can result in a range of sweet, sour, salty, umami, and bitter tastes. The brain appears to survey the overall pattern of incoming sensory impulses and take a "weighted average" of their taste messages as the perceived taste. Similarly, an odor contains many odor molecules, which activate a characteristic combination of receptor proteins. When this complex information is communicated to the cerebral cortex, we know we have smelled a rose—or an onion! Our senses of taste and smell have evolved to meet our physiological needs. Foods that are rich in nutrients we require, such as fruits, have a favorable taste. The smell of food cooking triggers a reflex that starts the release of digestive juices. A revolting or repulsive substance in the mouth can initiate the gag reflex or even vomiting. Smell is even more important to our survival. Smells associated with danger, such as smoke, can trigger the fight-or-flight reflex. Unpleasant smells can cause us to sneeze or choke. Have you ever noticed that a certain aroma vividly brings to mind a certain person or place? A person's perfume may remind you of someone else, or the smell of boxwood may remind you of your grandfather's farm. The olfactory bulbs have direct connections with the limbic system and its centers for emotions and memory. One researcher showed that, when subjects smelled an orange while viewing a painting, they not only remembered the painting when asked about it later but also had many deep feelings about it.

Breathing

In humans, the diaphragm is a muscular, membranous partition that divides the upper thoracic cavity from the lower abdominal cavity of the body. When humans breathe, the volume of the thoracic cavity and lungs is increased by muscle contractions that both lower the diaphragm and raise the ribs (Fig. 24.5). These movements create a negative pressure in the thoracic cavity and lungs, and air then flows into the lungs, a process called inspiration. It is important to realize that air comes in because the lungs have already opened up; air does not force the lungs open. When the rib and diaphragm muscles relax, the lungs recoil and air moves out as a result of increased pressure in the lungs, a process called expiration.

Mouth

In humans, the digestive system begins with the mouth, which chews food into pieces, beginning the process of mechanical digestion. Many vertebrates have teeth, an exception being birds, which lack teeth and depend on the churning of small pebbles within a gizzard to break up their food. The teeth (dentition) of mammals reflect their diet (Fig. 25.4). Carnivores eat other animals, and meat is easily digestible because the cells do not have a cellulose wall. Herbivores eat plant material, which needs a lot of chewing and other processing to break up the cellulose walls. Humans are omnivores; they eat both meat and plant material. The four front teeth (top and bottom) of humans are sharp, chisel-shaped incisors used for biting. On each side of the incisors are the pointed canines used for tearing food. The premolars and molars grind and crush food. It is as though humans were carnivores in the front of their mouths and herbivores in the back. Food contains cells composed of molecules of carbohydrates, proteins, nucleic acids, and lipids. Digestive enzymes break down these large molecules to smaller molecules. In the mouth, three pairs of salivary glands send saliva by way of ducts to the mouth, to begin the process of chemical digestion. One of these digestive enzymes is salivary amylase, which breaks down starch, a carbohydrate, to maltose, a disaccharide. While in the mouth, food is manipulated by the muscular tongue (mechanical digestion), mixing the chewed food with saliva (chemical digestion) and then forming this mixture into a mass called a bolus,which is swallowed.

The Human Nervous System

In humans, the nervous system controls the muscular system and works with the endocrine system to maintain homeostasis. The central nervous system (CNS)includes the brain and spinal cord, which have a central location along the midline of the body. The peripheral nervous system (PNS) consists of nerves that lie outside the central nervous system. The brain gives off paired cranial nerves (one on each side of the body), and the spinal cord gives off paired spinal nerves. The division between the central nervous system and the peripheral nervous system is arbitrary; the two systems work together and are connected to one another. While based on similar principles, the human nervous system is much more complex than the planarian system. Over the course of animal evolution, a number of important events occurred in the development of the nervous system: - A CNS developed that is able to summarize incoming messages before ordering outgoing messages. - Nerve cells (neurons) became specialized to send messages to the CNS, between neurons in the CNS, or away from the CNS. - A brain evolved that has special centers for receiving input from various regions of the body and for directing their activity. - The CNS became connected to all parts of the body by peripheral nerves. Therefore, the central nervous system can respond to both external and internal stimuli. - Complex sense organs, such as the human eye and ear, arose that can detect changes in the external environment.

Cardiovascular Disease

In the United States, cardiovascular disease, which includes hypertension, heart attack, and stroke, is among the leading causes of death. Cardiovascular disease is often due to blockage of arteries by plaque, which contains saturated fats and cholesterol. Cholesterol is carried in the blood by two types of lipoproteins: low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL is thought of as "bad" because it carries cholesterol from the liver to the cells, while HDL is thought of as "good" because it carries cholesterol from the cells to the liver, which takes it up and converts it to bile salts. Saturated fats, including trans fats, tend to raise LDL cholesterol levels, while unsaturated fats lower LDL cholesterol levels. Beef, dairy foods, and coconut oil are rich sources of saturated fat. Foods containing partially hydrogenated oils (e.g., vegetable shortening and stick margarine) are sources of trans fats. Unsaturated fatty acids in olive and canola oils, most nuts, and coldwater fish tend to lower LDL cholesterol levels. Furthermore, coldwater fish (e.g., herring, sardines, tuna, and salmon) contain polyunsaturated fatty acids, and especially omega-3 unsaturated fatty acids, which can reduce the risk for cardiovascular disease. Taking fish oil supplements to obtain omega-3 fatty acids is not recommended without a physician's approval, because too much of these fatty acids can interfere with normal blood clotting. Overall, dietary saturated fats and trans fats raise LDL cholesterol levels more than dietary cholesterol. A physician can determine if blood lipid levels are normal. If a person's cholesterol and triglyceride levels are elevated, modifying the fat content of the diet, losing excess body fat, and exercising regularly can reduce them. If lifestyle changes do not lower blood lipid levels enough to reduce the risk for cardiovascular disease, a physician may prescribe medication.

Transport in Humans

In the human cardiovascular system, like that of other vertebrates, the heart pumps blood into blood vessels, which take it to capillaries, where exchanges take place. In the lungs, carbon dioxide is exchanged for oxygen; in the tissues, nutrients and oxygen are exchanged for carbon dioxide and other wastes. These exchanges in the lungs and tissues are so important that, if the heart stops pumping, death results.

Three Parents—One Baby

In vitro fertilization (IVF) technology has been used for many years to help couples conceive a child. The process involves taking an egg from the mother and fertilizing it with paternal sperm to create embryos that are then implanted into the mother (or a surrogate) to carry the child to term. A new technique has been proposed that allows for a donor egg to be used in cases where the mother's cells contain defective genes in the mitochondria. Mitochondria are organelles inside a cell that generate the ATP that provides the energy the cell requires for its metabolism. These organelles contain genetic material (DNA) that encodes 37 genes, 14 of which are related to specific proteins involved in efficient ATP production. Any defect in these genes leads to serious conditions such as mitochondrial myopathies (muscle disorders). An innovative therapy to prevent genetic diseases directly tied to these mitochondrial genes is being developed. The new technique for in vitro fertilization creates what is sometimes called a "three-parent baby" because a third party donates an egg to be fertilized with a maternal nucleus and paternal sperm. The donor's nuclear genes are removed but the genes inside the mitochondria remain behind. The resulting baby has all of the nuclear genes of the mother and father but the 37 genes found in the mitochondria belong to the donor of the original egg cell. In this chapter, we will explore the structure and function of the reproductive system in humans, as well as some of the diseases that may cause individuals to need processes such as IVF.

Lipids

Like carbohydrates, triglycerides (the main components of fats and oils) supply energy for cells, but fat is stored for the long term in the body. Fat deposits under the skin, called subcutaneous fat, insulate the body from cold temperatures; deeper fat deposits in the trunk protect organs against injury. Nutritionists generally recommend that unsaturated rather than saturated fats (see Fig. 3.13) be included in the diet. Two unsaturated fatty acids (alpha-linolenic and linoleic acids) are essential dietary fatty acids. Delayed growth and skin problems can develop when the diet lacks these essential fatty acids, which can be supplied by eating fatty fish and by including plant oils, such as canola and soybean oils, in the diet. Animal foods such as butter, meat, whole milk, and cheeses contain saturated fats. Plant oils contain unsaturated fats. The differences between saturated and unsaturated fats are shown in Figure 3.13. Each type of oil has a particular percentage of monounsaturated and polyunsaturated fatty acids. Cholesterol, a lipid, can be synthesized by the body in sufficient quantities to meet daily needs. Cells use cholesterol to make various compounds, including bile, steroid hormones, and vitamin D. Cholesterol is also an important component of the plasma membrane. Plant foods do not contain cholesterol; only animal foods, such as cheese, egg yolks, liver, and certain shellfish (shrimp and lobster), are rich in cholesterol.

The Nerve Impulse

Like some other cellular processes, a nerve impulse is dependent on concentration gradients. In neurons, these concentration gradients are maintained by the sodium-potassium pump. This pump actively transports sodium ions (Na+) to the outside of the axon and actively transports potassium ions (K+) inside. Aside from ion concentration differences across the axon's membrane, a charge difference also exists. The inside of an axon is negative compared with the outside. This charge difference is primarily due in part to an unequal distribution of Na+ and K+ ions across the membrane. The charge difference across the axon's membrane plays an important role in the generation of a nerve impulse, which is also called an action potential. The nerve impulse is a rapid, short-lived, self-propagating reversal in the charge difference across the axon's membrane. Figure 27.4 shows how it works. A nerve impulse involves two types of gated channel proteins in the axon's membrane. In contrast to ungated channel proteins, which constantly allow ions to move across the membrane, gated channel proteins open and close in response to a stimulus, such as a signal from another neuron. One type of gated channel protein allows sodium (Na+) to pass through the membrane, and the other allows potassium (K+) to pass through the membrane. As an axon is conducting a nerve impulse, the Na+ gates open at a particular location, and the inside of the axon becomes positive as Na+ moves from outside the axon to the inside. The Na+ gates close, and then the K+ gates open. Now K+ moves from inside the axon to outside the axon, and the charge reverses. In Figure 27.4, the axon is unmyelinated, and the action potential at one locale stimulates an adjacent part of the axon's membrane to produce an action potential. In myelinated axons, an action potential at one node of Ranvier causes an action potential at the next node (Fig. 27.5). This type of conduction, called saltatory conduction, is much faster than conduction by unmylelinated axons. Imagine running down a long hallway as quickly as you can; then picture yourself able to get to the end of the same hall in just a few leaping bounds. Leaping would enable you to travel the same distance in a much shorter time; likewise, saltatory conduction greatly speeds the conduction of nerve impulses. In thin, unmyelinated axons, the nerve impulse travels about 1.0 meter/second, but in thick, myelinated axons, the rate is more than 100 meters/second due to saltatory conduction. In any case, action potentials are self-propagating; each action potential generates another along the length of an axon. The conduction of a nerve impulse (action potential) is an all-or-none event—that is, either an axon conducts a nerve impulse or it does not. The intensity of a message is determined by how many nerve impulses are generated within a given time span. An axon can conduct a volley of nerve impulses because only a small number of ions are exchanged with each impulse. As soon as an impulse has passed by each successive portion of an axon, it undergoes a short refractory period, during which it is unable to conduct an impulse. During a refractory period, the sodium gates cannot open. This period ensures that nerve impulses travel in only one direction and do not reverse.

Natural Killer Cells

Natural killer (NK) cells are large, granular lymphocytes that kill virus-infected cells and tumor (cancer) cells by cell-to-cell contact (see Table 26.1). What makes an NK cell attack and kill a cell? The cells of your body ordinarily have self proteins on their surface that bind to receptors on NK cells. Sometimes virus-infected cells and cancer cells undergo alterations and lose their ability to produce self proteins. When NK cells can find no self proteins to bind to, they kill the cell, using the same method as T lymphocytes (see Fig. 26.8). NK cells are not specific—their numbers do not increase when exposed to a particular antigen, and they have no means of "remembering" an antigen from previous contact with it.

During a kidney transplant, is the failing kidney removed?

Normally, no. When a patient needs a kidney transplant, the donor kidney is normally placed in the lower abdomen, below one of the two original kidneys, with the blood vessels from the donor kidney connected to the recipient's arteries and veins and the ureter from the donor kidney connected to the recipient's bladder. The patient's failing kidney is kept in most cases because, even if the nephrons are not functioning in cleansing the blood and producing urine, the kidney can still produce other chemicals and substances the body needs to function. The failing kidney is removed in cases of severe infection, cancer, or polycystic kidney disease (PKD).

Organs and Organ Systems

Organs are composed of a number of tissues, and the structure and function of an organ are dependent on those tissues. Since an organ has many types of tissues, it can perform a function that none of the tissues can do alone. For example, the function of the bladder is to store urine and to expel it when convenient. But this function is dependent on the individual tissues making up the bladder. The epithelium lining the bladder helps the organ store urine by stretching, while preventing urine from leaking into the internal body cavity. The muscles of the bladder propel the urine forward into the urethra, so that it can be removed from the body. Similarly, the functions of an organ system are dependent on its organs. The function of the urinary system is to produce urine, store it, and then transport it. The kidneys produce urine, the bladder stores it, and various tubes transport it from the kidneys to the bladder (the ureters) and out of the body (through the urethra). This text divides the systems of the body into those involved in (1) the transport of fluids throughout the body and the protection of the body, (2) maintenance of the body, (3) control of the body's systems, (4) sensory input and motor output, and (5) reproduction. All these systems have functions that contribute to homeostasis, the relative constancy of the internal environment (see Section 22.3).

Eating Disorders

People with eating disorders are dissatisfied with their body image. Social, cultural, emotional, and biological factors all contribute to the development of an eating disorder. These serious conditions can lead to malnutrition, disability, and death. Regardless of the eating disorder, early recognition and treatment are crucial. Treatment usually includes psychological counseling and antidepressant medications. Anorexia nervosa is a severe psychological disorder characterized by an irrational fear of getting fat, causing a refusal to eat enough food to maintain a healthy body weight (Fig. 25.22a). A self-imposed starvation diet is often accompanied by occasional binge eating, followed by purging and extreme physical activity to avoid weight gain. Binges usually include large amounts of high-calorie foods, and purging episodes involve self-induced vomiting and laxative abuse. About 90% of people suffering from anorexia nervosa are young women; an estimated 1 in 200 teenage girls is affected. A person with bulimia nervosa binge eats, then purges to avoid gaining weight (Fig. 25.22b). The binge-purge cyclic behavior can occur several times a day. People with bulimia nervosa can be difficult to identify because their body weight is often normal, and they tend to conceal their bingeing and purging practices. Women are more likely than men to develop bulimia; an estimated 4% of young women suffer from this condition. Other abnormal eating practices include binge-eating disorder and muscle dysmorphia. Many obese people suffer from binge-eating disorder, a condition characterized by episodes of overeating that are not followed by purging. Stress, anxiety, anger, and depression can trigger food binges. A person suffering from muscle dysmorphia thinks his or her body is underdeveloped. Body-building activities and a preoccupation with diet and body form accompany this condition. Each day, the person may spend hours in the gym, working out on muscle-strengthening equipment. Unlike anorexia nervosa and bulimia, muscle dysmorphia affects more men than women.

Understanding Nutrition Guidelines

Planning nutritious meals and snacks involves making daily food choices based on a wide variety of information about recommended amounts of nutrients. A day's food intake should provide the proper balance of nutrients—neither too much nor too little of each nutrient. Food guides can be helpful in planning your diet. Additionally, reading the "Nutrition Facts" panel on packaged foods can help you choose healthier sources of nutrients.

Transport and Internal Exchange of Gases

Recall from Section 23.3 that hemoglobin molecules within red blood cells carry oxygen to the body's tissues. If hemoglobin did not transport oxygen, it would take about 3 years for an oxygen molecule to move from your lungs to your toes by simple diffusion. Each hemoglobin molecule contains four polypeptide chains: two α and two β chains. Each polypeptide is folded around an iron-containing group called heme. It is actually the iron that bonds with oxygen and carries it to the tissues (Fig. 24.10). Since there are about 250 million hemoglobin molecules in each red blood cell, each cell is capable of carrying at least 1 billion molecules of oxygen. Hemoglobin gives up its oxygen in the tissues during internal exchange primarily because interstitial fluid always has a lower oxygen concentration than blood does. This difference occurs because cells take up and utilize oxygen when they carry on cellular respiration. Another reason hemoglobin gives up oxygen is the warmer temperature and lower pH in the tissues, environmental conditions that are also caused by cellular respiration. When cells respire, they give off heat and carbon dioxide as by-products. Carbon dioxide enters the blood during internal exchange because the interstitial fluid always has a higher concentration of carbon dioxide. Most of the carbon dioxide is transported in the form of the bicarbonate ion (HCO3-). First, carbon dioxide combines with water, forming carbonic acid, and then this acid dissociates to a hydrogen ion (H+) and HCO3-: The H+ does cause the pH to lower, but only slightly because much of the H+ is absorbed by the globin portions of hemoglobin. The HCO3- is carried in the plasma. What happens to the HCO3- in the lungs? As blood enters the pulmonary capillaries, carbon dioxide diffuses out of the blood into the alveoli. Hemoglobin gives up the H+ it has been carrying as this reaction occurs: Now, much of the carbon dioxide diffuses out of the blood into the alveoli of the lungs. Should the blood level of H+ rise, the breathing center in the brain increases the breathing rate, and as more CO2 leaves the blood, the pH of blood is corrected.

What is the 10,000-step program?

Research suggests that a minimum of 10,000 (10K) steps per day is necessary for weight maintenance and good health. That number of steps per day is roughly equivalent to the recommended 30 minutes of daily exercise. To get started, you'll need a pedometer. You'll probably find you need to increase the amount of walking you do to reach the goal of 10K steps a day. There are a number of easy ways to add steps to your routine. Park a little farther away from your office or the store. Take the stairs instead of the elevator, or go for a walk after a meal. If your goal is to lose weight, 12-15K steps a day have been shown to promote weight loss.

Vision

Sensory receptors that are sensitive to light are called photoreceptors. In planarians, eyespots allow these animals to determine only the direction of light. Other photoreceptors form actual images. Image-forming eyes provide information about an object, including how far away it is. Such detailed information is invaluable to an animal. Among invertebrates, arthropods have compound eyes composed of many, independent visual units, each of which possesses a lens to focus light rays on photoreceptors (Fig. 28.7a). The image that results from all the stimulated visual units is crude because the small size of compound eyes limits the number of visual units, which still might number as many as 28,000. Vertebrates (including humans) and certain molluscs, such as the squid and the octopus, have a camera-type eye (Fig. 28.7b). A single lens focuses light on photoreceptors, which number in the millions and are closely packed together within a retina. Since molluscs and vertebrates are not closely related, the camera-type eye evolved independently in each group. Insects have color vision, but they make use of a slightly shorter range of the electromagnetic spectrum than humans do. However, they can see the longest of the ultraviolet rays, and this enables them to be especially sensitive to the reproductive parts of flowers, which have particular ultraviolet patterns. Some fishes and most reptiles are believed to have color vision, but among mammals, only humans and other primates have expansive color vision. It would seem, then, that this trait was adaptive for a diurnal lifestyle (being active during the day), which accounts for its retention in only a few mammals.

Does the MMR vaccine cause autism?

Several large research studies have failed to find any connection among the measles, mumps, and rubella (MMR) vaccine; thimerosal (a preservative that was used for vaccines); and an increased risk of autism in children. While the cause of autism has not yet been identified, the evidence does not suggest that the vaccine or thimerosal is causing autism. The problem is that the first MMR vaccine is usually administered between 12 and 15 months of age, which is typically the age that an autistic child first presents symptoms. Most autism researchers believe that the factors causing autism are in place before this time frame and the timing with the MMR vaccine is coincidental. Several large-scale studies, including those sponsored by the Institute of Medicine and the National Alliance for Autism Research, have found no link between the MMR vaccine and an increased risk of autism. Most pediatricians agree that the threat of measles and rubella far outweighs the unsubstantiated risk of autism.

Seeing in the Dark

Some nocturnal animals rely on vision, but others use sonar (sound waves) to find their way in the dark. Bats flying in a dark room easily avoid obstacles in their path because they echolocate, as submarines do. When searching for food, they emit ultrasonic sound (sound above the range humans can hear) in chirps that bounce off their prey. Bats are able to determine the distance to their dinner by timing the echo's return—a long delay means the prey is far away. A bat-inspired sonar walking stick is being perfected to help visually impaired people sense their surroundings. It, too, emits ultrasonic chirps and picks up the echoes from nearby objects. Buttons on the cane's handle vibrate gently to warn a user to dodge a low ceiling or to sidestep objects blocking the path. A fast, strong signal means an obstacle is close by.

What is the Paleo diet?

The Paleo diet, also known as the caveman or warrior diet, is based on the idea that many modern nutritional problems, such as diabetes and cardiovascular disease, are due to the fact that humans are eating foods that their bodies have not had time to adjust to on an evolutionary scale. Supporters of the caveman diet suggest that a diet rich in nuts, meat, shellfish, vegetables, and berries—foods that were available to cavemen—reduces the risk for these diseases. While diets rich in these foods do reduce the risk for certain diseases, critics of the caveman diet state that the diet omits important foods, such as low-fat dairy, grains, and beans—all of which are important in a healthy diet. In all cases, diets should remove low-nutrient foods and replace them with healthier choices. Individuals should consult with their physician before undertaking any new diet.

ChooseMyPlate.gov

The U.S. Department of Agriculture (USDA) has developed a guideline called MyPlate (Fig. 25.18). This graphical representation replaced the older pyramids because most people found it easier to interpret. It can be used to help you decide how your daily calorie intake should be distributed among your food choices. MyPlate emphasizes the proportions of each food group that should be consumed daily. In addition, the USDA provides (on the ChooseMyPlate.gov website) recommendations concerning the minimum quantity of foods in each group that should be eaten daily. The site also contains an interactive component, SuperTracker, that allows you to track your own diet and set personal weight and activity goals. To support these decisions, the USDA also provides examples of daily food plans and information on how to follow a healthy diet on a budget.

Do women make testosterone?

The adrenal glands and ovaries of women make small amounts of testosterone. Women's low testosterone levels may affect libido, or sex drive. The use of supplemental testosterone to restore a woman's libido has not been well researched. By the way, men make estrogen, too. Some estrogen is produced by the adrenal glands. Androgens are also converted to estrogen by enzymes in the gonads and peripheral tissues. Estrogen may prevent osteoporosis in males.

