Chapter 22. Organization of the Body

Anatomical Terminology

Anatomical terms describe the location of body parts and various body regions. To correctly use these terms, it is assumed that the body is in the anatomical position. For example, picture yourself in the anatomical position: your body is standing upright and facing forward, and your arms are at your sides with the palms of your hands facing forward. Even if patients are lying down, for consistency and correct communication when you use anatomical terms, always refer to patients as if they are in the anatomical position. (Figure 22-9 demonstrates the anatomical position.) Directional Anatomical Terms The directional anatomical terms that identify the position of body structures compared to other body structures are: superior (cranial), inferior (caudal), anterior (ventral), posterior (dorsal), medial, lateral, proximal, distal, superficial, and deep. For example, the eyes are medial to the ears but lateral to the nose. See Table 22-2 and Figure 22-7 for an explanation and illustration of these important directional terms. Anatomical Terms That Describe Body Sections Sometimes in order to study internal body parts, it helps to imagine the body as being divided into sections. Medical professionals often use the following terms, defined in the bullets, to describe how the body is divided into sections: sagittal, midsagittal, transverse, and frontal (coronal). A sagittal plane divides the body into left and right portions. A midsagittal plane runs lengthwise down the midline of the body and divides it into equal left and right halves. Page 481 A transverse plane divides the body into superior (upper) and inferior (lower) portions. A frontal, or coronal, plane divides the body into anterior (frontal) and posterior (rear) portions. Anatomical Terms That Describe Body Parts Many other anatomical terms describe