The Autonomic System

The autonomic system of the PNS automatically and involuntarily regulates the activity of glands and cardiac and smooth muscle. The system is divided into the parasympathetic and sympathetic divisions (Fig. 27.13). Reflex actions, such as those that regulate blood pressure and breathing rate, are especially important to the maintenance of homeostasis. These reflexes begin when the sensory neurons in contact with internal organs send information to the CNS. They are completed by motor neurons within the autonomic system. The parasympathetic division includes a few cranial nerves (e.g., the vagus nerve) and axons that arise from the last portion of the spinal cord. The parasympathetic division, sometimes called the "housekeeper division," promotes all the internal responses we associate with a relaxed state. For example, it causes the pupil of the eye to constrict, promotes the digestion of food, and retards the heartbeat. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh). Axons of the sympathetic division arise from portions of the spinal cord. The sympathetic division is especially important during emergency situations and is associated with "fight or flight." If you need to fend off a foe or flee from danger, active muscles require a ready supply of glucose and oxygen. On one hand, the sympathetic division accelerates the heartbeat and dilates the airways. On the other hand, the sympathetic division inhibits the digestive tract, since digestion is not an immediate necessity if you are under attack. The sympathetic nervous system uses the neurotransmitter norepinephrine, which has a structure like that of epinephrine (adrenaline) released by the adrenal medulla.

Minerals

The body needs about 20 elements called minerals for numerous physiological functions, including regulation of biochemical reactions, maintenance of fluid balance, and incorporation into certain structures and compounds. Major minerals are needed in the body at higher levels than trace minerals. More than 100 milligrams (mg) per day of each major mineral is required in the diet, and less than 100 mg per day of each trace mineral is required in the diet. Table 25.4 lists the major minerals and a sample of the trace minerals. Their functions and food sources are given, as well as the health effects of too little or too much intake. Some individuals (especially women) do not get enough iron, calcium, magnesium, or zinc in their diets. Adult females need more iron in the diet than males (18 mg compared to 10 mg) if they are menstruating each month. Anemia, characterized by a run-down feeling due to insufficient red blood cells, results when the diet lacks iron. Also, many people take calcium supplements as directed by a physician to counteract osteoporosis, a degenerative bone disease (Fig. 25.17) that affects an estimated one-quarter of older men and one-half of older women in the United States. One mineral that people consume too much of is sodium. The recommended amount of sodium intake per day is 2,300 mg (1,500 mg if you have high blood pressure), although the average American takes in over 4,000 mg each day. The American Heart Association recommends that you aim to decrease your sodium intake to less than 1,500 mg per day. Table 25.5 gives recommendations for reducing the amount of sodium in the diet. TO REDUCE DIETARY SODIUM: - Use spices instead of salt to flavor foods. - Add little or no salt to foods at the table, and add only small amounts of salt when you cook. - Eat unsalted crackers, pretzels, potato chips, nuts, and popcorn. - Avoid hot dogs, ham, bacon, luncheon meats, smoked salmon, sardines, and anchovies. - Avoid processed cheese and canned or dehydrated soups. - Avoid brine-soaked foods, such as pickles and olives. - Read labels to avoid high-salt products.

The Complement System

The complement system, often simply called complement, is composed of a number of blood plasma proteins designated by the letter C and a number. The complement proteins "complement" certain immune responses, which accounts for their name. For example, they are involved in and amplify the inflammatory response, because certain complement proteins can bind to mast cells and trigger histamine release. Others can attract phagocytes to the scene. Some complement proteins bind to the surface of pathogens already coated with antibodies, which ensures that the pathogens will be phagocytized by a neutrophil or macrophage (Fig. 26.4a). Certain other complement proteins join to form a membrane attack complex, which produces holes in the surfaces of microbes (Fig 26.4b). Fluids and salts then enter the pathogen to the point that it bursts.

Endocrine System

The endocrine system consists of glands and tissues that secrete chemical signals called hormones (Fig. 27.14). Hormones are chemical messengers that are released by one gland in the body and regulate the activity of another organ, tissue, or gland in the body. Endocrine glands do not have ducts; they secrete their hormones directly into the bloodstream for distribution throughout the body. They can be contrasted with exocrine glands, which have ducts and secrete their products into these ducts for transport to body cavities. For example, the salivary glands (see Section 25.1) are exocrine glands, because they send saliva into the mouth by way of the salivary ducts. The endocrine system and the nervous system are intimately involved in homeostasis, the maintenance of relative stability in the body's internal environment. Hormones directly affect blood composition and pressure, body growth, and many more life processes. Certain hormones are involved in the maturation and function of the reproductive organs, and these are discussed in Section 29.2.

Types of Skeletons

The endoskeleton of vertebrates can be contrasted with the exoskeleton of arthropods. Both skeletons are jointed, which has helped members of both groups of animals live successfully on land. The endoskeleton of humans is composed of bone, which is living material and capable of growth. Like an exoskeleton, an endoskeleton protects vital internal organs, but unlike an exoskeleton, it need not limit the space available for internal organs, because it grows as the animal grows. The soft tissues that surround an endoskeleton protect it, and injuries to soft tissues are apt to be easier to repair than is the skeleton itself. The exoskeleton of arthropods is composed of chitin—a strong, flexible, nitrogenous polysaccharide. Besides providing protection against wear and tear and against enemies, an exoskeleton also prevents the animal from drying out. Although an arthropod exoskeleton provides support for muscle contractions, it does not grow with the animal, and arthropods molt to rid themselves of an exoskeleton that has become too small (Fig. 28.13a). This process makes them vulnerable to predators. a. Arthropods have an exoskeleton that must be shed as they grow. b. Worms have a hydrostatic skeleton, in which muscle contraction pushes against a fluid-filled internal cavity. One other type of skeleton is seen in the animal kingdom. In animals, such as worms, that lack a hard skeleton, a fluid-filled internal cavity can act as a hydrostatic skeleton (Fig. 28.13b). A hydrostatic skeleton offers support and resistance to the contraction of muscles, so that the animal can move. As an analogy, consider how a garden hose stiffens when filled with water and how a water-filled balloon changes shape when squeezed at one end. Similarly, an animal with a hydrostatic skeleton can change shape and perform a variety of movements.

How is Zika virus related to birth defects?

The first cases of Zika virus were reported in Africa in 1952, but the virus had been largely unknown in the Western Hemisphere until it was reported in Brazil in 2015. The virus is transmitted by a bite from an infected Aedes mosquito, and it can also be sexually transmitted in humans from infected males to females. For most people, infection with the Zika virus produces mild symptoms, such as fevers, rashes, or joint pain. Some individuals do not experience any symptoms. However, in a small number of cases, pregnant females who have been infected with the Zika virus have given birth to children with microcephaly. Microcephaly is a form of birth defect where the infant's head and brain are abnormally small. This can cause a number of developmental problems, including seizures, intellectual disabilities, and vision problems. There is no cure for microcephaly. The exact method by which Zika virus may cause microcephaly is still being investigated. Research is focusing on a group of neural stems cells that are associated with brain development.

Neurulation

The first organs to form are those of the central nervous system. The newly formed mesoderm cells coalesce to form a dorsal supporting rod called the notochord. The central nervous system develops from midline ectoderm located just above the notochord. During neurulation, a thickening of cells, called the neural plate, is seen along the dorsal surface of the embryo. Then, neural folds develop on both sides of a neural groove, which becomes the neural tube when these folds fuse. The anterior portion of the neural tube becomes the brain, and the posterior portion becomes the spinal cord. Development of the neural tube is an example of induction, the process by which one tissue or organ influences the development of another. Induction occurs because the tissue initiating the induction releases a chemical that turns on genes in the tissue being induced. Midline mesoderm cells that did not contribute to the formation of the notochord now become two masses of tissue. These two masses are then blocked off into somites, which are serially arranged on both sides along the length of the notochord. Somites give rise to the vertebrae and to muscles associated with the axial skeleton. The sequential order of the vertebrae and the muscles of the trunk testify that chordates are segmented animals. Lateral to the somites, the mesoderm splits and forms the mesodermal lining of the coelom. In addition, the neural crest consists of a band of cells that develops where the neural tube pinches off from the ectoderm. These cells migrate to various locations, where they contribute to the formation of skin and muscles, in addition to the adrenal medulla and the ganglia of the peripheral nervous system. At the end of the third week, over a dozen somites are evident, and the blood vessels and gut have begun to develop. At this point, the embryo is about 2 millimeters (mm) long.

Early Embryonic Development

The first two months of development are considered the embryonic period. Approximately 6 days of development occur in the uterine tube before the embryo implants itself in the uterine lining, or endometrium (Fig. 29.13). During the first stage of development, the embryo becomes multicellular. Following fertilization, the zygote undergoes cleavage, which is cell division without growth. DNA replication and mitotic cell division occur repeatedly, and the cells get smaller with each division. Notice that cleavage only increases the number of cells; it does not change the original volume of the egg cytoplasm. The resulting tightly packed ball of cells is called a morula. The cells of the morula continue to divide, but they also secrete a fluid into the center of a ball of cells. A hollow ball of cells, called the blastocyst, is formed, surrounding a fluid-filled cavity called the blastocoel. Within the ball is an inner cell mass that will go on to become the embryo. The outer layer of cells is the first sign of the chorion, the extraembryonic membrane that will contribute to the development of the placenta. As the embryo implants itself in the uterine lining (endometrium), the placenta begins to form and to secrete the hormone human chorionic gonadotropin (hCG). This hormone is the basis for the pregnancy test, and it maintains the corpus luteum past the time it normally disintegrates inside the ovary. Because of hCG, the endometrium is maintained until this function is taken over by estrogen and progesterone, produced by the placenta. Ovulation and menstruation do not normally occur during pregnancy.

B Cells and the Antibody Response

The general characteristics of a B cell are presented in Table 26.2. It is important to note that each B cell can bind only to a specific antigen—the antigen that fits the binding site of its receptor. The receptor is called a B-cell receptor (BCR). Some B cells never have anything to do, because an antigen that fits the binding site of their type of receptor is never encountered by the body. But if an antigen does bind to a BCR, that B cell is activated, and it divides, producing many plasma cells and memory B cells (Fig. 26.5). This mechanism is called the clonal selection model, since the antigen selects which B cell is activated and this cell then divides to produce many clones of itself. B cells are stimulated by cytokines to divide and produce plasma cells. Characteristics of B Cells - They provide an antibody response to a pathogen. - They are produced and become mature in bone marrow. - They reside in lymph nodes and the spleen; they circulate in blood and lymph. - They directly recognize antigens and then undergo cell division. - Cell division produces antibody-secreting plasma cells, as well as memory B cells. Cytokines are chemical signals that may be released by nonspecific defense mechanisms, such as the cells involved in the inflammatory response. Plasma cells are larger than regular B cells, because they have extensive rough endoplasmic reticulum for the mass production and secretion of antibodies specific to the antigen. The antibodies produced by plasma cells and secreted into the blood and lymph are identical to the BCR of the activated B cell. Memory B cells are the means by which long-term immunity is established. If the same antigen enters the system again, memory B cells quickly divide and give rise to more plasma cells capable of producing the correct antibodies. Once the threat of an infection has passed, the development of new plasma cells ceases, and those present undergo apoptosis. Apoptosis is a process of programmed cell death involving a cascade of cellular events, leading to the death and destruction of the cell (see Fig. 8.10).

What is the glycemic index?

The glycemic index is a reference used to indicate how a food item influences blood glucose levels. The index uses glucose as a reference value (100). In general, foods having a high amount of complex carbohydrates have a lower glycemic index than foods containing simple carbohydrates. For example, lentils and kidney beans have a glycemic index of 29, whereas most crackers have an index of 80. Originally designed to plan diets for diabetics, the glycemic index is useful for anyone who is interested in increasing the amount of complex carbohydrates in the diet. Many people mistakenly believe that children become hyperactive after eating sugar. There is no scientific basis for this belief, because sucrose is broken down into glucose and fructose in the small intestine, and these sugars are absorbed into the bloodstream. The spike caused by their absorption does not last long. Excess glucose and fructose enters the liver, and fructose is converted to glucose. As you know, the liver stores glucose as glycogen, and glycogen is broken down to maintain the proper glucose level.

The Classes of Nutrients

The human diet must contain the energy nutrients (macronutrients) in the correct proportions as well as adequate amounts of micronutrients. Generally, each type of nutrient has more than one function in the body and can be supplied by several different food sources (Table 25.1).

The Cardiac Cycle

The heart's pumping action, known as the heartbeat or cardiac cycle, consists of a series of events: First the atria contract, then the ventricles contract, and then they both rest. Figure 23.6 lists and depicts the events of a heartbeat, using the term systole to mean contraction and diastole to mean relaxation. When the heart beats, the familiar lub-dub sound is caused by the closing of the heart valves. The longer and lower-pitched lub occurs when the atrioventricular valves close, and the shorter and sharper dub is heard when the semilunar valves close. The pulse is a wave effect that passes down the walls of the arterial blood vessels when the aorta expands and then recoils following the ventricular systole. Because there is one pulse per ventricular systole, the pulse rate can be used to determine the heart rate. The heart beats about 70 times per minute, although a normal adult heart rate can vary from 60 to 100 beats per minute. The beat of the heart is regular because it has an intrinsic pacemaker, called the SA (sinoatrial) node. The nodal tissue of the heart, located in two regions of the atrial wall, is a unique type of cardiac muscle tissue. Every 0.85 second, the SA node automatically sends out an excitation impulse that causes the atria to contract (Fig. 23.7a). When this impulse is picked up by the AV (atrioventricular) node, it passes to the Purkinje fibers, which cause the ventricles to contract. If the SA node fails to work properly, the ventricles still beat, due to impulses generated by the AV node, but the beat is slower (40-60 beats per minute). To correct this condition, it is possible to implant an artificial pacemaker, which automatically gives an electrical stimulus to the heart every 0.85 second. In self-adjusting pacemakers, sensors generate variable electrical signals depending on the person's level of activity; the pacemakers change their output based on these signals. Although the beat of the heart is intrinsic, it is regulated by the nervous system and various hormones. Activities such as yoga and meditation lead to activation of the vagus nerve, which slows the heart rate. Exercise or anxiety leads to the release of the hormones norepinephrine and epinephrine by the adrenal glands, which causes the heart rate to speed up. An electrocardiogram (ECG) is a recording of the electrical changes that occur in the wall of the heart during a cardiac cycle. Body fluids contain ions that conduct electrical currents, and therefore the electrical changes in heart muscle can be detected on the skin's surface. When an electrocardiogram is being taken, electrodes placed on the skin are connected by wires to an instrument that detects these electrical changes (Fig. 23.7b). Various types of abnormalities can be detected by an electrocardiogram. One of them, called ventricular fibrillation, is due to uncoordinated contraction of the ventricles (Fig. 23.7c). Ventricular fibrillation is found most often in individuals with heart disease, but it may also occur as the result of an injury or a drug overdose. It is the most common cause of sudden cardiac death in a seemingly healthy person. To stop fibrillation, a defibrillator can be used to apply a strong electrical current for a short period. Then, the SA node may be able to reestablish a coordinated beat. Easy-to-use defibrillators are becoming increasingly available in public places, such as airports and college campuses.

Swallowing

The human digestive and respiratory passages come together in the pharynx and then separate (Fig. 25.5a). When food is swallowed, the soft palate, the rear portion of the mouth's roof, moves up to close off the nasal cavities (Fig. 25.5b). A flap of tissue called the epiglottis covers the glottis, an opening into the larynx. Ordinarily, the bolus must move through the pharynx and into the esophagus because the air passages are blocked. Unfortunately, we have all had the unpleasant experience of having food "go the wrong way." The wrong way may be either into the nasal cavities or into the trachea. If it is the latter, coughing will most likely force the food up out of the trachea and into the pharynx again. The esophagus is a muscular tube that takes food to the stomach, which lies below the diaphragm. When food enters the esophagus, peristalsis begins. Peristalsis is a series of rhythmic contractions of smooth muscles that move the contents along in tubular organs—in this case, those of the digestive tract.

Female Reproductive System

The human female reproductive system includes the ovaries, the uterine tubes, the uterus, and the vagina (Fig. 29.6a,b). The uterine tubes, also called the oviducts or fallopian tubes, extend from the ovaries to the uterus; however, the uterine tubes are not attached to the ovaries. Instead, the uterine tubes have fingerlike projections, called fimbriae (sing., fimbria), that sweep over the ovaries. When an oocyte, an immature egg cell, ruptures from an ovary during ovulation, it usually is swept into a uterine tube by the combined action of the fimbriae and the beating of cilia that line the uterine tube. Fertilization, if it occurs, normally takes place in the first one-third of the uterine tube, and the developing embryo is propelled slowly by ciliary movement and tubular muscle contraction to the uterus. The uterus is a thick-walled, muscular organ about the size and shape of an inverted pear. The narrow end of the uterus is called the cervix. An embryo completes its development after embedding itself in the uterine lining, called the endometrium. A small opening at the cervix leads to the vaginal canal. The vagina is a tube at a 45° angle with the body's vertical axis. The mucosal lining of the vagina lies in folds, and therefore the vagina can expand. This ability to expand is especially important when the vagina serves as the birth canal, and it can facilitate sexual intercourse, when the penis is inserted into the vagina. The external genital organs of a female are known collectively as the vulva (Fig. 29.6c). The mons pubis and two folds of skin called labia minora and labia majora are on each side of the urethral and vaginal openings. At the juncture of the labia minora is the clitoris, which is homologous to the penis of males. The clitoris has a shaft of erectile tissue and is capped by a pea-shaped glans. The many sensory receptors of the clitoris allow it to function as a sexually sensitive organ. Most females are born with a thin membrane, called the hymen, which partially obstructs the vaginal opening and has no apparent biological function. The hymen is typically ruptured by physical activities, including sexual intercourse, tampon insertion, and even athletics.

Complete and Incomplete Digestive Systems

The hydras and planarians (see Chapter 19) are both examples of animals that have an incomplete digestive system, meaning that a single opening serves as both an entrance and an exit. Most other animals, such as the earthworm, have a complete digestive tract. A complete digestive tract has a tube-within-a-tube configuration (Fig. 25.2). The inner tube (the digestive tract, sometimes called the alimentary canal) has both an entrance (the mouth) and an exit (the anus). Notice that the inner tube is separated from the outer tube (the body wall) by the coelom. Specialized organs that assist with digestion are located within the coelom. While you might think that the digestive tract of humans is very different from that of the earthworms, there are actually a number of important similarities. In all animals, including earthworms and humans, the digestion of food is an extracellular process. Digestive enzymes, produced by glands in the wall of the digestive tract or by accessory glands that lie nearby, are released into the tract. Food is never found within these accessory glands, only within the digestive tract itself.

Anterior Pituitary

The hypothalamus controls the anterior pituitary by producing hypothalamic-releasing hormones, most of which act by stimulating the action of other glands (Fig. 27.16). 1. Thyroid-stimulating hormone (TSH) stimulates the thyroid to produce triiodothyronine (T3) and thyroxine (T4). 2. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce the glucocorticoids. 3. Gonadotropic hormones (FSH and LH) stimulate the gonads—the testes in males and the ovaries in females—to produce gametes and sex hormones. The hypothalamic-releasing hormones are kept in balance by a three-tiered control system that uses negative-feedback mechanisms (Fig. 27.17). For example, the secretion of thyroid-releasing hormone (TRH) by the hypothalamus stimulates the thyroid to produce the thyroid-stimulating hormone (TSH), and the thyroid produces its hormones (T3 and T4), which gives feedback to inhibit the release of the first two hormones mentioned. Two other hormones produced by the anterior pituitary do not affect other endocrine glands. Prolactin (PRL) is produced in quantity during pregnancy and after childbirth. It causes the mammary glands in the breasts to develop and produce milk. It also plays a role in carbohydrate and fat metabolism. Growth hormone (GH) promotes skeletal and muscular growth. It stimulates the rate at which amino acids enter cells and protein synthesis occurs. Underproduction of growth hormone leads to pituitary dwarfism, and overproduction can lead to pituitary gigantism.

Hormonal Regulation in Males

The hypothalamus has ultimate control of the testes' sexual function, because it secretes a hormone called gonadotropin-releasing hormone, or GnRH, that stimulates the anterior pituitary to produce the gonadotropic hormones (see Section 27.2). Both males and females have two gonadotropic hormones—follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In males, FSH promotes spermatogenesis in the seminiferous tubules. LH controls the production of testosterone by the interstitial cells, which are scattered in the spaces between the seminiferous tubules. Testosterone, the main sex hormone in males, is essential for the normal development and functioning of the sexual organs. Testosterone is also necessary for the maturation of sperm. In addition, testosterone brings about and maintains the male secondary sex characteristics that develop at puberty, the time of life when sexual maturity is attained. Males are generally taller than females and have broader shoulders and longer legs relative to trunk length. The deeper voice of males compared with females is due to males' having a larger larynx with longer vocal cords. Because the Adam's apple is a part of the larynx, it is usually more prominent in males than in females. Testosterone causes males to develop noticeable hair on the face, chest, and occasionally other regions of the body, such as the back. Testosterone also leads to the receding hairline and contributes to the pattern baldness that frequently occurs in males. Testosterone is responsible for the greater muscular development in males.

Posterior Pituitary

The hypothalamus produces two hormones, antidiuretic hormone (ADH) and oxytocin (see Fig. 27.16). The axons of hypothalamic secretory neurons extend into the posterior pituitary, where hormones are stored in the axon terminals. When the hypothalamus determines that the blood is too concentrated, ADH is released from the posterior pituitary. On reaching the kidneys, ADH causes water to be reabsorbed. As the blood becomes dilute, ADH is no longer released. This is also an example of control by negative feedback, because the effect of the hormone (to dilute blood) is to shut down the hormone's release. Negative feedback, as discussed in Section 22.3, maintains homeostasis. Oxytocin, the other hormone made in the hypothalamus, causes uterine contraction during childbirth and milk letdown (ejection) when a baby is nursing.

Hypothalamus and Pituitary Gland

The hypothalamus, a part of the brain (see Fig. 27.8b), helps regulate the internal environment. For example, it is on the receiving end of information about the heartbeat and body temperature. And to correct any abnormalities, the hypothalamus communicates with the medulla oblongata, where the brain centers that control the autonomic system are located. The hypothalamus is also a part of the endocrine system, containing specialized hormone-secreting neurons. It controls the glandular secretions of the pituitary gland, a small gland connected to the brain by a stalklike structure. The pituitary has two portions, the anterior pituitary and the posterior pituitary, which are distinct from each other.

Cells of the Immune System

The immune system not only contains a network of lymphatic organs and lymphatic tissues but also includes a wide variety of cells (Table 26.1). These cells play a role in the immune system's ability to distinguish between the cells of our body (self) and pathogens in the body (nonself). Pathogens are identified by the presence of antigens. An antigen is any molecule, usually a protein or carbohydrate from a pathogen, that stimulates the immune system. By being able to distinguish between self and nonself, the cells of the immune system provide us with immunity. Immunity is the body's ability to repel foreign substances, pathogens, and cancer cells. There are different levels of immunity: nonspecific immunity indiscriminately repels pathogens, while specific (adaptive) immunity requires that a certain antigen be present.

Overview of the Immune System

The immune system plays an important role in keeping us healthy, because it fights infections and cancer. The immune system contains the lymphatic organs: the red bone marrow, thymus, lymph nodes, and spleen (Fig. 26.1). Lymphoid tissue may also be found in the tonsils and appendix.

Disorders of the Immune System

The immune system usually protects us from disease because it can distinguish self from nonself. Sometimes, however, it responds in a manner that harms the body, as when individuals develop allergies or have an autoimmune disease.

The Stages of Birth

The latest findings suggest that, when the fetal brain is sufficiently mature, the hypothalamus causes the pituitary to stimulate the adrenal cortex, so that androgens are released into the bloodstream. The placenta uses androgens as a precursor for estrogen, a hormone that stimulates the production of oxytocin and prostaglandin (a molecule produced by many cells that acts as a local hormone). All three of these molecules—estrogen, oxytocin, and prostaglandin—cause the uterus to contract and expel the fetus. The process of birth (parturition) has three stages (Fig. 29.18). During the first stage, the cervix dilates to allow passage of the baby's head and body. The amnion usually bursts about this time. During the second stage, the baby is born and the umbilical cord is cut. During the third stage, the placenta is delivered.

The Limbic System

The limbic system is a complex network that includes the diencephalon and areas of the cerebrum (Fig. 27.10). The limbic system blends higher mental functions and primitive emotions into a united whole. It accounts for why activities such as sexual behavior and eating seem pleasurable and why, for instance, mental stress can cause high blood pressure. Two significant structures within the limbic system are the hippocampus and the amygdala, which are essential for learning and memory. The hippocampus, a seahorse-shaped structure that lies deep in the temporal lobe, is well situated in the brain to make the prefrontal area aware of past experiences stored in sensory association areas. The amygdala, in particular, can cause these experiences to have emotional overtones. A connection between the frontal lobe and the limbic system means that reason can keep us from acting out strong feelings. Learning and Memory Memory is the ability to hold a thought in mind or to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives. Learning takes place when we retain and utilize past memories. The prefrontal area in the frontal lobe is active during short-term memory, as when we temporarily recall a telephone number. Some telephone numbers go into long-term memory. Think of a telephone number you know by heart, and see if you can bring it to mind without also thinking about the place or person associated with that number. Most likely, you cannot, because typically long-term memory is a mixture of what is called semantic memory (numbers, words, and so on) and episodic memory (persons, events, and other associations). Skill memory is a type of memory that can exist independently of episodic memory. Skill memory is what allows us to perform motor activities, such as riding a bike or playing ice hockey. A person who has Alzheimer disease experiences a progressive loss of memory, particularly for recent events. Gradually, the person loses the ability to perform any type of daily activity and becomes bedridden. In Alzheimer patients, clusters of abnormal tissue develop among degenerating neurons, especially in the hippocampus and amygdala. Major research efforts are devoted to seeking a cure for Alzheimer disease. What parts of the brain are functioning when we remember something from long ago? Our long-term memories are stored in bits and pieces throughout the sensory association areas of the cerebral cortex. The hippocampus gathers this information for use by the prefrontal area of the frontal lobe when we remember, for example, Uncle George or a past summer's vacation. Why are some memories so emotionally charged? The amygdala is responsible for conditioning fear and for associating danger with sensory information received from the thalamus and the cortical sensory areas.

Liver

The liver (see Fig. 25.8) has numerous functions, including the following: 1. Detoxifies the blood by removing and metabolizing poisonous substances 2. Produces plasma proteins, such as albumin and fibrinogen 3. Destroys old red blood cells and converts hemoglobin to the breakdown products in bile (bilirubin and biliverdin) 4. Produces bile, which is stored in the gallbladder before entering the small intestine, where it emulsifies fats 5. Stores glucose as glycogen and breaks down glycogen to glucose between meals to maintain a constant glucose concentration in the blood 6. Produces urea from amino acids and ammonia Blood vessels from the large and small intestines, as well as others, merge to form the hepatic portal vein, which leads to the liver (Fig. 25.12). The liver helps maintain the glucose concentration in blood at about 0.1% by removing excess glucose from the hepatic portal vein and storing it as glycogen. Between meals, glycogen (see Section 3.2) is broken down to glucose, and glucose enters the hepatic veins. If the supply of glycogen and glucose runs short, the liver converts amino acids to glucose molecules. Amino acids contain nitrogen in their amino groups, whereas glucose contains only carbon, oxygen, and hydrogen. Therefore, before amino acids can be converted to glucose molecules, deamination, the removal of amino groups from the amino acids, must occur. By a complex metabolic pathway, the liver converts the amino groups to urea, the most common nitrogenous (nitrogen-containing) waste product of humans. After urea is formed in the liver, it is transported in the bloodstream to the kidneys, where it is excreted. Liver Disorders When a person has jaundice, the skin has a yellowish tint due to an abnormally large quantity of bile pigments in the blood. In hemolytic jaundice, red blood cells are broken down in abnormally large amounts; in obstructive jaundice, the bile duct is obstructed or liver cells are damaged. Obstructive jaundice often occurs when crystals of cholesterol precipitate out of bile and form gallstones. Jaundice can also result from a viral infection of the liver, called hepatitis. Hepatitis A is most often caused by eating contaminated food. Hepatitis B and C are commonly spread by blood transfusions, kidney dialysis, or injection with an unsterilized needle. These three types of hepatitis can also be spread by sexual contact. Cirrhosis is a chronic liver disease in which the organ first becomes fatty, and then the liver tissue is replaced by inactive, fibrous scar tissue. Many alcoholics get cirrhosis, most likely due at least in part to the excessive amounts of alcohol the liver is forced to break down.

Lungs and External Exchange of Gases

The lungs of humans and other mammals are more elaborately subdivided than those of amphibians and reptiles. Frogs and salamanders have a moist skin that allows them to use the surface of their body for gas exchange in addition to the lungs. It has been estimated that human lungs have a total surface area at least 50 times the skin's surface area because of the presence of alveoli. An alveolus (Fig. 24.8), like the capillary that surrounds it, is bounded by squamous epithelium. Diffusion alone accounts for gas exchange between the alveolus and the capillary. Carbon dioxide, being more plentiful in the pulmonary vein, diffuses from a pulmonary capillary to enter an alveolus, while oxygen, being more plentiful in the lungs, diffuses from an alveolus into a pulmonary capillary. The process of diffusion requires a gas-exchange region that is not only large but also moist and thin. The alveoli are lined with surfactant, a film of lipoprotein that lowers the surface tension of water, thereby preventing the sides of the alveoli from sticking together during exhalation. Some newborns, especially if premature, lack this film. Surfactant replacement therapy is used to treat this condition. The blood within pulmonary capillaries is indeed spread thin, and the red blood cells are pressed up against their narrow walls. The alveolar epithelium and the capillary epithelium are so close that together they are called the respiratory membrane. Hemoglobin in the red blood cells quickly picks up oxygen molecules as they diffuse into the blood. Emphysema is a serious lung condition in which the walls of many alveoli have been destroyed. This greatly reduces the surface area for gas exchange to occur. Individuals with emphysema are unable to supply their cells with enough oxygen to conduct cellular respiration, and therefore usually the individual has very low energy levels. Emphysema is nonreversible and usually fatal, although individuals with early emphysema can use supplemental oxygen to increase oxygen delivery to their cells.

Nervous System

The nervous system and the endocrine system work together to regulate the activities of the other systems. Both control systems use chemical signals when they respond to changes that might threaten homeostasis, but they have different means of delivering these signals (Fig. 27.1). The nervous system quickly sends a message along a nerve fiber directly to a target organ or tissue, such as skeletal or smooth muscle. Once a chemical signal is released, the muscle brings about an appropriate response. The endocrine system uses the blood vessels of the cardiovascular system to send hormones as chemical messengers to a target organ, such as the liver. The endocrine system is slower-acting because it takes time for the hormone molecules to move through the bloodstream to the target organ. Also, hormones change the metabolism of cells, and this takes time; however, the response is longer-lasting. Cellular metabolism tends to remain the same for at least a limited period of time. As we examine the human nervous system, we will also compare it with the nervous systems of other animals.

The Central Nervous System

The organizations of vertebrate brains in reptiles (alligator), birds (goose) and mammals (horse and human) are compared in Figure 27.8. If we divide the brain into a hindbrain, midbrain, and forebrain, we can see that the forebrain is most prominent in humans. The forebrain of mammals also has an altered function in that it becomes the last depository for sensory information. This change accounts for why the forebrain carries on much of the integration for the entire nervous system before it sends out motor instructions to glands and muscles. In humans, the spinal cord provides a means of communication between the brain and the spinal nerves, which are a part of the PNS. Spinal nerves leave the spinal cord and take messages to and from the skin, glands, and muscles in all areas of the body, except the head and face. Long, myelinated fibers of interneurons in the spinal cord run together in bundles called tracts. These tracts connect the spinal cord to the brain. Because these tracts cross over, the left side of the brain controls the right side of the body, and vice versa. In our discussion of the peripheral nervous system, we will see that the spinal cord is involved in reflex actions, which are programmed, built-in circuits that allow for protection and survival. They are present at birth and require no conscious thought to take place.

Pancreas

The pancreas (see Fig. 25.8) functions as both an endocrine gland and an exocrine gland. Endocrine glands are ductless and secrete their products into the blood. The pancreas acts as an endocrine gland when it produces and secretes the hormones insulin and glucagon into the bloodstream. These hormones are involved in the regulation of blood glucose levels (see Section 27.2). Exocrine glands secrete their products into ducts. The pancreas is an exocrine gland when it produces and secretes pancreatic juice (see the content on "Digestive Enzymes" below) into the duodenum of the small intestine through the common bile duct.

Pancreas

The pancreas is composed of two types of tissue. Exocrine tissue produces and secretes digestive juices that pass through ducts to the small intestine. Endocrine tissue, called the pancreatic islets (islets of Langerhans), produces and secretes the hormones insulin and glucagon directly into the blood. Insulin is secreted when there is a high blood glucose level, which usually occurs just after eating. Insulin stimulates the uptake of glucose by cells, especially liver cells, muscle cells, and adipose tissue cells. In liver and muscle cells, glucose is then stored as glycogen. In muscle cells, glucose supplies energy for ATP production, leading to protein metabolism and muscle contraction. In fat cells, the breakdown of glucose supplies glycerol and acetyl groups for the formation of fat. In these ways, insulin lowers the blood glucose level. Glucagon is usually secreted between meals, when the blood glucose level is low. The major target tissues of glucagon are the liver and adipose tissue. Glucagon stimulates the liver to break down glycogen to glucose and to use fat and protein in preference to glucose as energy sources. The use of fat and protein spares glucose and makes more available to enter the blood. In these ways, glucagon raises the blood glucose level.

Placenta

The placenta has a fetal side contributed by the chorion, the outermost extraembryonic membrane, and a maternal side consisting of uterine tissues. Notice in Figure 29.16 that the chorion has treelike projections, called chorionic villi. The chorionic villi are surrounded by maternal blood, yet maternal and fetal blood never mix, because exchange always takes place across the walls of the villi. Carbon dioxide and other wastes move from the fetal side to the maternal side of the placenta, and nutrients and oxygen move from the maternal side to the fetal side. The umbilical cord stretches between the placenta and the fetus. The umbilical blood vessels are an extension of the fetal circulatory system and simply take fetal blood to and from the placenta. Harmful chemicals can also cross the placenta. This is of particular concern during the embryonic period, when various structures are first forming. Each organ or part seems to have a sensitive period, during which a substance can alter its normal development. For example, if a woman takes the drug thalidomide, a tranquilizer, between days 27 and 40 of her pregnancy, the infant is likely to be born with deformed limbs. After day 40, however, the limbs will develop normally.

Why a muscle shortens when it contracts.

The presence of calcium (Ca2+) sets in motion a chain of events (1-3) that causes myosin heads to attach to and pull an actin filament toward the center of a sarcomere. After binding to other ATP molecules, myosin heads return to their resting position. Then, the chain of events (1-3) occurs again, except that the myosin heads reattach farther along the actin filament.

The Peripheral Nervous System

The purpose of the peripheral nervous system (PNS) is to relay sensory information to the CNS for processing and to relay motor responses to the tissues of the body. Some of this information comes from sensory neurons located within the sense organs (Section 28.1), while other information comes from sensory neurons within the tissues of the body. The peripheral nervous system (PNS) lies outside the central nervous system and contains nerves, which are bundles of axons. The cell bodies of neurons are found in the CNS—that is, the brain and spinal cord—or in ganglia. Ganglia (sing., ganglion) are collections of cell bodies within the PNS. Humans have 12 pairs of cranial nerves that arise from the brain (Fig. 27.11). Cranial nerves are largely concerned with the head, neck, and facial regions of the body. However, the vagus nerve is a cranial nerve that has branches not only to the pharynx and larynx but also to most of the internal organs. Humans also have 31 pairs of spinal nerves, and each of these contains many sensory and motor axons. The dorsal root of a spinal nerve contains the axons of sensory neurons, which conduct impulses to the spinal cord from sensory receptors. The cell body of a sensory neuron is in the dorsal root ganglion. The ventral root contains the axons of motor neurons, which conduct impulses away from the cord, largely to skeletal muscles. Each spinal nerve serves the region of the body in which it is located.

Thymus

The soft, bilobed thymus varies in size, but it is larger in children and shrinks as we get older. The thymus plays a role in the maturing of T lymphocytes. Immature T lymphocytes migrate from the bone marrow through the bloodstream to the thymus, where they develop, or "mature," into functioning T lymphocytes. Only about 5% of T lymphocytes ever leave the thymus. These T lymphocytes have survived a critical test: If any show the ability to react with the cells of our own body, or "self," they die. If they have the potential to attack a foreign cell, they leave the thymus and enter lymphatic vessels and organs.

The Somatic System

The somatic system of the PNS includes the nerves that take information about external stimuli from sensory receptors to the CNS and motor commands away from the CNS to skeletal muscles. Voluntary control of skeletal muscles always originates in the brain. Involuntary responses to stimuli, called reflexes, can involve either the brain or just the spinal cord. Flying objects cause our eyes to blink, and sharp pins cause our hands to jerk away even without our having to think about it. Figure 27.12 illustrates the path of a reflex that involves only the spinal cord. If your hand touches a sharp pin, sensory receptors in the skin generate nerve impulses that move along sensory axons toward the spinal cord. Sensory neurons that enter the spinal cord pass signals on to many interneurons. Some of these interneurons synapse with motor neurons. The short dendrites and the cell bodies of motor neurons are in the spinal cord, but their axons leave the cord. Nerve impulses travel along motor axons to an effector, which brings about a response to the stimulus. In this case, a muscle contracts, so that you withdraw your hand from the pin. Various other reactions are possible—you will most likely look at the pin, wince, and cry out in pain. This whole series of responses is explained by the fact that some of the interneurons involved carry nerve impulses to the brain. Also, sense organs send messages to the brain that make us aware of our actions. Your brain makes you aware of the stimulus and directs these other reactions to it.

Spleen

The spleen, which is about the size of a fist, is in the upper left abdominal cavity. The spleen's unique function is to filter the blood. This soft, spongy organ contains tissue called red pulp and white pulp. Blood passing through the many sinuses in the red pulp is filtered of pathogens and debris, including worn-out red blood cells, because the sinuses are lined by macrophages. The white pulp contains lymphocytes, which are actively engaged in fighting infections and cancer. The spleen's outer capsule is relatively thin, and an infection or a severe blow can cause the spleen to burst. The spleen's functions can be fulfilled by other organs, but individuals without a spleen are often slightly more susceptible to infections. As a result, they will receive certain vaccinations and may have to receive antibiotic therapy indefinitely.

Neurons

The structure of a nerve cell, or neuron, is well suited to its function as the primary cell of the nervous system (Fig. 27.3). The cell body contains the nucleus and other organelles that allow a cell to function. The neuron's many short dendrites fan out to receive signals from sensory receptors or other neurons. These signals can result in nerve impulses carried by an axon. The axon, an extension of the neuron that is typically longer than a dendrite, is the location in the neuron that is responsible for conducting nerve impulses to their targets. Axons can deliver nerve impulses great distances, for example, there are axons that extend from your toes to your spinal cord. Long axons are covered by a white myelin sheath formed from the membranes of tightly spiraled cells that leave gaps called nodes of Ranvier. The axons of neurons are often organized as nerves, which frequently appear white due to the myelin sheaths. The nervous system has three types of neurons specific to its three functions (Fig. 27.3): 1. The nervous system receives sensory input. Sensory neurons perform this function. They take nerve impulses from sensory receptors to the CNS. The sensory receptor, which is the distal end of the axon of a sensory neuron, may be as simple as a naked nerve ending (such as a pain receptor), or it may be built into a highly complex organ, such as the eye or ear. In any case, the axon of a sensory neuron can be quite long if the sensory receptor is far from the CNS. 2. The nervous system performs integration—in other words, the CNS sums up the input it receives from all over the body. Interneurons occur entirely within the CNS and take nerve impulses between its various parts. Some interneurons lie between sensory neurons and motor neurons, and some take messages from one side of the spinal cord to the other or from the brain to the spinal cord, and vice versa. Interneurons also form complex pathways in the brain, where processes that account for thinking, memory, and language occur. 3. The nervous system generates motor output. Motor neurons take nerve impulses from the CNS to muscles or glands. Motor neurons cause muscle fibers to contract or glands to secrete, and therefore they are said to innervate these structures.

Calcium Regulation

The thyroid gland also produces calcitonin, a hormone that helps regulate the blood calcium level. Calcium (Ca2+) plays a significant role in both nervous conduction and muscle contraction. It is also necessary for blood clotting. Calcitonin temporarily reduces the activity and number of osteoclasts, cells that break down bone. Therefore, more calcium is deposited in bone. When the blood calcium level returns to normal, the thyroid's release of calcitonin is inhibited by negative feedback. However, a low level of blood calcium stimulates the parathyroid glands' release of parathyroid hormone (PTH). The parathyroid glands are embedded in the posterior surface of the thyroid gland. Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Parathyroid hormone promotes the activity of osteoclasts and the release of calcium from the bones. PTH also promotes the reabsorption of calcium by the kidneys, where it activates vitamin D. Vitamin D, in turn, stimulates the absorption of calcium from the small intestine. These effects bring the blood calcium level back to the normal range, so that the parathyroid glands no longer secrete PTH. When insufficient parathyroid hormone production leads to a dramatic drop in the blood calcium level, tetany results. In tetany, the body shakes from continuous muscle contraction. This effect is brought about by increased excitability of the nerves, which in turn initiates nerve impulses spontaneously and without rest.

Thyroid and Parathyroid Glands

The thyroid gland is a large gland located in the neck. It produces the hormones calcitonin, triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four iodine atoms. As we will see below, calcitonin is involved in calcium homeostasis. Triiodothyronine and thyroxine are produced by the thyroid gland using iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that in the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by the anterior pituitary, the thyroid enlarges, resulting in an endemic goiter (Fig. 27.18a). Some years ago, it was discovered that the use of iodized salt (table salt to which iodine has been added) helps prevent endemic goiter. Thyroid hormones increase the metabolic rate. They do not have a single, specific target organ; instead, they stimulate all the cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is used. In the case of hyperthyroidism (oversecretion of thyroid hormone), or Graves disease, the thyroid gland is overactive, and bulging of the eyes, called exophthalmos, results (Fig. 27.18b). The eyes protrude because of swelling in the eye socket tissues and in the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. The removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition.

Adrenal Cortex

The two major types of hormones produced by the adrenal cortex are the mineralocorticoids, such as aldosterone, and the glucocorticoids, such as cortisol. Aldosterone acts on the kidneys and thereby regulates salt and water balance, leading to increases in blood volume and blood pressure. Cortisol regulates carbohydrate, protein, and fat metabolism, leading to an increase in the blood glucose level. It is also an anti-inflammatory agent. The adrenal cortex also secretes small amounts of both male and female sex hormones in both sexes. When the level of adrenal cortex hormones is low due to hyposecretion, a person develops Addison disease (Fig. 27.19). ACTH may build up as more is secreted to attempt to stimulate the adrenal cortex. The excess can cause bronzing of the skin, because ACTH in excess stimulates melanin production. Without cortisol, glucose cannot be replenished when a stressful situation arises. Even a mild infection can lead to death. The lack of aldosterone results in the loss of sodium and water by the kidneys, low blood pressure, and possibly severe dehydration. Left untreated, Addison disease can be fatal. When the level of adrenal cortex hormones is high due to hypersecretion, a person develops Cushing syndrome. The excess cortisol results in a tendency toward diabetes mellitus as muscle protein is metabolized and subcutaneous fat is deposited in the midsection. Excess production of adrenal male sex hormones in women may result in masculinization, including an increase in body hair, deepening of the voice, and beard growth. An excess of aldosterone and reabsorption of sodium and water by the kidneys lead to a basic blood pH and hypertension. The face swells and takes on a moon shape. Masculinization may occur in women because of excess adrenal male sex hormones.

Large Intestine

The word bowel technically means the part of the digestive tract between the stomach and the anus, but it is sometimes used to mean only the large intestine. The large intestine (also called the colon) absorbs water, salts, and some vitamins. It also stores indigestible material until it is eliminated at the anus. The large intestine takes its name from its diameter rather than its length, which is shorter than that of the small intestine. The large intestine has a blind pouch, the cecum, below the entry of the small intestine, with a small projection containing lymphatic tissue called the appendix. In humans, the appendix may play a role in fighting infections by acting as a reservoir of beneficial bacteria. In the condition called appendicitis, the appendix becomes infected and so filled with fluid that it may burst. If an infected appendix bursts before it can be removed, a serious, generalized infection of the abdominal lining, called peritonitis, may result. The large intestine has a large population of bacteria, notably Escherichia coli. The bacteria break down indigestible material, and they produce some vitamins, including vitamin K. Vitamin K is necessary for blood clotting. Digestive wastes (feces) eventually leave the body through the anus, the opening of the anal canal. Feces are about 75% water and 25% solid matter. Almost one-third of this solid matter is made up of intestinal bacteria. The remainder is indigestible plant material (also called fiber), fats, waste products (such as bile pigments), inorganic material, mucus, and dead cells from the intestinal lining. A diet that includes fiber adds bulk to the feces, improves the regularity of elimination, and prevents constipation. About 1.5 liters of water enter the digestive tract daily as a result of eating and drinking. An additional 8.5 liters also enter the digestive tract each day, carrying the various substances secreted by the digestive glands. About 95% of this water is absorbed by the small intestine, and much of the remaining portion is absorbed by the large intestine. If this water is not reabsorbed, diarrhea can occur, leading to serious dehydration and ion loss, especially in children. Because the cells of the large intestine have a longer exposure to chemicals in our foods, this area is more subject to the development of polyps, which are small growths arising from the mucosa. Polyps, whether benign or cancerous, can be removed surgically.

Adrenal Glands

Two adrenal glands sit atop the kidneys. Each adrenal gland consists of an inner portion called the adrenal medulla and an outer portion called the adrenal cortex. These portions, like the anterior pituitary and the posterior pituitary, have no functional connection with one another. The hypothalamus exerts control over the activity of both portions of the adrenal glands. It initiates nerve impulses that travel by way of the brain stem, spinal cord, and sympathetic nerve fibers to the adrenal medulla, which then secretes its hormones. The hypothalamus, by means of ACTH-releasing hormone, controls the anterior pituitary's secretion of ACTH, which in turn stimulates the adrenal cortex. Stress of all types—including emotional and physical trauma, and even vigorous exercise—prompts the hypothalamus to stimulate both the adrenal medulla and the adrenal cortex.

What are varicose veins?

Veins are thin-walled tubes, divided into many separate chambers by vein valves. Excessive stretching occurs if veins are overfilled with blood. For example, if a person stands in one place for a long time, leg veins can't drain properly and blood pools in them. As the vein expands, vein valves become distended and fail to function. These two mechanisms cause the veins to bulge and be visible on the skin's surface. Hemorrhoids are varicose veins in the rectum. Obesity, a sedentary lifestyle, female gender, genetic predisposition, and increasing age are risk factors for varicose veins.

Water

Water constitutes about 60% of an adult's body. Water participates in many chemical reactions; in addition, watery fluids lubricate joints, transport other nutrients, and help maintain body temperature. Beverages, soups, fruits, and vegetables are sources of water, and most solid foods contain some water. The amount of totalwater (water from beverages and foods) that you need to consume depends on your physical activity level, your diet, and environmental conditions. On average, men should consume about 125 ounces (oz) and women should consume about 90 oz of total water each day. Thirst is a healthy person's best guide for meeting water needs and avoiding dehydration. Too much water can also be a problem. In water toxication (hyponatremia), individuals who consume excessive amounts of water upset the balance of electrolytes, usually sodium and potassium, in their blood. This can lead to irregular heartbeat, and in some cases, death.

T Cells and the Cellular Response

When T cells leave the thymus, they have unique T-cell receptors (TCRs), just as B cells do. Unlike B cells, however, T cells are unable to recognize an antigen without help. The antigen must be presented to them by an antigen-presenting cell (APC), such as a macrophage. A macrophage becomes an APC by ingesting and destroying a pathogen. The APC then travels to a lymph node or the spleen, where T cells also congregate. When a macrophage phagocytizes a virus, it is digested in a lysosome. An antigen from the virus is combined with a protein called a major histocompatability complex (MHC). MHC proteins play a major role in identifying self cells. After the antigen binds to the MHC protein, the complex appears on the cell surface. Then the combined MHC + antigen complex is presented to a T cell. The importance of self proteins in plasma membranes was first recognized when it was discovered that they contribute to the specificity of tissues and make it difficult to transplant tissue from one human to another. The general characteristics of T cells are provided in Table 26.4. The two main types of T cells are helper T cells (TH cells) and cytotoxic T cells (TC cells). Each of these types has a TCR that can recognize an antigen fragment in combination with an MHC molecule. However, the major difference in antigen recognition by these two types of cells is that the TH cells recognize only antigens presented by APCs with MHC class II molecules on their surface, while TCcells recognize only antigens presented by APCs with MHC class I molecules on their surface. In Figure 26.7, the antigen is represented by a red triangle, and the helper T cell that binds to the antigen has the specific TCR that can combine with this particular MHC II + antigen. Now the helper T cell is activated and divides to produce more helper T cells. Characteristics of T Cells - They provide a cellular response to virus-infected cells and cancer cells. - They are produced in bone marrow and mature in the thymus. - Antigen must be presented to a T cell in groove of an MHC¹ protein. - Cytotoxic T cells destroy nonself antigen-bearing cells. - Helper T cells secrete cytokines that control the immune response. ¹MHC = major histocompatibility complex. As an illness disappears, the immune reaction wanes and the activated T cells become susceptible to apoptosis. Apoptosis contributes to homeostasis by regulating the number of cells present in an organ, or in this case, in the immune system. When apoptosis does not occur as it should, T-cell cancers (e.g., lymphomas and leukemias) can result. Also, in the thymus gland, any T cell that has the potential to destroy the body's own cells undergoes apoptosis.

Structure of a Bone

When a long bone is split open, as in Figure 28.15, a cavity is revealed that is bounded on the sides by compact bone. Compact bone contains many osteons(Haversian systems), where bone cells called osteocytes lie in tiny chambers arranged in concentric circles around central canals. The cells are separated by an extracellular matrix, which contains mineral deposits—primarily, calcium and phosphorus salts. Two other cell types are constantly at work in bones. Osteoblastsdeposit bone, and osteoclasts secrete enzymes that digest the matrix of bone and release calcium into the bloodstream. When a person has osteoporosis (weak bones subject to fracture), osteoclasts are working harder than osteoblasts. Intake of high levels of dietary calcium, especially when a person is young and more active, encourages denser bones and lessens the chance of getting osteoporosis later in life. A bone has a central cavity that is usually filled with yellow bone marrow. Both spongy bone and compact bone are living tissues composed of bone cells within a matrix that contains calcium. The spaces of spongy bone contain red bone marrow. A long bone has spongy bone at each end. Spongy bone has numerous bony bars and plates separated by irregular spaces. The spaces are often filled with red bone marrow, a specialized tissue that produces red blood cells. The cavity of a long bone is filled with yellow bone marrow and stores fat. Beyond the spongy bone are a thin shell of compact bone and a layer of hyaline cartilage, which is important to healthy joints.

The Inflammatory Response

When an injury is not serious, the inflammatory response is short-lived and the healing process quickly returns the affected area to a normal state. Nearby cells secrete chemical factors to ensure the growth (and repair) of blood vessels and new cells to fill in the damaged area. If, on the other hand, the neutrophils are overwhelmed, they call for reinforcements by secreting chemical mediators called cytokines. Cytokines attract more white blood cells to the area, including monocytes. Monocytes are longer-lived cells that become macrophages, even more powerful phagocytes than neutrophils. Macrophages can enlist the help of lymphocytes to carry out specific defense mechanisms. Inflammation is the body's natural response to an irritation or injury and serves an important role. Once the healing process has begun, inflammation rapidly subsides. However, in some cases, chronic inflammation lasts for weeks, months, or even years if an irritation or infection cannot be overcome. Inflammatory chemicals may cause collateral damage to the body, in addition to killing the invaders. Should an inflammation persist, anti-inflammatory medications, such as aspirin, ibuprofen, or cortisone, can minimize the effects of various chemical mediators.The inflammatory response plays an important role in the defense against invasion by pathogens. Inflammation employs mainly neutrophils and macrophages to surround and kill (engulf by phagocytosis) pathogens trying to get a foothold inside the body. Protective proteins are also involved. Inflammation is usually recognized by its four hallmark symptoms: redness, heat, swelling, and pain. The four signs of the inflammatory response are due to capillary changes in the damaged area, and all serve to protect the body. Chemical mediators, such as histamine, released by damaged tissue cells and mast cells, cause the capillaries to dilate and become more permeable. Excess blood flow due to enlarged capillaries causes the skin to redden and become warm. Increased temperature in an inflamed area tends to inhibit the growth of some pathogens. Increased blood flow brings white blood cells to the area. Increased permeability of capillaries allows fluids and proteins, including blood-clotting factors, to escape into the tissues. Clot formation in the injured area prevents blood loss. The excess fluid in the area presses on nerve endings, causing the familiar pain associated with swelling. Together, these events summon white blood cells to the area. As soon as the white blood cells arrive, they move out of the bloodstream into the surrounding tissue. The neutrophils are first and actively phagocytize debris, dead cells, and bacteria they encounter. The many neutrophils attracted to the area can usually localize any infection and keep it from spreading. If neutrophils die off in great quantity, they become a yellow-white substance called pus. When an injury is not serious, the inflammatory response is short-lived and the healing process quickly returns the affected area to a normal state. Nearby cells secrete chemical factors to ensure the growth (and repair) of blood vessels and new cells to fill in the damaged area. If, on the other hand, the neutrophils are overwhelmed, they call for reinforcements by secreting chemical mediators called cytokines. Cytokines attract more white blood cells to the area, including monocytes. Monocytes are longer-lived cells that become macrophages, even more powerful phagocytes than neutrophils. Macrophages can enlist the help of lymphocytes to carry out specific defense mechanisms. Page 501Inflammation is the body's natural response to an irritation or injury and serves an important role. Once the healing process has begun, inflammation rapidly subsides. However, in some cases, chronic inflammation lasts for weeks, months, or even years if an irritation or infection cannot be overcome. Inflammatory chemicals may cause collateral damage to the body, in addition to killing the invaders. Should an inflammation persist, anti-inflammatory medications, such as aspirin, ibuprofen, or cortisone, can minimize the effects of various chemical mediators.

Autoimmune Diseases

When cytotoxic T cells or antibodies mistakenly attack the body's own cells as if they bore antigens, the resulting condition is known as an autoimmune disease.Exactly what causes autoimmune diseases is not known. However, sometimes they occur after an individual has recovered from an infection. In the autoimmune disease myasthenia gravis, neuromuscular junctions do not work properly, and muscular weakness results. In multiple sclerosis (MS), the myelin sheath of nerve fibers breaks down, causing various neuromuscular disorders. A person with systemic lupus erythematosus has various symptoms prior to death due to kidney damage. In rheumatoid arthritis, the joints are affected (Fig. 26.11). Researchers suggest that rheumatic fever and type 1 diabetes are autoimmune illnesses. As yet, there are no cures for autoimmune diseases, but they can be controlled with drugs.

The Human Eye

When looking straight ahead, each of our eyes views the same object from a slightly different angle. This slight displacement of the images permits binocular vision, the ability to perceive three-dimensional images and to sense depth. Like the human ear, the human eye has numerous parts, many of which are involved in preparing the stimulus for the sensory receptors. In the case of the ear, sound wave energy is magnified before it reaches the sensory receptors. In the case of the eye, light rays are brought to a focus on the photoreceptors within the retina. As shown in Figure 28.8, the cornea and especially the lens are involved in focusing light rays on the photoreceptors. The iris, the colored part of the eye, regulates the amount of light that enters the eye by way of the pupil. The retina generates nerve impulses, which are sent to the visual part of the cerebral cortex; from this information, the brain forms an image of the object. The shape of the lens is controlled by the ciliary muscles. When we view a distant object, the lens remains relatively flat, but when we view a near object, the lens rounds up. With normal aging, the lens loses its ability to accommodate for near objects; therefore, many people need reading glasses when they reach middle age. Aging, and exposure to UV rays from the sun, also makes the lens subject to cataracts; the lens becomes opaque, incapable of transmitting light rays. Currently, surgery is the only viable treatment for cataracts.

Action of muscles.

When muscles contract, they shorten. Therefore, muscles only pull—they cannot push. This limitation means that they need to work in antagonistic pairs; each member of the pair pulls on a bone in the opposite direction. For example, (a) when the biceps contracts, the forearm flexes (raises), and (b) when the triceps contracts, the forearm extends (lowers).

Specific Defenses and Adaptive Immunity

When nonspecific defenses have failed to prevent an infection, specific defenses, or adaptive immunity, come into play. Specific defenses respond to antigens, which may be components of a pathogen or cancer cell. An antigen acts as a "marker" that a pathogen may be present in the body. If the marker is detected by the immune system, the adaptive immune responses begin to actively look for cells that possess the antigen, such as bacterial cells, viruses, or even the cells of our own body that may be infected by the pathogen. When the body learns to destroy a particular antigen, it develops immunity to that pathogen. Because our immune system does not ordinarily respond to the proteins on the surface of our own cells (as if they were antigens), the immune system is able to distinguish self from nonself. Only in this way can the immune system aid, rather than disrupt, homeostasis. Lymphocytes are capable of recognizing antigens because their plasma membranes have receptor proteins whose shapes allow them to combine with specific antigens. Because we encounter millions of different antigens during our lifetime, we need a great diversity of lymphocytes to protect us against them. Remarkably, diversification occurs to such an extent during the maturation process that a lymphocyte type potentially exists for any possible antigen. Immunity usually lasts for some time. For example, once we recover from the measles, we usually do not get the illness a second time. Immunity is primarily the result of the action of the B lymphocytes and T lymphocytes. B lymphocytes mature in the bone marrow, and T lymphocytes mature in the thymus. B lymphocytes (also called B cells) give rise to plasma cells, which produce antibodies. These antibodies are secreted into the blood, lymph, and other body fluids. In contrast, T lymphocytes (also called T cells) do not produce antibodies. Some T lymphocytes regulate the immune response, and other T lymphocytes directly attack cells that bear antigens (see Table 26.1).

Skeletal Muscle Contraction

When skeletal muscle fibers contract, they shorten. Let's look at details of the process, beginning with the motor axon (Fig. 28.17). When nerve impulses travel down a motor axon and arrive at an axon terminal, synaptic vesicles release the neurotransmitter acetylcholine (ACh) into a synaptic cleft. ACh quickly diffuses across the cleft and binds to receptors in the plasma membrane of a muscle fiber, called the sarcolemma. The sarcolemma generates impulses, which travel along its T tubules to the endoplasmic reticulum, which is called the sarcoplasmic reticulum in muscle fibers. The release of calcium from calcium storage sites causes muscle fibers to contract. A muscle fiber contains many myofibrils divided into sarcomeres, which are contractile. When innervated by a motor neuron, the myofibrils contract and the sarcomeres shorten because actin filaments slide past the myosin filaments. The contractile portions of a muscle fiber are many parallel, threadlike myofibrils (Fig. 28.17). An electron microscope shows that myofibrils (and therefore skeletal muscle fibers) are striated because of the placement of protein filaments within contractile units called sarcomeres. A sarcomere extends between two dark lines called Z lines. Sarcomeres contain thick filaments made up of the protein myosin and thin filaments made up of the protein actin. As a muscle fiber contracts, the sarcomeres within the myofibrils shorten because actin (thin) filaments slide past the myosin (thick) filaments and approach one another. The movement of actin filaments in relation to myosin filaments is called the sliding filament model of muscle contraction. During the sliding process, the sarcomere shortens, even though the filaments themselves remain the same length. Why the Filaments Slide The thick filament is a bundle of myosin molecules, each having a globular head with the capability of attaching to the actin filament when calcium (Ca2+) is present. First, myosin binds to and hydrolyzes ATP. The energized myosin head attaches to an actin filament. The release of ADP and phosphorus causes myosin to shift its position and pull the actin filament to the center of the sarcomere. The action is similar to the movement of your hand when you flex your forearm. In the presence of another ATP, a myosin head detaches from actin. Now the heads attach farther along the actin filament. The cycle occurs again and again, and the actin filaments move nearer and nearer the center of the sarcomere each time the cycle is repeated. Contraction continues until nerve impulses cease and calcium ions are returned to their storage sites. The membranes of the sarcoplasmic reticulum contain active transport proteins that pump calcium ions back into the calcium storage sites, and the muscle relaxes. When a person or an animal dies, ATP production ceases. Without ATP, the myosin heads cannot detach from actin, nor can calcium be pumped back into the sarcoplasmic reticulum. As a result, the muscles remain contracted, a phenomenon called rigor mortis.

Does "0% trans fat" on a food label really mean that the item is free of trans fat?

When the Food and Drug Administration required that trans fat information be added to food labels in 2006, many food companies created labels touting their products as "trans fat free." A check of the label details would reveal 0 grams (0 g) of trans fat listed under "Nutrition Facts." But a more thorough check of the list of ingredients might reveal some trans fats lurking in the food. If you see "partially hydrogenated oil" listed with the ingredients, there are some trans fats in the food. Trans fats have to be listed with the breakdown of fat grams only when they constitute 0.5 g or more per serving. Limiting trans fats to 1% of daily calories is recommended by the American Heart Association. Unfortunately, eating more than one serving of a food with "hidden" trans fats might push some people over the recommended daily intake of trans fats.

Energy Intake Versus Energy Output

While genetics and physiological factors such as the types of bacteria in the large intestine are known to contribute to being overweight, a person cannot become fat without taking in more food energy (calories) than are expended. Energy Intake The energy value of food is often reported in kilocalories (kcal). A kilocalorie is the amount of heat that raises the temperature of a liter of water by 1°C. For practical purposes, you can estimate a food's caloric value if you know how many grams of carbohydrate, fat, protein, and alcohol it contains. Each gram (g) of carbohydrate or protein supplies 4 kcal, and each gram of fat supplies 9 kcal. Although alcohol is not a nutrient, it is considered a food, and each gram supplies 7 kcal. Therefore, if a serving of food contains 30 g of carbohydrate, 9 g of fat, and 5 g of protein, it supplies 221 kcal: carb 30 g x 4 kcal = 120 kcal fat 9 g x 9 kcal = 81 kcal protein 5 g x 4 kcal = 20 kcal Total = 221 kcal Energy Output The body expends energy primarily for (1) metabolic functions; (2) physical activity; and (3) digestion, absorption, and processing of nutrients from food. Scientists can assess a person's energy expenditure for a particular physical activity by measuring oxygen intake and carbon dioxide output during performance of the activity For practical purposes, here's how to estimate your daily energy needs: 1. Kcal needed daily for metabolic functions: Multiply your weight in kilograms (weight in pounds divided by 2.2) times 1.0 if you are a man, and times 0.9 if you are a woman. Then multiply that number by 24. Example: Meghan, a woman who weighs 130 pounds (about 59 kg), would calculate her daily caloric need for metabolic functions as follows: 0.9 kcal × 59 kg × 24 hours = approximately 1,274 kcal/day 2. Kcal needed daily for physical activity: Choose a multiplication factor from one of the follow categories: - Sedentary (little or no physical activity) = 0.20 to 0.40 - Light (walk daily) = 0.55 to 0.65 - Moderate (daily vigorous exercise) = 0.70 to 0.75 - Heavy (physical labor/endurance training) = 0.80 to 1.20 Multiply this factor times the kcal value you obtained in step 1. Example: Meghan performs light physical activity daily. She multiplies 0.55 × 1,274 kcal to determine her daily caloric need for physical activity, which is 701 kcal. 3. Kcal needed for digestion, absorption, and processing of nutrients: Multiply the total kcal from steps 1 and 2 by 0.1, and add that value to the total kcal from steps 1 and 2 to get your total daily energy needs. Example: Meghan adds 1,274 and 701 and then multiplies 1,975 kcal by 0.1 and adds that value (197.5) to 1,975 to obtain her total daily energy needs of 2,172 kcal. Therefore, Meghan will maintain her weight of 130 pounds if she continues to consume about 2,170 kcal a day and to perform light physical activities.

Examples of Nervous Systems

While many organisms have mechanisms that enable them to respond to factors in the environment, the presence of a nervous system is a characteristic that is unique to animals. Most animals (except the parazoans—see Section 19.2) utilize a nervous system to detect stimuli in the evironment and then perform coordinated reactions in response to the stimuli. For example, animals may use their nervous system to detect chemical signals that allow them to move toward a food source or away from a predator. While there are varying levels of complexity in animal nervous systems, they all receive sensory input, which is processed to direct a coordinated reaction. An early example of a nervous system is found in the planarian, a bilateral organism with a simple body plan (see Section 19.3). Planarians possess two lateral nerve cords (bundles of nerves) joined together by transverse nerves. The arrangement is called a ladderlike nervous system. The simple brain receives sensory information from the eyespots and sensory cells in the auricles. The two lateral nerve cords allow a rapid transfer of information from the cerebral ganglia to the posterior end, and the transverse nerves between the nerve cords keep the movement of the two sides coordinated. The nervous organization in planarians is a foreshadowing of the central and peripheral nervous systems found in more complex invertebrates, such as earthworms, and in vertebrates, including humans (Fig. 27.2).

Synovial joints.

a. The synovial joints of the human skeleton allow the body to be flexible and move with precision even when bearing a weight. b. Generalized synovial joint. Problems arise when menisci or ligaments are torn, bursae become inflamed, and articular cartilage wears away. c. The shoulder is a ball-and-socket joint that permits movement in three planes. d. The elbow is a hinge joint that permits movement in a single plane

Why is coconut part of so many processed foods?

A coconut is a large nut, and the white, milky substance on the inside is the endosperm. Humans have been using this versatile nut for food, oil, and shredded fiber from the husk. Health food stores sell electrolyte-packed coconut water as a hydrating energy drink, and coconut oil is favored by cooks who do not use dairy products to achieve a butterlike consistency. In the Philippines and Papua New Guinea, a coconut oil blend is being used to power ships, trucks, and cars!

Mechanical Example

A home heating system is often used to illustrate how a more complicated negative feedback mechanism works (Fig. 22.16). Suppose you set the thermostat at 20°C (68°F). This is the set point. The thermostat contains a thermometer, a sensor that detects when the room temperature is above or below the set point. The thermostat also contains a control center; it turns the furnace off when the room is warm and turns it on when the room is cool. When the furnace is off, the room cools a bit, and when the furnace is on, the room warms a bit. In other words, a negative feedback system results in fluctuation above and below the set point.

Sugar Transport in Phloem

A plant must make and store sugar, then provide that sugar to its developing parts, such as the roots, flowers, and fruit. The location where the sugar is made or stored is called the source, and the location where the sugar will be used is the sink. How sugar is transported from the source to the sink can be explained using the pressure-flow model (Fig. 20.20). Leaves undergoing photosynthesis make sugar and become the source. The sugar is actively transported from the cells in the leaf mesophyll into the sieve tubes of the phloem. Recall that, like xylem, phloem is a continuous "pipeline" throughout the plant. High concentrations of sugar in the sieve tubes cause water to follow by osmosis. Like turning the nozzle on a hose, there is an increase in positive pressure as water flows in. The sugar (sucrose) solution at the source is forced to move to areas of lower pressure at a sink, like a root. When the sugar arrives at the root, it is actively transported out of the sieve tubes into the root cells. There, the sugar is used for cellular respiration or other metabolic processes. The high concentration of sugar in the root cells causes water to follow by osmosis, where it is later reclaimed by the xylem tissue. Although leaves are generally the source, modified roots and stems such as carrots, beets, and potatoes are also examples of sources that provide much needed sugar during winter or periods of dormancy. The high pressure of sucrose in phloem has resulted in a very interesting mutualistic relationship between aphids and some species of ants. Aphids are tiny insects with needlelike mouthparts. These mouthparts can poke a stem and tap into a sieve tube (Fig. 20.21). The high-pressure sucrose solution is forced through their digestive tracts very quickly, resulting in a droplet of sucrose at the rear called honeydew. Ants stroke the aphids with their antennae to induce honeydew production and then drink the beads of sucrose from the aphids' rear ends! Their "aphid farms" provide whole colonies of ants with all the sugar they need, and in turn the ants protect the aphids against predators.

Roots

A plant's roots support the plant by anchoring it in the soil, as well as absorbing water and minerals from the soil for the entire plant. As a rule of thumb, the root system is at least equivalent in size and extent to the plant's shoot system. Therefore, an apple tree has a much larger root system than a corn plant. Also, the extent of a root system depends on the environment. A single corn plant may have roots that extend as deep as 2.5 meters (m), while a mesquite tree in the desert may have roots that penetrate to a depth of 20 m. Roots have a cylindrical shape and a slimy surface. These features allow roots to penetrate the soil as they grow and permit water to be absorbed from all sides. In a special zone near the root tip, there are many root hairs that greatly increase the absorptive capacity of the root. Root hairs are so numerous that they increase the absorption of water and minerals tremendously. It has been estimated that a single rye plant has about 14 billion root-hair cells! Root-hair cells are constantly being replaced, so a rye plant forms about 100 million new root-hair cells every day. You probably know that a plant yanked out of the soil will not fare well when transplanted. This is because small lateral roots and root hairs are torn off. Transplantation is more apt to be successful if you take a part of the surrounding soil along with the plant, leaving as many of the lateral roots and root hairs intact as possible.

Nonwoody Stems

A stem that experiences only primary growth is nonwoody. Plants that have nonwoody stems, such as zinnias, mint, and daisies, are termed herbaceous plants. As in leaves, the outermost tissue of a herbaceous stem is the epidermis, which is covered by a waxy cuticle to prevent water loss. Beneath the epidermis of eudicot stems is the cortex, a narrow band of parenchyma cells. The cortex is sometimes green and carries on photosynthesis. The ground tissue in the center of a eudicot stem is called the pith. A monocot stem lacks an organized cortex or pith. Herbaceous stems have distinctive vascular bundles, where xylem and phloem are found. In each bundle, xylem is typically found toward the inside of the stem, and phloem is found toward the outside. In the stems of herbaceous eudicots, the vascular bundles are arranged in a distinct ring that separates the cortex from the pith. In the stems of herbaceous monocots, the vascular bundles are scattered throughout the stem and have a characteristic "monkey face" appearance. Figure 20.11 contrasts herbaceous eudicot and monocot stems. Imagine placing a heavy object on top of a straw. The straw would bend and buckle under the weight. Stems resist breaking during growth because of the internal strength provided by vessel elements, tracheids, and sclerenchyma cells impregnated with lignin. The herbaceous stem of the mammoth sunflower holds up a flower head that can weigh up to 5 pounds! Stems may have functions other than support and transport. In a cactus, the stem is the primary photosynthetic organ and serves as a water reservoir (see Fig. 20.10c), and the tuber of a potato plant is a food storage portion of an underground stem (Fig. 20.10d). Perennial plants are able to regrow each season from varied underground stems, such as tubers and rhizomes, all of which bear nodes that can produce a new shoot system.

Organ Systems and Homeostasis

All the systems of the body contribute to maintaining homeostasis (Fig. 22.14). The cardiovascular system conducts blood to and away from capillaries, where exchange occurs. Red blood cells transport oxygen and participate in the transport of carbon dioxide. White blood cells fight infection, and platelets participate in the clotting process. Lymphatic capillaries collect excess interstitial fluid and return it via lymphatic vessels to the cardiovascular system. Lymph nodes help purify lymph and keep it free of pathogens. The digestive system takes in and digests food, providing nutrient molecules that enter the blood to replace those that are constantly being used by the body's cells. The respiratory system removes carbon dioxide from and adds oxygen to the blood. The kidneys are extremely important in homeostasis, not only because they remove metabolic wastes but also because they regulate blood volume, salt balance, and pH. The liver, among other functions, regulates the glucose concentration of the blood. After a meal, the liver removes excess glucose from the blood for storage as glycogen. Later, the glycogen is broken down to replace the glucose that was used by body cells. The liver makes urea, a nitrogenous end product of protein metabolism. The nervous and endocrine systems regulate the other systems of the body. In the nervous system, sensory receptors send nerve impulses to control centers in the brain, which then direct effectors (muscles or glands) to become active. Muscles bring about an immediate change. Endocrine glands secrete hormones that bring about slower, more lasting changes that keep the internal environment relatively stable.

Closed Circulatory Systems

All vertebrates and some invertebrates have a closed circulatory system (Fig. 23.3), which is more commonly called a cardiovascular system because it consists of a strong, muscular heart and blood vessels. In a closed circulatory system, the blood remains within the blood vessels at all times. In humans, the heart has two receiving chambers, called atria (sing., atrium), and two pumping chambers, called ventricles. There are three kinds of vessels: arteries, which carry blood away from the heart; capillaries, which exchange materials with interstitial fluid; and veins, which return blood to the heart. Blood is always contained within these vessels and never runs freely into the body unless an injury occurs. As blood passes through capillaries, the pressure of blood forces some water out of the blood and into the interstitial fluid. Some of this fluid returns directly to a capillary, and some is picked up by lymphatic capillaries in the vicinity. The fluid, now called lymph, is returned to the cardiovascular system by lymphatic vessels. The function of the lymphatic system is discussed in Section 23.2.

Plant Hormones

Animals respond to environmental stimuli (danger, food, etc.) by moving toward or away from the stimulus. Plants respond to environmental stimuli (light, water, etc.) by growing toward or away from the stimulus. Most of these growth responses occur at the cellular level and are mediated by hormones. Much like our own hormones, plant hormones are small, organic molecules produced by a plant that serve as chemical signals between cells and tissues. Figure 21.1 shows how most hormones are signals that bind to receptor proteins. Generally, the process involves the binding of a hormone to a receptor, which in turn signals the cell to respond to the stimulus. The five commonly recognized groups of plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene (Table 21.1).

Blood Vessels

Arteries transport blood away from the heart. When the heart contracts, blood is sent under pressure into the arteries; thus, blood pressure accounts for the flow of blood in the arteries. Arteries have a much thicker wall than veins because of a well-developed middle layer composed of smooth muscle and elastic fibers. The elastic fibers allow arteries to expand and accommodate the sudden increase in blood volume that results after each heartbeat. The smooth muscle strengthens the wall and prevents overexpansion. Arteries branch into arterioles, small arteries just visible to the naked eye. Their diameter can be regulated by the nervous system, depending on the needs of the body. When arterioles are dilated, more blood flows through them; when they are constricted, less blood flows. The constriction of arterioles can also raise blood pressure. Arterioles branch into capillaries, which are extremely narrow, microscopic tubes with a wall composed of only epithelium, often called endothelium (Fig. 23.8b). Capillaries, which are usually so narrow that red blood cells pass through in single file, allow the exchange of nutrient and waste molecules across their thin walls. Capillary beds (many interconnected capillaries) are so widespread that, in humans, all cells are less than a millimeter from a capillary. Because the number of capillaries is so extensive, blood pressure drops and blood flows slowly along. The slow movement of blood through the capillaries also facilitates efficient exchange of substances between the blood and the interstitial fluid. The entrance to a capillary bed is controlled by bands of muscle called precapillary sphincters. During muscular exercise, these sphincters relax, and the capillary beds of the muscles are open. Also, after an animal has eaten, the capillary beds in the digestive tract are open. Otherwise, blood moves through a shunt that takes the blood from arteriole to venule (see Fig. 23.10b). Venules collect blood from capillary beds and join as they deliver blood to veins. Veins carry blood back to the heart. Blood pressure is much reduced by the time blood reaches the veins. The walls of veins are much thinner and their diameters are wider than those of arteries (Fig. 23.8c). The thin walls allow skeletal muscle contraction to push on the veins, forcing the blood past a valve (Fig. 23.9). Valves within the veins point, or open, toward the heart, preventing a backflow of blood when they close. When inhalation occurs and the chest expands, the thoracic pressure falls and abdominal pressure rises. This action also aids the flow of venous blood back to the heart, because blood flows in the direction of reduced pressure.

Plant Organs

As you learned in Section 18.1, the earliest plants were simple and lacked true leaves, stems, and roots. As plants gained vascular tissue and began moving onto land away from water, organs developed to facilitate living in drier environments. Even though all vascular plants have vegetative organs, this chapter focuses on the organs commonly found in flowering plants, or angiosperms. A flowering plant, whether a cactus, a daisy, or an apple tree, has a shoot system and a root system (Fig. 20.5). The shoot system consists of the stem, leaves, flowers, and fruit. A stem supports the leaves, transports materials between the roots and leaves, and produces new tissue. Lateral (side) branches grow from a lateral bud located at the angle where a leaf joins the stem. A node is the location where leaves, or the buds for branches, are attached to the stem. An internode is the region between nodes. At the end of a stem, a terminal bud contains an apical meristem and produces new leaves and other tissues during primary growth (Fig. 20.6).Vascular tissue transports water and minerals from the roots through the stem to the leaves and transports the products of photosynthesis, usually in the opposite direction. The root system simply consists of the roots. The root tip also contains an apical meristem, which produces primary growth downward. Primary growth at the terminal bud and root tip would be equivalent to your increasing in height by growing from your head and your feet! Ultimately, the three vegetative organs—the leaf, the stem, and the root—perform functions that allow a plant to live and grow. Flowers and fruit are reproductive organs and will be discussed in Section 21.3.

When Breathing Becomes Difficult

Asthma is a disease in which the airways become constricted (narrowed) and inflamed (swollen), both of which can result in difficulty breathing. The symptoms often include wheezing and shortness of breath, a frequent cough (often at night), and a feeling of being very tired or weak, especially when exercising. Over 34 million children and adults in the United States have asthma, and the incidence seems to be increasing. Experts offer various explanations for this. One hypothesis is that we may be "too clean," in the sense that we are not exposed to enough common bacteria, viruses, and parasites as children. As a result, our immune systems may react to harmless material we inhale. There are many harmful forms of air pollution, but of increasing concern are tiny, "ultrafine" particles, those less than 0.1 micrometer (μm) across, which are produced at high levels by diesel engines. These particles can bypass the normal defenses of the upper respiratory tract and end up lodging deep in the lungs, with damaging effects. Recently, there have been breakthroughs in asthma research. In addition to environmental factors, there appear to be a number of genes that increase susceptibility to asthma. For example, one set of genes on chromosome 5 has been linked to asthma and is associated with the body's inflammatory response. Inflammation can constrict the airways and increase fluid flow to the lungs. Variations in a gene on chromosome 17 have also been shown to increase susceptibility, especially in individuals who have been exposed to respiratory viruses. Although there is not currently a cure, the identification of these genes sheds hope for those with asthma.

Ethylene

At one time, it was believed that abscisic acid functioned in the process of abscission, which is the dropping of leaves, fruits, or flowers from a plant. Although the external application of abscisic acid promotes abscission, this hormone is no longer believed to function naturally in this process. The hormone ethylene is now known to be responsible for fruit abscission and fruit ripening in plants. Ethylene is a gas that can move freely in the air. The hormone stimulates certain enzymes, such as cellulase, that cause leaf, fruit, or flower drop (Fig. 21.6a,b). Cellulase hydrolyzes cellulose in plant cell walls and weakens that part of the plant. In the early twentieth century, it was common practice to prepare citrus fruits for market by placing them in a room with a kerosene stove. Only later did researchers realize that ethylene, an incomplete combustion product of kerosene, was ripening the fruit. Because it is a gas, ethylene can act from a distance, and is often used commercially to ripen fruit just before it is delivered to the grocery store (Fig. 21.6c). A barrel of ripening apples can induce ripening in a bunch of bananas, even if they are in different containers. If a plant is wounded due to physical damage or infection, ethylene is released at the wound site. This is why one rotten apple spoils the whole barrel.

Blood

Blood is composed of several types of cells suspended in a liquid matrix called plasma. Blood is unlike other types of connective tissue in that the matrix is not made by the cells of the connective tissue (Fig. 22.6). Even though the blood is liquid, it is a connective tissue by definition; it consists of cells within a matrix. Blood has many functions for overall homeostasis of the body. It transports nutrients and oxygen to cells and removes their wastes. It helps distribute heat and plays a role in fluid, ion, and pH balance. Also, various components of blood help protect us from disease, and blood's ability to clot prevents fluid loss. Blood contains three formed elements: red blood cells, white blood cells, and platelets. Red blood cells are small, biconcave, disk-shaped cells without nuclei. The presence of the red pigment hemoglobin makes the cells red, as well as making blood as a whole red. Hemoglobin binds oxygen and allows the red blood cells to transport oxygen to the cells of the body. White blood cells can be distinguished from red blood cells by the fact that they are usually larger, have a nucleus, and would appear translucent without staining. White blood cells fight infection in two primary ways: (1) Some are phagocytic and engulf infectious pathogens; (2) others produce antibodies, molecules that combine with foreign substances to inactivate them. Platelets are another component of blood, but they are not complete cells; rather, they are fragments of giant cells present only in bone marrow. When a blood vessel is damaged, platelets form a plug that seals the vessel; along with injured tissues, platelets release molecules that help the clotting process.

Growth Zones of a Root

Both monocot and eudicot roots contain the same growth zones. In Figure 20.13, a longitudinal section of a eudicot root tip reveals these zones, where cells are in various stages of differentiation as primary growth occurs. The apical meristem of a root contains actively dividing cells and is surrounded by a root cap. The root cap is covered with a slimy substance to help it penetrate downward into the abrasive soil, and it is meant to be damaged to protect the apical meristem. The dividing cells of the apical meristem are found in the zone of cell division. As more cells are made, older cells are pushed into the zone of elongation, where cells lengthen as they become specialized. In the zone of maturation, which contains fully differentiated cells, many of the epidermal cells have root hairs.

What trees are used to produce cork bottle stoppers?

Cork stoppers, such as those traditionally found in wine bottles, are manufactured almost exclusively in Spain and Portugal from the cork oak tree (Quercus suber). Cork from these trees can be harvested once the tree reaches an age of about 20 years. Then, every 9 years, the outer 1-2 inches of cork may be removed from the tree without harming it. This yields about 16 kilograms (35 pounds) of cork per tree, and cork trees can continue to produce cork for around 150 years. Cork is considered to be an environmentally friendly forest product because the trees are not harmed during its production.

Cytokinins

Cytokinins were discovered as a result of attempts to grow plant tissues and organs in culture vessels in the 1940s. It was found that cell division occurs when coconut milk (a liquid endosperm) and yeast extract are added to the culture medium. Although the effective agents could not be isolated at the time, they were collectively called cytokinins because, as you may recall, cytokinesis means "division of the cytoplasm." A naturally occurring cytokinin was not isolated until 1967. Because it came from the kernels of maize (Zea), it was called zeatin. Cytokinins influence plant growth by promoting cell division. Cytokinins are found in plant meristems, young leaves, root tips, and seeds and fruits. Whenever a plant grows, cytokinins are involved. Cytokinins also prevent senescence, or aging, of plant organs. As cytokinin levels drop within a plant organ, such as a leaf, growth slows or even stops. Then, the leaf loses its natural color as large molecules are broken down and transported to other parts of the plant. Senescence is a necessary part of a plant's growth. For example, as some plants grow taller, they naturally lose their lower leaves. Researchers are aware that the ratio of auxin to cytokinin and the acidity of the culture medium determine whether a plant tissue forms an undifferentiated mass, called a callus (see Fig 21.23a), or differentiates to form roots, vegetative shoots, leaves, or floral shoots. Research indicates that the presence of auxin and cytokinins in certain ratios leads to the activation of an enzymatic pathway that releases from the cell walls chemicals that influence the specialization of plant cells.

Synthetic Blood

Every 2 seconds, someone in the United States needs a transfusion of blood. The need may have been caused by an injury, or a surgical procedure, or even a disease that requires regular transfusions. To answer these needs, blood donors provide over 36,000 units of blood every day, or 15 million units of blood every year. Historically, the need for blood transfusions has been met by blood donors. Unfortunately, supply does not always meet demand. Natural disasters may place a strain on supplies, especially considering that blood can be stored for only about 42 days, which makes stockpiling difficult. Furthermore, some people are not able to receive blood from donors due to religious beliefs or medical conditions. To meet the demand for transfusions, scientists have been developing synthetic, or artificial, blood. The use of an oxygen-carrying blood substitute, called oxygen therapeutics, has the goal of providing a patient's tissues with enough oxygen. In some cases, the blood substitute is completely synthetic, and contains chemicals that mimic the oxygen-carrying hemoglobin found in red blood cells. In other cases, scientists are applying biotechnology to manufacture replacement red blood cells. While several biotech companies are currently conducting clinical trials using synthetic blood, most medical professionals believe that we are still several years away from being able to produce a synthetic blood supply to meet the needs of society.

Capillary Exchange in the Tissues

Figure 23.1 showed that in some animals, exchange is carried out by each cell individually because there is no cardiovascular system. When an animal has a cardiovascular system, the interstitial fluid is involved with exchanging materials between the capillaries and the cells. Notice in Figure 23.13 that amino acids, oxygen, and glucose exit a capillary and enter interstitial fluid, to be used by cells. On the other hand, carbon dioxide and wastes leave the interstitial fluid and enter a capillary, to be taken away and excreted from the body. A chief purpose of the cardiovascular system is to take blood to the capillaries, where exchange occurs. Without this exchange, homeostasis is not maintained, and the cells of the body perish. Figure 23.13 illustrates certain mechanics of capillary exchange. Blood pressure and osmotic pressure are two opposing forces at work along the length of a capillary. Blood pressure is caused by the beating of the heart, while osmotic pressure is due to the salt and protein content of the blood. Blood pressure holds sway at the arterial end of a capillary and water exits. Blood pressure is reduced by the time blood reaches the venous end of a capillary, and osmotic pressure now causes water to enter. Midway between the arterial and venous ends of a capillary, blood pressure pretty much equals osmotic pressure, and passive diffusion alone causes nutrients to exit and wastes to enter. Diffusion works because interstitial fluid always contains fewer nutrients and more wastes than blood does. After all, cells use nutrients and thereby create wastes. The exchange of water at a capillary is not exact, and the result is always excess interstitial fluid. Excess interstitial fluid is collected by lymphatic capillaries, and in this way it becomes lymph (see Fig. 23.12). Lymph contains all the components of plasma, except it has much less protein. Lymph is returned to the cardiovascular system when the major lymphatic vessels enter the subclavian veins in the shoulder region (see Fig. 23.11). In addition to nutrients and wastes, blood distributes heat to body parts. When you are warm, many capillaries that serve the skin are open, and your face is flushed. This helps rid the body of excess heat. When you are cold, skin capillaries close, conserving heat.

Photoperiodism

Flowering in angiosperms is a striking response to environmental seasonal changes. In some plants, flowering occurs according to the photoperiod, which is the ratio of the length of day to the length of night over a 24-hour period. You may know someone who suffers from seasonal allergies. In the spring, the days are longer and the nights are shorter, triggering long-day (short-night) plants to flower, produce pollen, and affect individuals with spring allergies. In the fall, the days are shorter and the nights are longer, triggering short-day (long-night) plants to flower, produce pollen, and affect individuals with fall allergies. The spring-flowering and fall-flowering plants are responding to a critical length—a period of light specific in length for any given species, which appears to initiate flowering (Fig. 21.9a,b). Experiments have shown that the length of continuous darkness, not light, is what actually controls flowering in many plants. Nurseries use these kinds of data to make all types of flowers available throughout the year (Fig. 21.9b).

Overview of the Plant Life Cycle

Flowering plants have an alternation-of-generations life cycle, but with the modifications shown in Figure 21.11. After this overview of the flowering plant life cycle, we will discuss the life cycle in more depth. In flowering plants, the sporophyte is dominant, and it is the generation that produces flowers. The flower is the reproductive organ of angiosperms. The flower of the sporophyte produces two types of spores: microspores and megaspores. A microsporedevelops into a male gametophyte, which is a pollen grain. A megaspore develops into a female gametophyte, the embryo sac, which is microscopic and housed deep within the flower. A pollen grain is either windblown or carried by an animal to the vicinity of the embryo sac. At maturity, a pollen grain contains two nonflagellated sperm. The embryo sac contains an egg. A pollen grain develops a pollen tube, and the sperm move down the pollen tube to the embryo sac. After a sperm fertilizes an egg, the zygote becomes an embryo, still within the flower. The structure that houses the embryo develops into a seed. The seed also contains stored food and is surrounded by a seed coat. The seeds are often enclosed by a fruit, which aids in dispersing the seeds. When a seed germinates, a new sporophyte emerges and develops. The life cycle of flowering plants is well adapted to a land existence (see Chapter 18). No external water is needed to transport the pollen grain to the embryo sac, or to enable the sperm to reach the egg. All stages of the life cycle are protected from drying out.

Fruit Types and Seed Dispersal

Flowering plants have seeds enclosed by a fruit (Fig. 21.18). Seeds develop from ovules, and fruits develop from ovaries and sometimes other parts of a flower. Technically, market produce, such as cucumber, tomatoes, and sugar snap peas, are fruits—not vegetables—because they contain seeds. A vegetable is an edible plant part without seeds, such as celery (a stem), lettuce (leaves), and carrots (roots). Fruits are quite diverse. Dry fruits are generally a dull color with a thin, dry ovary wall, so that the potential food for animals is largely confined to the seeds. In grains such as wheat, corn, and rice, the fruit looks like a seed. Nuts (e.g., walnuts, pecans) have a hard, outer shell covering a single seed. A legume, such as a pea, has a several-seeded fruit that splits open to release the seeds. In contrast to dry fruits, fleshy fruits have a juicy portion that is sometimes brightly colored to attract animals. A drupe (e.g., peach, cherry, olive) is a "stone fruit"—the outer part of the ovary wall is fleshy, but there is an inner, stony layer. Inside the stony layer is the seed. A berry, such as a tomato, contains many seeds. An apple is a pome, in which a dry ovary covers the seeds, and the fleshy part is derived from the receptacle of the flower. A strawberry is an interesting fruit, because the flesh is derived from the receptacle, and what appear to be the seeds are actually dry fruits!

Germination of Seeds

Following dispersal, if all goes well, the seeds will germinate. As growth occurs, a seedling appears. Germination does not usually take place until there is sufficient water, warmth, and oxygen to sustain growth. In deserts, germination does not occur until there is adequate moisture. These requirements help ensure that seeds do not germinate until the most favorable growing season has arrived. Some seeds do not germinate until they have been dormant for a period of time. For seeds, dormancy is the time during which no growth occurs, even though conditions may be favorable for growth. In the temperate zone, seeds often have to be exposed to a period of cold weather before dormancy is broken. Fleshy fruits (e.g., apples, pears, oranges, and tomatoes) contain inhibitors, so that germination does not occur until the seeds are removed and washed. Aside from water, bacterial action and even fire can act on the seed coat, allowing it to become permeable to water. The uptake of water causes the seed coat to burst and germination to occur.

Gibberellins

Gibberellins were discovered in 1926 when a Japanese scientist was investigating a fungal disease of rice plants called "foolish seedling disease" which caused rapid stem elongation that weakened the plants and caused them to collapse. The fungus infecting the plants produced an excess of a chemical called gibberellin, named after the fungus, Gibberella fujikuroi. It wasn't until 1956 that a form of gibberellin now known as gibberellic acid was isolated from a flowering plant rather than from a fungus. Over 130 different gibberellins have been identified. The most common of these is gibberellic acid, GA3 (the subscript 3 distinguishes it from other gibberellins). Young leaves, roots, embryos, seeds, and fruits are places where natural gibberellins can be found. Gibberellins are growth-promoting hormones that bring about elongation of cells. When gibberellins are applied externally to plants, the most obvious effect is stem elongation between the nodes (Fig. 21.4a). Gibberellins can cause dwarf plants to grow, cabbage plants to become as much as 2 meters tall, and bush beans to become pole beans. There are many commercial uses for gibberellins that promote growth in a variety of crops such as apples, cherries, and sugarcane. A notable example is their use on many of the table grapes grown in the United States. Commercial grapes are a genetically seedless variety that would naturally produce small fruit on very small bunches. Treatment with GA3 substitutes for the presence of seeds, which would normally be the source of native gibberellins for fruit growth. Treatments increase both fruit stem length (producing looser clusters) and fruit size (Fig. 21.4b). In the brewing industry, the production of beer relies on the breakdown of starch in barley grains (seeds). Barley grains would naturally be dormant—a period in which a seed does not grow. Gibberellins are used to break the dormancy of barley grains, yielding fermentable sugars, mainly maltose, which are then fermented by yeast to produce ethanol.

Ground Tissue

Ground tissue forms the internal bulk of leaves, stems, and roots. Ground tissue contains three types of cells (Fig. 20.3). Parenchyma cells are the least specialized of the cell types and are found in all the organs of a plant. They may contain chloroplasts and carry on photosynthesis, or they may contain colorless organelles that store the products of photosynthesis. Collenchyma cells are like parenchyma cells except that they have irregularly shaped corners and thicker cell walls. Collenchyma cells often form bundles just beneath the epidermis and give flexible support to immature regions of a plant body. The familiar strands in celery stalks are composed mostly of collenchyma cells. Sclerenchyma cells have thick secondary cell walls containing lignin, which makes plant cell walls tough and hard. If we compare a cell wall to reinforced concrete, cellulose fibrils would play the role of steel rods, and lignin would be analogous to the cement. Most sclerenchyma cells are nonliving; their primary function is to support the mature regions of a plant. The hard outer shells of nuts are made of sclerenchyma cells. The long fibers in plants, composed of strings of sclerenchyma, make them useful for a number of purposes. For example, cotton and flax fibers can be woven into cloth, and hemp fibers can make strong rope.

Phytochrome

If flowering is dependent on night length, plants must have a way to detect these periods. In some plants, this appears to be the role of a leaf pigment called phytochrome. Phytochrome can detect the wavelengths of light from the sun (see Fig. 6.4) and can distinguish between red wavelengths and far-red wavelengths of light. Figure 21.10 shows how the presence of red light or far-red light determines the particular form of phytochrome: - Pr (phytochrome red) absorbs red light and is converted into Pfr in the daytime. - Pfr (phytochrome far-red) absorbs far-red light and is converted into Pr in the evening During the day, sunlight contains more red light than far-red light, and Pr is converted into Pfr. But at dusk, more far-red light is available, and Pfris converted to Pr. There is also a slow metabolic replacement of Pfr by Pr during the night. Plant spacing is another interesting function of phytochrome. Next time you are at a garden center, read the instructions on a seed packet and notice the specific details on spacing the seeds placed in the ground. In nature, red and far-red light also signal spacing. Leaf shading increases the amount of far-red light relative to red light. Plants somehow measure the amount of far-red light bounced back to them from neighboring plants. The closer together plants are, the more far-red relative to red light they perceive and the more likely they are to grow tall, a strategy for outcompeting others for sunshine! 21.2

Eudicot Versus Monocot Germination

If the two cotyledons of a bean seed are parted, you can see the cotyledons and a rudimentary plant with immature leaves. As the eudicot seedling emerges from the soil, the shoot is hook-shaped to protect the immature leaves as they start to grow. The cotyledons shrivel up as the true leaves of the plant begin photosynthesizing (Fig. 21.20). A corn kernel is actually the fruit of a monocot. The outer covering is the fruit and seed coat combined (Fig. 21.21). Inside is the single cotyledon. Also, both the immature leaves and the root are covered by sheaths. The sheaths are discarded when the seedling begins growing, and the immature leaves become the first true leaves of the corn plant.

Propagation of Plants in a Garden

If you wanted to create a bed of tulips, irises, or gladiolas in your garden, you would not plant seeds but instead would rely on bulbs, rhizomes, or corms and reproduce the plants asexually (Fig. 21.22). Baking potatoes are modified stems called tubers, and each "eye" has a bud that can become a new plant. All of these structures are typically fleshy, underground food-storage tissues that contain buds that will sprout in the spring. Runners, or stolons, such as those found in strawberries, are horizontal stems that can also result in new clonal plants.

The Human Heart

In humans, the heart is a double pump: The right side of the heart pumps O2-poor blood to the lungs, and the left side of the heart pumps O2-rich blood to the tissues (Fig. 23.5). The heart acts as a double pump because a septum separates the right side from the left. Further, the septum prevents O2-poor blood from mixing with O2-rich blood. Each side of the heart has two chambers. The upper, thin-walled chambers are called atria (sing., atrium), and they receive blood. The lower chambers are the thick-walled ventricles, which pump the blood away from the heart. Valves are located between the atria and the ventricles, and between the ventricles and attached vessels. Because these valves close after the blood moves through, they keep the blood moving in the correct direction. The valves between the atria and ventricles are called the atrioventricular valves, and the valves between the ventricles and their attached vessels are called semilunar valves because their cusps look like half-moons. The following sequence traces the path of blood through the heart: The right atrium receives blood from the attached veins, called the venae cavae, that are returning O2-poor blood to the heart from the tissues. After the blood passes through an atrioventricular valve (also called the tricuspid valve because of its three flaps), the right ventricle pumps it through the pulmonary semilunar valve into the pulmonary trunk and pulmonary arteries, which take it to the lungs. The pulmonary veins take O2-rich blood to the left atrium. After this blood passes through an atrioventricular valve (also called the bicuspid valve), the left ventricle pumps it through the aortic semilunar valve into the aorta, which takes it to the tissues. O2-poor blood is often associated with all veins and O2-rich blood with all arteries, but this idea is incorrect: Pulmonary arteries and pulmonary veins are just the reverse. That is why the pulmonary arteries in Figure 23.5 are colored blue and the pulmonary veins are colored red. The correct definitions are that an arteryis a vessel that takes blood away from the heart, and a vein is a vessel that takes blood toward the heart.

Breathing in Other Animals

In mammals and most reptiles, air moves in and out by the same route; therefore, some residual air, low in oxygen, is always left in the lungs of humans. Birds, on the other hand, use a one-way ventilation mechanism (Fig. 24.6). Incoming air is carried past the lungs by a trachea, which takes it to a set of abdominal air sacs. Then air passes forward through the lungs into a set of thoracic air sacs. Fresh air never mixes with used air in the lungs of birds, and thereby gas-exchange efficiency is greatly improved.

Dispersal of Seeds

In order for plants to be successful in their environments, their seeds have to be dispersed—that is, moved long distances from the parent plant. There are various methods of seed dispersal. For example, birds and mammals sometimes eat fruits, including the seeds, which then pass out of the digestive tract with the feces some distance from the parent plant (Fig. 21.19a). Squirrels and other animals gather seeds and fruits, which they bury some distance away. Some plants have evolved unusual ways to ensure dispersal. The hooks and spines of clover, cocklebur, and burdock fruits attach to the fur of animals and the clothing of humans, which carry them far away from the parent plant (Fig. 21.19b). Other seeds are dispersed by wind. Woolly hairs, plumes, and wings are all adaptations for this type of dispersal. The dandelion fruit is attached to several hairs that function as a parachute and aid dispersal (Fig. 21.19c). The winged fruit of a maple tree, which contains two seeds, has been known to travel up to 10 kilometers from its parent (Fig. 21.19d). Different still, a touch-me-not plant has seed pods that swell as they mature. A passing animal may cause the swollen pods to burst, hurling the ripe seeds some distance away from the plant.

Development of the Seed in a Eudicot

It is now possible to account for the three parts of a seed: seed coat, embryo, and endosperm. The seed coat is a protective covering that once was the ovule wall. Double fertilization results in an endosperm nucleus and a zygote. Cell division produces a multicellular embryo and a multicellular endosperm, which is the stored food of a seed. Figure 21.17 shows the stages in the development of the seed. Tissues become specialized until eventually a shoot and root tip containing apical meristems develop. Notice that the cotyledons, or embryonic leaves, absorb the developing endosperm and become large and fleshy (Fig. 21.17f). The food stored by the cotyledons will nourish the embryo when it resumes growth. Cotyledons wither when the first true leaves grow and become functional. The common garden bean is a good example of a eudicot seed with large cotyledons and no endosperm (see Fig. 21.20).

Leaves

Leaves are the chief organs of photosynthesis, and as such they require a supply of solar energy, carbon dioxide, and water. Broad and thin foliage leaves maximize the surface area to collect sunlight and absorb carbon dioxide. Leaves receive water from the root system by way of vascular tissue that terminates in the leaves. Deciduous plants lose their leaves, often due to a yearly dry season or the onset of winter. Other trees, called evergreens, retain their leaves for the entire year. Figure 20.8 shows the general structure of a leaf. The wide portion of a foliage leaf is called the blade. The petiole is a stalk that attaches the blade to the stem. The blade of a leaf is often undivided, or simple, such as a cottonwood leaf (Fig. 20.8a). But in some leaves, the blade is divided, or compound, as in a shagbark hickory or honey locust (Fig. 20.8b,c). Notice that it is possible to tell from the placement of the lateral bud whether you are looking at several individual leaves or one compound leaf. Figure 20.9 shows a cross section of a typical eudicot leaf. The outermost structure, the waxy cuticle, prevents water loss. Next, the epidermal layer can be found above and below. The lower epidermis contains the stomata, allowing gas exchange. The interior area of a leaf is composed of mesophyll, the tissue that carries out photosynthesis. Vascular tissue terminating at the mesophyll transports water and minerals to a leaf and transports the product of photosynthesis, a sugar (sucrose), away from the leaf. The mesophyll has two distinct regions: Palisade mesophyll contains tightly packed cells, thereby increasing the surface area for the absorption of sunlight, and spongy mesophyll contains irregular cells surrounded by air spaces. The loosely packed arrangement of the cells in the spongy layer increases the amount of surface area for gas exchange and water loss. Water taken into a leaf by xylem tissue evaporates from spongy mesophyll and exits at the stomata. Leaves may have several other functions aside from photosynthesis (Fig. 20.10). Leaves may be modified as tendrils that allow the plant to attach to objects (Fig. 20.10a) or as traps for catching insects (Fig. 20.10b). The leaves of a cactus are spines that reduce water loss and protect the plant from hungry animals (Fig. 20.10c).

Loose Fibrous and Related Connective Tissues

Let's consider loose fibrous connective tissue first and then compare the other types with it. This tissue occurs beneath an epithelium and connects it to the other tissues within an organ. It also forms a protective covering for many internal organs, such as muscles, blood vessels, and nerves. Its cells are called fibroblastsbecause they produce a matrix that contains fibers, including collagen fibers and elastic fibers, that stretch under tension and return to their original shape when released. The presence of loose fibrous connective tissue in the walls of lungs and arteries gives these organs resilience, the ability to expand and then return to their original shape without damage. Adipose tissue (see Fig. 22.2) is a type of loose connective tissue in which the fibroblasts enlarge and store fat, and there is limited matrix. Adipose tissue is located beneath the skin and around organs, such as the heart and kidneys, where it cushions and protects the organs and serves as long-term energy storage. Compared with loose fibrous connective tissue, dense fibrous connective tissue contains more collagen fibers, which are packed closely together. This type of tissue has more specific functions than does loose fibrous connective tissue. For example, dense fibrous connective tissue is found in tendons, which connect skeletal muscles to bones, and in ligaments, which connect bones to other bones at joints. In cartilage, the cells lie in small, open cavities called lacunae, separated by a matrix that is semisolid yet flexible. Hyaline cartilage, the most common type of cartilage, contains only very fine collagen fibers (Fig. 22.5). The matrix has a white, translucent appearance when unstained. Hyaline cartilage is found in the nose and at the ends of the long bones and the ribs, and it forms rings in the walls of respiratory passages. The human fetal skeleton is also made of this type of cartilage, which makes it easier for the baby to pass through the birth canal. Most of the cartilage is later replaced by bone. Cartilaginous fishes, such as sharks, have a cartilaginous skeleton throughout their lives. Bone is the most rigid connective tissue (Fig. 22.5). It consists of an extremely hard matrix of inorganic salts, primarily calcium salts, which are deposited around collagen fibers. The inorganic salts give bone rigidity, and the collagen fibers provide elasticity and strength, much as steel rods do in reinforced concrete. The inorganic salts found in bone also act as storage for calcium and phosphate ions for the entire body. Compact bone, the most common type of bone in humans, consists of cylindrical structural units called osteons. The central canal of each osteon is surrounded by rings of hard matrix. Bone cells are located in lacunae between the rings of matrix. Blood vessels in the central canal carry nutrients that allow bone to renew itself.

What is a heart murmur?

Like mechanical valves, the heart valves are sometimes leaky; they may not close properly, and there is a backflow of blood. A heart murmur is often due to leaky atrioventricular valves, which allow blood to pass back into the atria after the valves have closed. These may be caused by a number of factors: high blood pressure (hypertension), heart disease, or rheumatic fever. Rheumatic fever is a bacterial infection that begins in the throat and spreads throughout the body. The bacteria attack various organs, including the heart valves. When damage is severe, the valve can be replaced with a synthetic valve or one taken from a pig's heart.

Adaptations of Roots for Mineral Uptake

Minerals enter a plant at its root system, and two mutualistic relationships assist roots in fulfilling this function. Air contains about 78% nitrogen (N2), but plants can't make use of it. Most plants depend on bacteria in the soil to fix nitrogen—that is, the bacteria change atmospheric nitrogen (N2) to nitrate (NO3−) or ammonium (NH4+), both of which plants can take up and use. Legume plants, such as soybean and peas, have roots colonized by bacteria that are able to take up atmospheric nitrogen and reduce it to a form suitable for incorporation into organic compounds (Fig. 20.17a). The bacteria live in root nodules (see Fig. 17.13), and the plant supplies the bacteria with carbohydrates, while the bacteria in turn furnish the plant with nitrogen compounds. Another mutualistic relationship involves mycorrhizal fungi and almost all plant roots (Fig. 20.17b). The hyphae of the fungus increase the surface area available for water uptake and break down organic matter, releasing inorganic nutrients that the plant can use. In return, the root furnishes the fungus with sugars and amino acids. Plants are extremely dependent on mycorrhizal fungi. For example, orchid seeds, which are quite small and contain limited nutrients, do not germinate until a mycorrhizal fungus has invaded their cells.

Auxins

More than a century ago, an organic substance known as auxin was the first plant hormone to be discovered. All plant cells have a rigid cell wall. Auxin's role is to soften the cell wall so that plant growth can occur. Auxin is involved in phototropism, a trait of plants that results in the bending of the stem in the direction of a light source (Fig. 21.2a). When a plant is exposed to light on one side, auxin moves to the shady side. On that side, cells become longer, causing the stem to bend toward the light. Elongation of cells in the shade occurs due to a series of events (Fig. 21.2b): 1. Auxin binds to a protein receptor. 2. Hydrogen ions (H+) are actively pumped out of the cell (requiring ATP). 3. The increased concentration of H+ ions creates an acidic environment. 4. The acid triggers other enzymes to soften the cell wall. 5. Growth and elongation of the cell takes place. Auxin is also responsible for a phenomenon called apical dominance (Fig. 21.3). Experienced gardeners know that, to produce a bushier plant, they must remove the terminal bud. Normally, auxin is produced in the apical meristem of the terminal bud and is transported downward in the plant. The presence of auxin inhibits the growth of lateral buds. When the terminal bud is removed, auxin is not produced, allowing the lateral buds to grow and the plant to take on a fuller appearance. Interestingly, if auxin were to be applied to the broken terminal stem, apical dominance would be restored. Synthetic auxins are used today in a number of applications. These auxins are sprayed on plants, such as tomatoes, to induce the development of fruit without pollination, creating seedless varieties. Synthetic auxins have been used as herbicides to control broadleaf weeds, such as dandelions and other plants. These substances have little effect on grasses. Agent Orange is a powerful synthetic auxin that was used in extremely high concentrations to defoliate the forests of Vietnam during the Vietnam War. This powerful auxin proved to be carcinogenic and harmed many of the local people.

How is paper made?

Most paper is made from the cellulose fibers of trees. Recall from Section 3.2 that cellulose is a component of the cell wall of plants. After a tree is harvested, it is debarked and cut into small chips to increase the surface area. However, before the cellulose can be extracted from the cells, the lignin and resins must be removed. This is done either mechanically or by using a combination of steam and sulfur-based chemicals to separate the lignin and resins from the cellulose fibers. In the next step, the lignins and resins are removed by bleaching with chlorine gas (or chlorine dioxide), which also makes the fibers white. In the final steps, fibers are dried and treated in a machine that forms rolls of paper for commercial use.

Muscular Tissue Moves the Body

Muscular tissue and nervous tissue work together to enable animals to move. Muscular tissue contains contractile protein filaments, called actin and myosin filaments, that interact to produce movement. The three types of vertebrate muscles are skeletal, cardiac, and smooth. Skeletal muscle, which works under voluntary movement, is attached by tendons to the bones of the skeleton, and when it contracts, bones move. Contraction of skeletal muscle, being under voluntary control, occurs faster than in the other two muscle types. The cells of skeletal muscle, called fibers, are cylindrical and quite long—sometimes they run the entire length of the muscle (Fig. 22.7a). They arise during development when several cells fuse, resulting in one fiber with multiple nuclei. The nuclei are located at the edge of the cell, just inside the plasma membrane. The fibers have alternating light and dark bands running across the cell, giving them a striated appearance. These bands are due to the arrangement of actin filaments and myosin filaments in the cell. Cardiac muscle is found only in the walls of the heart, and its contraction pumps blood and accounts for the heartbeat. Like skeletal muscle, cardiac muscle has striations, but the contraction of the heart is autorhythmic (occurring at a set pace) and involuntary (Fig. 22.7b). Cardiac muscle cells also differ from skeletal muscle cells in that they have a single, centrally placed nucleus. The cells are branched and seemingly fused with one another. The heart appears to be composed of one large, interconnected mass of muscular cells. Actually, cardiac muscle cells are separate and individual, but they are bound end to end at intercalated disks, areas where folded plasma membranes allow the contraction impulse to spread from cell to cell. Smooth muscle is named because the cells lack striations. The spindle-shaped cells form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells. Consequently, the nuclei form an irregular pattern in the tissue (Fig. 22.7c). Like cardiac muscle, smooth muscle is involuntary. Smooth muscle is also sometimes called visceral muscle because it is found in the walls of the viscera (intestines, stomach, and other internal organs) and blood vessels. Smooth muscle contracts more slowly than skeletal muscle but can remain contracted for a longer time. When the smooth muscle of the intestines contracts, food moves along the lumen. When the smooth muscle of the blood vessels contracts, the vessels constrict, helping raise blood pressure.

Negative Feedback

Negative feedback is the primary homeostatic mechanism that allows the body to keep the internal environment relatively stable. A negative feedback mechanism has at least two components: a sensor and a control center (Fig. 22.15). The sensor detects a change in the internal environment (a stimulus); the control center initiates an effect that brings conditions back to normal. At that point, the sensor is no longer activated. In other words, a negative feedback mechanism is present when the output of the system dampens (reduces) the original stimulus. Consider a simple example. When the pancreas detects that the blood glucose level is too high, it secretes insulin, a hormone that causes cells to take up glucose. Then the blood sugar level returns to normal, and the pancreas is no longer stimulated to secrete insulin. When conditions exceed their limits and feedback mechanisms cannot compensate, illness results. For example, if the pancreas is unable to produce insulin, as in diabetes mellitus, the blood sugar level becomes dangerously high and the individual can become seriously ill. The study of homeostatic mechanisms is therefore medically important.

Nervous Tissue Communicates

Nervous tissue coordinates the functions of body parts and allows an animal to respond to external and internal environments. The nervous system depends on (1) sensory input, (2) integration of data, and (3) motor output to carry out its functions. Nerves conduct impulses from sensory receptors, such as pain receptors in the skin, to the spinal cord and brain, where integration occurs. The phenomenon called sensation occurs only in the brain, however. Nerves then conduct nerve impulses away from the spinal cord and brain to the muscles and glands, causing them to contract or secrete in response. In this way, a coordinated response to both internal and external stimuli is achieved. A nerve cell is called a neuron. Every neuron has three parts: dendrites, a cell body, and an axon (Fig. 22.8). A dendrite is an extension of the neuron cell body that conducts signals toward the cell body. The cell body contains the major concentration of the cytoplasm and the nucleus of the neuron. An axon is an extension that conducts nerve impulses away from the neuron cell body to other cells. The brain and spinal cord contain many neurons, whereas nerves contain only bundles of axons of neurons. The dendrites and cell bodies of these neurons are located in the spinal cord or brain, depending on whether the nerve is a spinal nerve or a cranial nerve. In addition to neurons, nervous tissue contains neuroglia, cells that support and nourish neurons. They outnumber neurons nine to one and take up more than half the volume of the brain. Although their primary function is support, research is currently being conducted to determine how much neuroglia directly contribute to brain function. Schwann cells are a type of neuroglia that encircle long nerve fibers within nerves, forming a protective coating on the axons called a myelin sheath. The presence of myelin sheaths insulates the axon and thus allows nerve impulses to travel much more quickly down its length. 22.1

From Spores to Fertilization

Now let's examine the flowering plant life cycle in more detail. As you may recall, the sporophyte of seed plants produces two types of spores: microspores, which become male gametophytes (mature pollen grains), and megaspores, which become the female gametophyte, or embryo sac. Just exactly where does this happen, and how do these events contribute to the life cycle of flowering plants? Microspores develop into pollen grains in the anthers of stamens (Fig. 21.15). A pollen grain at first consists of two cells. The larger cell will eventually produce a pollen tube. The smaller cell divides, either right away or later, to become two sperm. This is why the stamen is called the "male" portion of the flower. Pollen grains are distinctive to the particular plant (Fig. 21.16), and pollination in flowering plants is simply the transfer of pollen from the anther to the stigma of a carpel. Plants often have adaptations that favor cross-pollination, which occurs when the pollen landing on the stigma is from a different plant of the same species. For example, the carpels may mature only after the anthers have released their pollen. Cross-pollination may also be brought about with the assistance of an animal pollinator. If a pollinator, such as a bee, goes from flower to flower of only one type of plant, cross-pollination is more likely to occur in an efficient manner. The secretion of nectar is one way that plants attract insects; over time, certain pollinators have become adapted to reach the nectar of only one type of flower. In the process, pollen is inadvertently picked up and taken to another plant of the same type. Plants attract particular pollinators in still other ways. For example, through the evolutionary process, some species have flowers that smell of rotting flesh; they are called carrion flowers, or stinking flowers. The putrid smell attracts flies, which in turn pick up the pollen and move it to another "rotting" plant! Figure 21.15 also shows the development of the megaspore. In an ovule, within the ovary of a carpel, meiosis produces four megaspores. One of the cells develops into the female gametophyte, or so-called embryo sac, which is a seven-celled structure containing a single egg cell. Fertilization of the egg occurs after a pollen grain lands on the stigma of a carpel and develops a pollen tube. A pollen grain that has germinated and produced a pollen tube is the mature male gametophyte (see Fig. 21.15, middle). A pollen tube contains two sperm. Once the sperm reaches the ovule, double fertilization occurs. One sperm unites with the egg, forming a 2n (diploid) zygote. The other sperm unites with two nuclei centrally placed in the embryo sac, forming a 3n (triploid) endosperm cell.

How quickly does epithelial tissue renew?

On average, if there is no injury and the body is simply replacing worn-out cells, a new epithelial cell, such as the kind in your skin, can renew and move to the top of the five layers of (thick) skin in about a month. When there is an injury or damage, various hormones, such as epidermal growth factor, will speed up the renewal process during wound healing, and it may take only a week or two.

Why does your nose run when you are cold?

One of the functions of the mucosal lining in the nose is that the mucus helps to warm the air entering the lungs. The tissue lining the nose is highly vascularized (lots of blood vessels), and combining the circulation of the blood with the mucus warms the cold air in the nostrils, so that it will be closer to body temperature when it gets inside the lungs. Warming the inhaled air means that the internal body temperature does not fluctuate too greatly and cause adverse effects on homeostasis.

Propagation of Plants in Tissue Culture

One of the major disadvantages of most asexual propagation techniques is that they also propagate pathogens. Plant pathogens can be viruses, bacteria, or fungi, and clones created from an infected parent will also be infected. However, it is possible to maintain plants in a disease-free status if clones from an uninfected parent are made in sterile test tubes through tissue culture. Hence, tissue culture is simply plant propagation done in a laboratory under sterile conditions. If you were to take one cell from an animal and try to grow it in a test tube, you would not be able to make another whole animal. On the other hand, if you take one cell from a plant you can grow an entirely new plant. This capacity of one plant cell to give rise to a mature plant is called totipotency and is the reason plant tissue culture is so successful. Techniques for tissue culture vary, but most begin with cells from the meristem of the parent plant. Meristematic cells are grown in a sterile, jellylike substrate in flasks, tubes, or petri dishes. The substrate, also called a medium, contains growth hormones, vitamins, and macro- and micronutrients and provides dividing plant cells with all the support, nutrients and water they need. Initially, a mass of undifferentiated (unspecialized) cells, called a callus, forms (Fig. 21.23a). The addition of more nutrients and hormones will initiate organ formation until a fully developed plantlet is formed (Fig. 21.23b,c). Plantlets can then be shipped off in their sterile containers to growers for transplantation into soil pots or the field (Fig. 21.23d). Tissue culture is an important technique for propagating many fruits and vegetables found in local supermarkets. As described in the chapter opener, bananas are sterile fruit that do not produce seeds. The only way to provide this commercially important fruit for the whole world is through tissue culture. Asparagus is a dioecious plant, and all commercial stalks are male. The female stalks favor the production of flowers and are undesirable for eating. Tissue culture is therefore a more efficient means of producing disease-free male asparagus for growers. Many botanical gardens and universities use tissue culture for plant conservation. The Atlanta Botanical Garden has a tissue culture lab that propagates native species of orchids and lilies. The propagated plants are shared with local nurseries, resulting in fewer plants being removed from the wild by collectors. Tissue culture of rare species is also used to help increase their populations in the wild, as they are replanted in native habitats (Fig. 21.24). When the desired product is not an entire plant but merely a substance the plant produces, scientists can use a technique called cell suspension culture. In one method, rapidly growing calluses are cut into small pieces and shaken in a liquid nutrient medium, so that single cells or small clumps of cells break off and form a suspension. The target chemical is then extracted from the liquid the cells are growing in. Cell suspension cultures of cells from the quinine tree produce quinine, a drug used to combat malaria, and those of the woolly foxglove produce digitoxin, used to treat certain types of heart disease.

Carnivorous Plants Are Adapted to Harsh Conditions

Pitcher plants are unusual in that they like to grow in areas that lack nitrogen and phosphorus, which are required nutrients for plants. What's their secret? Pitcher plants feed on animals, particularly insects, to get those necessary nutrients. That's right—like a few other types of plants, pitcher plants are carnivorous. A pitcher plant is named for its leaves, which are shaped like a container we call a pitcher. The pitcher attracts flying insects because it has a scent, is brightly colored, and provides nectar that insects can eat. When an insect lands on the lip of the pitcher, a slippery substance and downward-pointing hairs encourage it to slide into a pit. There, juices secreted by the leaf begin digesting the insect. The plant then absorbs these nutrients into its tissues. Although the pitcher is designed to attract insects, animals as large as rats have been found in pitcher plants! The unique adaptations of pitcher plants allow them to thrive in hostile environments where few plants can survive. Some animals are able to turn the tables on the pitcher plant by taking advantage of the liquid it provides. Larvae of mosquitoes and flies have been known to develop safely inside a pitcher plant. Carnivorous plants, such as the pitcher plant, are fairly unusual in the plant world, but so are many others, such as the coastal redwoods—the tallest trees on planet Earth. In this chapter, you will learn about the organs and structures of flowering plants, the nutrients they need, and how they transport those nutrients within their tissues.

Plant Nutrition

Plant nutrition is remarkable to us because plants require only inorganic nutrients, and from these they make all the organic compounds that compose their bodies. Of course, they require carbon, hydrogen, and oxygen, which they can acquire from carbon dioxide and water, but the other elements, or minerals, that they need they obtain from the environment. An element is termed an essential nutrient if a plant cannot live without it. The essential nutrients are divided into macronutrients and micronutrients, according to their relative concentrations in plant tissue. The following diagram indicates which are the macronutrients and which are the micronutrients: look a picture Notice that iron (Fe) is considered to be a micronutrient for some plants and a macronutrient for others. All of these elements play vital roles in plant cells, but deficiencies of the macronutrients tend to be the most devastating. Nitrogen, for example, is necessary for protein production. The carnivorous plants featured in the chapter opener have evolved to capture insects because of the lack of nitrogen in boggy soils. Traditional farming depletes the nitrogen in soil, and many farmers turn to crop rotation to keep soils healthy. Figure 20.16 shows how an insufficient supply of the macronutrient nitrogen can affect the growth of a plant. The micronutrients are often cofactors for enzymes in various metabolic pathways. Cofactors are elements that ensure that enzymes have the correct shape. It's interesting to observe that humans make use of a plant's ability to take minerals from the soil. For example, we depend on plants for supplies of iron to help carry oxygen to our cells. Eating plants provides minerals such as copper and zinc, which are also cofactors for our own enzymes.

Monocots Versus Eudicots

Plant organs are arranged in different patterns depending on the type of flowering plant. Figure 20.7 shows how flowering plants can be divided into two major groups. One of the differences between the two groups concerns the cotyledons, which are embryonic leaves present in seeds. The cotyledons wither after the first true leaves appear. Plants whose embryos have one cotyledon are known as monocotyledons, or monocots. Other plant embryos have two cotyledons, and these plants are known as eudicotyledons, or eudicots (true dicots). The cotyledons of eudicots supply nutrients for seedlings, but the cotyledons of monocots store some nutrients and act as a transfer tissue for nutrients stored elsewhere. Although the distinction between monocots and eudicots may seem minimal at first glance, there are many differences in their structures. For example, a microscopic view would show that the location and arrangement of vascular tissue differ between monocots and eudicots. Recall that vascular plants contain two main types of transport tissue: the xylem for water and minerals and the phloem for organic nutrients. In a sense, xylem and phloem are to plants what veins and arteries are to animals. In the monocot root, vascular tissue occurs in a ring around the center. But in the eudicot root, vascular tissue is located in the center. The xylem forms a star shape, and phloem is located between the points of the star. In a stem, vascular tissue occurs in vascular bundles. The vascular bundles are scattered in the monocot stem, and they occur in a ring in the eudicot stem. To the naked eye, the vascular tissue forms leaf veins. In monocots, the veins are parallel, while in eudicots, the leaf veins form a netlike pattern. Monocots and eudicots also have different numbers of flower parts, and this difference will be discussed further in Section 21.3. The eudicots are the larger group and include some of our most familiar flowering plants—from dandelions to oak trees. The monocots include grasses, lilies, orchids, and palm trees, among others. Some of our most significant food sources are monocots, including rice, wheat, and corn. 20.2

Plant Cells and Tissues

Plants have levels of biological organization similar to those of animals (see Fig. 1.2). As in animals, a cell is a basic unit of life, and a tissue is composed of specialized cells that perform a particular function. An organ is a structure made up of multiple tissues. When a plant embryo first begins to develop, the first cells are called meristem cells (Fig. 20.1). Meristem cells organize into meristem tissue, allowing a plant to grow its entire life. Even a 5,000-year-old tree is still growing! Early on, meristem tissue is present at the very top and the very bottom of a plant. Because these areas of tissue are located at the ends, they are called the apical meristems. Apical meristems develop (differentiate) into the three types of specialized tissues of the plant body: 1. Epidermal tissue forms the outer protective covering of a plant. 2. Ground tissue fills the interior of a plant and helps carry out the functions of a particular organ. 3. Vascular tissue transports water and nutrients in a plant and provides support. Plants not only grow up and down, but they can also grow wider. The vascular cambium is another type of meristem that gives rise to new vascular tissue called secondary growth. Secondary growth causes a plant to increase in girth.

Woody Stems

Plants with woody stems, such as trees and shrubs, experience both primary and secondary growth. Secondary growth increases the girth of stems, branches, and roots. It occurs because of a difference in the location and activity of vascular cambium, which, as you may recall from Section 20.1, is a type of meristem tissue. In herbaceous eudicots, vascular cambium is usually present between the xylem and phloem of each vascular bundle. In woody plants, the vascular cambium forms a ring of meristem that produces new xylem and phloem each year. In an older woody stem, vascular cambium occurs between the bark and the wood (Fig. 20.12). Bark The bark of a tree contains cork, cork cambium, cortex, and phloem. It is very harmful to remove the bark of a tree, because without phloem, organic nutrients cannot be transported. Although new phloem tissue is produced each year by vascular cambium, it does not build up in the same manner as xylem. Cork cambium is located beneath the epidermis and is another region of active cell division. When cork cambium first begins to divide, it produces tissue that disrupts the epidermis and replaces it with cork cells. Cork cells are impregnated with suberin, a waxy layer that makes them waterproof but also causes them to die. In a woody stem, gas exchange is impeded, except at lenticels, which are pockets of loosely arranged cork cells not impregnated with suberin. Wood When a plant first begins growing, the xylem is made by the apical meristem. Later, as the plant matures, xylem is made by the vascular cambium and is called secondary xylem. Wood is secondary xylem that builds up year after year, thereby increasing the girth of a tree. In trees that have a growing season, vascular cambium is dormant during the winter and becomes active again in the spring when temperatures increase and water becomes more available. In the spring, vascular cambium produces secondary xylem tissue that contains wide vessels with thin walls. This is called spring wood, and the wider vessels transport sufficient water to the growing leaves. In summer, there is less rain and the wood at this time, called summer wood, has a lower proportion of vessels. Strength is required because the tree is growing larger, and summer wood contains numerous thick-walled tracheids. At the end of the growing season, just before the vascular cambium becomes dormant again, only heavy fibers with especially thick secondary walls may develop. When the trunk of a tree has spring wood followed by summer wood, the two together make up one year's growth, or an annual ring (see Fig. 20.12). A dendrochronologist is a scientist who studies annual rings to determine the ages of trees. Annual rings can also give clues on water availability and forest fires in the past, and they support predictions about global climate change. Wood is one of the most useful and versatile materials known. It is used for building structures and for making furniture. Much wood is also used for heating and cooking, as well as for producing paper, chemicals, and pharmaceuticals. The resins derived from wood are used to make turpentine and rosin.

Plasma

Plasma is composed mostly of water (90-92%) and proteins (7-8%), but it also contains smaller quantities of many types of molecules, including nutrients, wastes, and salts. The salts and proteins are involved in buffering the blood, effectively keeping the pH near 7.4, slightly basic. They also maintain the blood's osmotic pressure, so that water has an automatic tendency to enter blood capillaries. Several plasma proteins are involved in blood clotting, and others transport large organic molecules in the blood. Albumin, the most plentiful of the plasma proteins, transports bilirubin, a breakdown product of hemoglobin. Lipoproteins transport cholesterol.

Platelets and Blood Clotting

Platelets (also called thrombocytes) result from the fragmentation of certain large cells, called megakaryocytes, in the red bone marrow. Platelets are produced at a rate of 200 billion a day, and a small drop of whole blood contains 150,000 to 300,000. Platelets are involved in blood clotting, or coagulation. Blood contains at least 12 clotting factors that participate in clot formation. Hemophilia is an inherited clotting disorder in which the liver is unable to produce one of the clotting factors. The slightest bump can cause the affected person to bleed into the joints, and this leads to degeneration of the joints. Bleeding into muscles can lead to nerve damage and muscular atrophy. The most frequent cause of death due to hemophilia is bleeding into the brain. Prothrombin and fibrinogen, two proteins involved in blood clotting, are manufactured and deposited in blood by the liver. Vitamin K, found in green vegetables and formed by intestinal bacteria, is necessary for the production of prothrombin; if this vitamin is missing from the diet, hemorrhagic disorders can develop. A series of reactions leads to the formation of a blood clot (Fig. 23.16). When a blood vessel in the body is damaged, platelets clump at the site of the puncture and form a plug, which temporarily seals the leak. Platelets and the injured tissues release a clotting factor, called prothrombin activator, that converts prothrombin to thrombin. This reaction requires calcium ions (Ca2+). Thrombin, in turn, acts as an enzyme that severs two short amino acid chains from each fibrinogen molecule. These activated fragments then join end to end, forming long threads of fibrin. Fibrin threads wind around the platelet plug in the damaged area of the blood vessel and provide the framework for the clot. Red blood cells also are trapped within the fibrin threads; these cells make a clot appear red. Clot retraction follows, during which the clot gets smaller as platelets contract. A fluid called serum is squeezed from the clot. A fibrin clot is present only temporarily. As soon as blood vessel repair is initiated, an enzyme called plasmin destroys the fibrin network and restores the fluidity of the plasma.

Do "magical" plants really exist?

Strange and poisonous plants have been made famous by popular series such as Harry Potter and The Hunger Games. But do these types of plants really exist? Yes! Mandrake roots look like a human body, and giant hogweed leaves will cause painful blisters to sprout on the skin. Cuckoopint ("bloody man's finger") will cause the tongue to swell, and the black berries of the deadly nightshade are indeed lethal.

How big are neurons and how quickly do they communicate?

The average adult human brain weighs about 3 pounds and contains over 100 billion neurons. Some of the neurons—specifically, the axons of the neurons—are less than a millimeter in length; others, such as the axons from the spinal cord to a muscle in the foot, can be 3 feet long or more. The speed at which transmission occurs can vary as well. It can be as slow as 0.5 meter/second or as fast as 120 meters/second—268 mph!

Transport and Protection

The cardiovascular system (Fig. 22.9a) consists of blood, the heart, and the blood vessels that carry blood throughout the body. The body's cells are surrounded by a liquid called interstitial fluid. Blood transports nutrients and oxygen to interstitial fluid for the cells and removes waste molecules, excreted by cells, from the interstitial fluid. The lymphatic system (Fig. 22.9b) consists of lymphatic vessels, lymph, lymph nodes, and other lymphatic organs. Lymphatic vessels absorb fat from the digestive system and collect excess interstitial fluid, which is returned to the blood in the cardiovascular system. The cardiovascular and lymphatic systems are also involved in the protection of the body against disease. Along with the thymus and spleen, certain cells in the lymph and blood are part of the immune system, which specifically protects the body from disease.

Respiratory System

The cells of your body are bathed in a fluid, called interstitial fluid. The cells acquire oxygen and nutrients and get rid of carbon dioxide and other wastes through exchanges with the interstitial fluid. In turn, the interstitial fluid exchanges these compounds with the blood (see Section 23.2). Blood is refreshed because the respiratory, urinary, and digestive systems make exchanges with the external environment. Only in this way is blood cleansed of waste molecules and supplied with the oxygen and nutrients the cells require. In this section we will focus on the role of the respiratory system in the exchange of gases. Notice in Figure 24.1 that, when blood enters the lungs of the respiratory system, it gives up carbon dioxide (CO2) and picks up oxygen (O2). Carbon dioxide exits the body through exhalation, and oxygen, obtained through inhalation, is delivered to the body's cells. In this way, respiration, also referred to as ventilation, contributes to homeostasis. Respiration in terrestrial vertebrates (including humans) requires these steps: 1. Breathing: inspiration (entrance of air into the lungs) and expiration (exit of air from the lungs) 2. External exchange of gases between the air and the blood within the lungs 3. Internal exchange of gases between blood and interstitial fluid and the exchange of gases between the cells and interstitial fluid Regardless of the particular gas-exchange surfaces of animals and the manner in which gases are delivered to the cells, in the end oxygen enters mitochondria, where aerobic cellular respiration takes place. Without the delivery of oxygen to the body's cells, ATP is not produced, and life ceases. Carbon dioxide, a waste molecule given off by cells, is a by-product of cellular respiration (see Section 7.1).

Open and Closed Circulatory Systems

The circulatory system of an animal delivers oxygen and nutrients to cells and removes carbon dioxide and waste materials. In some animals, the body plan makes a circulatory system unnecessary. In a hydra (Fig. 23.1a), cells either are part of a single layer of external cells or line the gastrovascular cavity. In either case, each cell is exposed to water and can independently exchange gases and rid itself of wastes. The cells that line the gastrovascular cavity are specialized to carry out digestion. They pass nutrient molecules to other cells by diffusion. In a planarian (Fig. 23.1b), the digestive cavity branches throughout the small, flattened body. No cell is very far from one of the digestive branches, so nutrient molecules can diffuse from cell to cell. Similarly, diffusion meets the respiratory and excretory needs of the cells. Some other animals, such as nematodes and echinoderms, rely on the movement of fluid within a body cavity (or coelom) to circulate gases, nutrients, and wastes.

Tissues of a Root

The five tissues of a root are as follows: 1. Vascular tissue. Both monocot and eudicot roots have vascular cylinders that contain xylem and phloem, but the cylinders are arranged differently. In a eudicot root (Fig. 20.13), the xylem is star-shaped because several xylem arms radiate from a common center. Phloem is found in separate regions between the points of the star. In a monocot root (Fig. 20.14), the vascular cylinder consists of alternating xylem and phloem bundles that surround a pith. Pith can function as a storage site. 2. Endodermis. The endodermis of a root is a single layer of rectangular cells that fit snugly together. A layer of impermeable material on all but two sides forces water and minerals to pass through endodermal cells. In this way, the endodermis regulates the entrance of minerals into the vascular tissue of the root. 3. Pericycle. The pericycle is the first layer of cells inside the endodermis of the root. These cells can continue to divide and form lateral roots. 4. Cortex. Large, thin-walled parenchyma cells make up the cortex of the root. The cells contain starch granules, and the cortex may function in food storage. 5. Epidermis. The epidermis, which forms the outer layer of the root, consists of only a single layer of largely thin-walled, rectangular cells. In the zone of maturation, many epidermal cells have root hairs. Just as stems and leaves are diverse, so are roots. A carrot plant has one main taproot, which stores the products of photosynthesis (Fig. 20.15a). Grass has fibrous roots that cling to the soil (Fig. 20.15b), and corn plants have prop roots that grow from the stem to provide additional support (Fig. 20.15c).

Flowers

The flower is a reproductive structure that is unique to the angiosperms (Fig. 21.12). Flowers produce the spores and protect the gametophytes. They often attract pollinators, which help transport pollen from plant to plant. Flowers also produce the fruits that enclose the seeds. The success of angiosperms, with over 270,000 species, is largely attributable to the evolution of the flower. In monocots, flower parts occur in threes and multiples of three; in eudicots, flower parts are in fours or fives and multiples of four or five (Fig. 21.13). A typical flower has four whorls of modified leaves attached to a receptacle at the end of a flower stalk. 1. The sepals are the most leaflike of all the flower parts. They are usually green but some resemble petals (see Figs. 21.12 and 21.13a). Sepals protect the bud as the flower develops. 2. An open flower also has a whorl of petals, whose color accounts for the attractiveness of many flowers. The size, shape, and color of petals are attractive to specific pollinators. Wind-pollinated flowers may have no petals at all. 3. Stamens are the "male" portion of the flower. Each stamen has two parts: the anther, a saclike container, and the filament, a slender stalk. Pollen grains develop from the microspores produced in the anther. 4. At the very center of a flower is the carpel, a vaselike structure that represents the "female" portion of the flower. A carpel usually has three parts: the stigma, an enlarged, sticky knob; the style, a slender stalk; and the ovary, an enlarged base that encloses one or more ovules. The ovule becomes the seed, and the ovary becomes the fruit. A flower can have a single carpel or multiple carpels. Sometimes several carpels are fused into a single structure, in which case the ovary has several chambers, each of which contains ovules. A carpel usually contains many ovules, which increases the number of seeds the plant may produce. Not all flowers have sepals, petals, stamens, and a carpel. Those that do are said to be complete, and those that do not are said to be incomplete. Flowers that have both stamens and carpels are called bisexual flowers; those with only stamens are male flowers. Those with only carpels are female flowers. If both male and female flowers are on one plant, the plant is called monoecious (Fig. 21.14). But if male and female flowers occur on separate plants, the plant is called dioecious. Holly trees are dioecious, and if red berries are a priority, it is necessary to acquire a plant with male flowers and another plant with female flowers.

Formed Elements

The formed elements are red blood cells, white blood cells, and platelets. Among the formed elements, red blood cells, also called erythrocytes, transport oxygen. Red blood cells are small, biconcave disks that at maturity lack a nucleus and contain the respiratory pigment hemoglobin. There are 6 million red blood cells in a small drop of whole blood, and each one of these cells contains about 250 million hemoglobin molecules. Hemoglobin contains iron, which combines loosely with oxygen; in this way, red blood cells transport oxygen. If the number of red blood cells is insufficient, or if the cells do not have enough hemoglobin, the individual suffers from anemia and has a tired, run-down feeling. At high altitudes, where oxygen levels are lower, red blood cell formation is stimulated. If an individual lives and works in a high altitude long enough, he or she will develop a greater number of red blood cells. Red blood cells are manufactured continuously within certain bones, namely the skull, the ribs, the vertebrae, and the ends of the long bones. The hormone erythropoietin (EPO) stimulates the production of red blood cells. The kidneys produce erythropoietin by acting on a precursor made by the liver. Now available as a drug, erythropoietin is helpful to persons with anemia, but it has also been abused by athletes to enhance their performance. Before they are released from the bone marrow into the blood, red blood cells synthesize hemoglobin and lose their nuclei. After living about 120 days, they are destroyed, chiefly in the liver and spleen, where they are engulfed by large, phagocytic cells. When red blood cells are destroyed, hemoglobin is released. The iron is recovered and returned to the red bone marrow for reuse. Another portion of the molecules (i.e., heme) undergoes chemical degradation and is excreted by the liver as bile pigments in the bile. The bile pigments are primarily responsible for the color of feces. White blood cells, also called leukocytes, help fight infections. White blood cells differ from red blood cells in that they are usually larger and have a nucleus, they lack hemoglobin, and they appear translucent if unstained. Figure 23.15 shows the appearance of the various types of white blood cells. When they are stained, white blood cells appear light blue unless they have granules that bind with certain stains. There are approximately 5,000-11,000 white blood cells in a small drop of whole blood. Growth factors are available to increase the production of all white blood cells, and these are helpful to people with low immunity, such as AIDS patients. Red blood cells are confined to the blood, but white blood cells are able to squeeze between the cells of a capillary wall. Therefore, they are found in interstitial fluid, lymph, and lymphatic organs. When an infection is present, white blood cells greatly increase in number. Many white blood cells live only a few days—they probably die while engaging pathogens. Others live months or even years. When microbes enter the body due to an injury, the body's response is called an inflammatory response because swelling, reddening, heat, and pain occur at the injured site (see Section 26.2). Damaged tissue releases kinins, which dilate capillaries, and histamines, which increase capillary permeability. White blood cells called neutrophils, which are amoeboid, squeeze through the capillary wall and enter the interstitial fluid, where they phagocytize foreign material. White blood cells called monocytes come on the scene next and are transformed into macrophages—large, phagocytizing cells that release white blood cell growth factors. Soon the number of white blood cells increases explosively. A thick, yellowish fluid called pus contains a large proportion of dead white blood cells that have fought the infection. Lymphocytes, another type of white blood cell, also play an important role in fighting infection. Lymphocytes called T cells attack the body's cells that are infected with viruses. Lymphocytes called B cells produce antibodies to protect the body against certain types of antigens, which don't belong to the body. An antigen is most often a protein but sometimes a polysaccharide. Antigens are present in the outer covering of parasites or in their toxins. When antibodies combine with antigens, the complex is often phagocytized by a macrophage. An individual is actively immune when a large number of B cells are all producing the antibody needed to fight a particular infection. The role of lymphocytes in the immune response will be explored in greater detail in Section 26.3.

The Pulmonary and Systemic Circuits

The human cardiovascular system includes two major circulatory pathways, the pulmonary circuit and the systemic circuit. The pulmonary circuit moves blood to and from the lungs, where O2-poor blood becomes O2-rich blood. The systemic circuit moves blood to and from the other tissues of the body. The function of the systemic circuit is to serve the metabolic needs of the body's cells. Figure 23.10 traces the path of blood in both circuits. Pulmonary Circuit O2-poor blood from all regions of the body collects in the right atrium and then passes into the right ventricle. The pulmonary circuit begins when the right ventricle pumps blood to the lungs via the pulmonary trunk and the pulmonary arteries. As blood passes through pulmonary capillaries, carbon dioxide is given off and oxygen is picked up. O2-rich blood returns to the heart via the pulmonary veins. The pulmonary veins enter the left atrium. Systemic Circuit O2-rich blood enters the left atrium from the lungs and passes into the left ventricle. The systemic circuit begins when the left ventricle pumps the blood into the aorta. Arteries branching from the aorta carry blood to all areas and organs of the body, where it passes through capillaries and collects in veins. Veins converge on the venae cavae, which return the O2-poor blood to the right atrium. In the systemic circuit, arteries contain O2-rich blood and are bright red; veins contain O2-poor blood and appear dull red or, when viewed through the skin, blue. A portal system begins and ends in capillaries. For example, the hepatic portal vein takes blood from the intestines to the liver. The liver, an organ of homeostasis, modifies substances absorbed by the intestines and monitors the normal composition of the blood. The hepatic veins (see Fig. 25.12) carry blood out of the liver into the inferior vena cava.

The Human Respiratory Tract

The human respiratory system includes all of the structures that conduct air in a continuous pathway to and from the lungs (Fig. 24.2), the major organ of gas exchange in the body. As air moves through the respiratory tract, it is filtered, so that it is free of debris, warmed, and humidified. By the time the air reaches the lungs, it is at body temperature and saturated with water. In the nose, hairs and cilia act as filtering devices. In the respiratory passages, cilia beat upward (Fig. 24.3), carrying mucus, dust, and other particles in the air into the throat, where the accumulation may be swallowed or spit out. Smoking cigarettes and cigars inactivates and eventually destroys these cilia, so that the lungs become laden with soot and debris. This is the first step toward various lung disorders. As the air moves out of the respiratory tract, it cools and loses its moisture. As air cools, it deposits its moisture on the lining of the tract, and the nose may even drip as a result of this condensation. However, the air still retains so much moisture that, on a cold day, it forms a small cloud when we breathe out.

Lower Respiratory Tract

The lower respiratory tract contains the respiratory tree, consisting of the trachea, bronchi, and bronchioles (see Fig. 24.2). The trachea, commonly called the windpipe, is a tube connecting the larynx to the bronchi. The trachea is held open by a series of C-shaped, cartilaginous rings that do not completely meet in the rear. This arrangement allows food to pass down through the esophagus, which lies right behind the trachea in the neck, without the rings of cartilage damaging the outer tissue of the esophagus. The trachea divides into two primary bronchi, which enter the right and left lungs. Bronchitis is an infection of the bronchi. As bronchitis develops, a nonproductive cough becomes a deep cough that produces mucus and perhaps pus. The deep cough of smokers indicates that they have bronchitis and the respiratory tract is irritated. When a person stops smoking, this progression reverses and the airways become healthy again. Chronic bronchitis is the second step toward emphysema and lung cancer caused by smoking cigarettes. Lung cancer often begins in the bronchi, and from there it spreads to the lungs. The bronchi continue to branch until there are a great number of smaller passages called bronchioles. The two bronchi resemble the trachea in structure, but as the passages divide and subdivide, their walls become thinner, the rings of cartilage disappear, and the amount of smooth muscle increases. During an attack of asthma, the smooth muscle of the bronchioles contracts, causing constriction of the bronchioles and characteristic wheezing (see the chapter opener). Each bronchiole terminates in an elongated space enclosed by a multitude of little air pockets, or sacs, called alveoli (sing., alveolus), which make up the lungs (see Fig. 24.8). Respiration in Other Animals Whereas humans have one trachea, insects have many tracheae, little air tubes supported by rings of chitin that branch into every part of the body (Fig. 24.4). The tracheal system begins at spiracles, openings that perforate the insect's body wall, and ends in very fine, fluid-filled tubules, which may actually indent the plasma membranes of cells to come close to mitochondria. Ventilation is assisted by the presence of air sacs that draw in the air. Since no cell is very far from the site of gas exchange, the bloodstream does not transport oxygen, and no oxygen-carrying pigment is required.

Lymphatic System

The lymphatic system consists of lymphatic vessels and various lymphatic organs (Figure 23.11). The lymphatic system serves many functions in the body. The lymphatic vessels take up fat in the form of lipoproteins from the digestive tract and transport it to the circulatory system. As you will see in Section 26.1, the lymphatic system also works with the immune system to help defend the body against disease. In this chapter, we are interested in the lymphatic vessels that take up excess interstitial fluid and return it to cardiovascular veins in the shoulders, namely the subclavian veins. Lymphatic vessels are quite extensive; most regions of the body are richly supplied with lymphatic capillaries. The construction of the larger lymphatic vessels is similar to that of cardiovascular veins, including the presence of valves. Also, the movement of lymph within these vessels is dependent on skeletal muscle contraction. When the muscles contract, the lymph is squeezed past a valve that closes, preventing the lymph from flowing backward. The lymphatic vessels work very closely with the cardiovascular system (Fig. 23.12). The lymphatic system is a one-way system that begins at the lymphatic capillaries. These capillaries take up excess interstitial fluid. This is fluid that has diffused from the cells and capillaries, but has not been reabsorbed back into the capillaries. Once the interstitial fluid enters the lymphatic vessels, it is called lymph. The lymphatic capillaries join to form larger lymphatic vessels that merge before entering one of two ducts: the thoracic duct or the right lymphatic duct. The thoracic duct is much larger than the right lymphatic duct. It serves the lower limbs, abdomen, left arm, and left sides of both the head and the neck. The right lymphatic duct serves the right arm, the right sides of both the head and the neck, and the right thoracic area. The lymphatic ducts enter the subclavian veins.

Connective Tissue Connects and Supports

The many types of connective tissue (Fig. 22.5) are all involved in binding structures of the body together and providing support and protection. As a rule, connective tissue cells are widely separated by a matrix, a noncellular material that varies from solid to semifluid to fluid. The matrix usually has fibers—notably, collagen fibers. Collagen, used mainly for structural support, is the most common protein in the human body, which gives you some idea of the prevalence of connective tissue.

Control

The nervous system (Fig. 22.11a) consists of the brain, the spinal cord, and associated nerves. The nerves conduct nerve impulses from receptors to the brain and spinal cord. They also conduct nerve impulses from the brain and spinal cord to the muscles and glands, allowing us to respond to both external and internal stimuli. Sensory receptors and sense organs are sometimes considered a part of the nervous system. The endocrine system (Fig. 22.11b) consists of the hormonal glands, such as the thyroid and adrenal glands, which secrete hormones, chemicals that serve as messengers between body parts. Both the nervous and endocrine systems coordinate and regulate the functions of the body's other systems. The nervous system tends to cause quick responses in the body, while the body's responses to hormones released by the endocrine system tend to last much longer. The endocrine system also helps maintain the proper functioning of the male and female reproductive organs.

Reproduction

The reproductive system (Fig. 22.13) involves different organs in the male and female. The male reproductive system consists of the testes, other glands, and various ducts, such as the ductus deferens, that conduct semen to and through the penis. The testes produce sex cells called sperm. The female reproductive system consists of the ovaries, uterine tubes, uterus, vagina, and external genitals. The ovaries produce sex cells called eggs. When a sperm fertilizes an egg, an offspring begins development.

Human Example: Regulation of Body Temperature

The thermostat for body temperature is located in the part of the brain called the hypothalamus. When the core body temperature falls below normal, the control center directs (via nerve impulses) the blood vessels of the skin to constrict (Fig. 22.17). This action conserves heat. If the core body temperature falls even lower, the control center sends nerve impulses to the skeletal muscles, and shivering occurs. Shivering generates heat, and gradually body temperature rises toward 37°C (98.6°F). When the temperature rises to normal, the control center is inactivated. When body temperature is higher than normal, the control center directs the blood vessels of the skin to dilate. More blood is then able to flow near the surface of the body, where heat can be lost to the environment. In addition, the nervous system activates the sweat glands, and the evaporation of sweat helps lower body temperature. Gradually, body temperature decreases to 37°C (98.6°F). Notice that a negative feedback mechanism prevents continued change in the same direction; body temperature does not get warmer and warmer, because warmth stimulates changes that decrease body temperature. Also, body temperature does not get colder and colder, because a body temperature below normal causes changes that bring body temperature up.

Bananas—Mules of the Fruit World

The typical supermarket banana is sterile, meaning that it contains no viable seeds; there is nothing to plant if you want to grow more. This is because in the mid-nineteenth century two varieties of wild bananas were crossed to form the sweet, but sterile, banana that we all enjoy today. So how do farmers grow bananas if there are no seeds? The answer lies in three alternative techniques: asexual reproduction, tissue culture, and genetic engineering. Sometimes, farmers use asexual reproduction—simply cutting a piece of the banana stem or root and planting it directly in the ground. This creates an identical plant, but it may pass on diseases from the first plant. In an effort to obtain new disease-free banana plants, many plantations have turned to tissue culture. In this procedure, a technician scrapes a piece of the apical meristem and puts it in a petri dish containing nutrients and growth hormones. Eventually, new plants form and are transplanted in the field. However, fungi and insects often attack these disease-free plants. These pests can destroy entire banana plantations. In the 1950s, a fungus wiped out banana plantations throughout the Caribbean and Central America. Another technique for growing disease-resistant bananas involves creating genetically engineered plants. Scientists are taking antifungal genes from rice plants and inserting them into banana plants. These modified banana plants are better able to fight off infections. In this chapter, we will explore methods of sexual versus asexual reproduction in plants.

Upper Respiratory Tract

The upper respiratory tract consists of the nasal cavities, pharynx, and larynx (see Fig. 24.2). The nose, a prominent feature of the face, is the only external portion of the respiratory system. The nose contains the nasal cavities, narrow canals separated from one another by a septum composed of bone and cartilage. Tears from the eyes drain into the nasal cavities by way of tear ducts. For this reason, crying produces a runny nose. The nasal cavities are connected to the sinuses, air-filled spaces that reduce the weight of the skull and act as resonating chambers for the voice. If the ducts leading from the sinuses become inflamed, fluid may accumulate, causing a sinus headache. The nasal cavities are separated from the mouth by a partition called the palate. The palate has two portions. Anteriorly, the hard palate is supported by bone; posteriorly, the soft palate is made solely of soft tissue and muscle. The pharynx is a funnel-shaped passageway that connects the nasal cavity and mouth to the larynx, or voice box. The tonsils form a protective ring of lymphatic tissue at the junction of the mouth and the pharynx. Tonsillitis occurs when the tonsils become inflamed and enlarged. If tonsillitis occurs frequently and enlargement makes breathing difficult, the tonsils can be removed surgically, a procedure called a tonsillectomy. In the pharynx, the air passage and food passage cross because the larynx, which receives air, is anterior to the esophagus, which receives food. This arrangement may seem inefficient, since there is danger of choking if food accidentally enters the trachea, but it does have the advantage of letting you breathe through your mouth if your nose is plugged. In addition, it permits greater intake of air during heavy exercise, when a higher rate of gas exchange is required. When swallowing occurs, the epiglottis, an elastic flap of cartilage, covers the glottis, the opening into the larynx, and this helps prevent choking. Air passes from the pharynx through the glottis. The larynx is always open because it is formed by a complex of cartilages, among them the one that forms the "Adam's apple." At the edges of the glottis, embedded in mucous membrane, are the vocal cords. These flexible bands of connective tissue vibrate and produce sound when air is expelled past them through the glottis from the larynx. Laryngitis is an infection of the larynx with accompanying hoarseness, leading to the inability to speak audibly.

Maintenance of the Body

Three systems (respiratory, urinary, and digestive) either add or remove substances from the blood. If the composition of the blood remains constant, so does that of the interstitial fluid. The respiratory system (Fig. 22.10a) consists of the lungs, the trachea, and other structures that take air to and from the lungs. The respiratory system brings oxygen into the body and takes carbon dioxide out through the lungs. It also exchanges gases with the blood. The urinary system (Fig. 22.10b) consists of the kidneys and the urinary bladder, along with the structures that transport urine. This system rids blood of wastes and helps regulate the fluid level and chemical content of the blood. The digestive system (Fig. 22.10c) consists of the organs along the digestive tract, together with associated organs, including the teeth, salivary glands, liver, and pancreas. This system receives food and digests it into nutrient molecules, which then enter the blood.

Homeostasis

To understand homeostasis, there are two types of environments to consider: the external environment, which includes everything outside the body, and the internal environment, which includes our cells, tissues, fluids, and organs. Recall that cells live in a liquid environment called the interstitial fluid. This fluid is constantly renewed with nutrients and gases via exchanges with the blood. Therefore, blood and interstitial fluid constitute part of the body's internal environment. The volume and composition of interstitial fluid remain relatively constant only as long as blood composition remains near normal levels. We say "relatively constant" because the composition of both interstitial fluid and blood varies within an acceptable range. Maintenance of the relatively constant condition of the internal environment within these ranges is called homeostasis. Because of homeostasis, even though external conditions may change dramatically, internal conditions stay within a narrow range. For example, the temperature of the body is maintained near 37°C (97°F to 99°F), even if the surrounding temperature is lower or higher. If you eat acidic foods, the pH of your blood still stays about 7.4, and even if you eat a candy bar, the amount of sugar in your blood remains at just about 0.1%.

Vascular Tissue

Vascular tissue extends from the root through the stem to the leaves, and vice versa. In the root, the vascular tissue is located in a central cylinder; in the stem, vascular tissue can be found in multiple vascular bundles; and in the leaves, it is found in leaf veins. Although both types of vascular tissue are usually found together, they have different functions. Xylem transports water and minerals from the roots to the leaves. Phloem transports sugar, in the form of sucrose, and other organic compounds, such as hormones, often from the leaves to the roots. Xylem contains two types of conducting cells: vessel elements and tracheids (Fig. 20.4a). Both types of conducting cells are hollow and nonliving, but the vessel elements are larger, have perforated end walls, and are arranged to form a continuous pipeline for water and mineral transport. The end walls and side walls of tracheids have pits that allow water to move from one tracheid to another. The conducting cells of phloem are sieve-tube members, which are named because they contain a cluster of pores in their end walls. The pores are collectively known as a sieve plate. The sieve-tube members are arranged to form a continuous sieve tube (Fig. 20.4b). Sieve-tube members contain cytoplasm but no nuclei. Each sieve-tube member has a companion cell, which does have a nucleus. The companion cells may very well be involved in the transport function of phloem.

What do the numbers on a bag of fertilizer mean?

When you buy a bag of fertilizer, you will notice three numbers on the outside of the bag (for example, 20-20-20). These numbers are called the NPK ratio, and they refer to the amounts of nitrogen (N), phosphorus (P), and potassium (K) in the fertilizer. All three of these are macronutrients that are important for plant health and growth. Nitrogen promotes the vegetative growth of the plant, and the plant needs phosphorus to maintain a healthy root system. Potassium is also involved in the general health of a plant and is needed for the formation of the chlorophyll molecules involved in photosynthesis. The optimal amount of each nutrient is dependent on the type of plant; thus, larger numbers do not always signify a better fertilizer.

Monocots Vs Eudicots

Whereas eudicot embryos have two cotyledons, monocot embryos have only one cotyledon. In monocots, the cotyledon stores food, and it absorbs food molecules from the endosperm and passes them to the embryo. In other words, the endosperm is retained in monocot seeds. In eudicots, the cotyledons usually store all the nutrient molecules the embryo uses. Therefore, the endosperm disappears, because it has been taken up by the two cotyledons. A corn plant is a monocot; consequently, in a corn kernel, there is only one cotyledon and the endosperm is present (see Fig. 21.21).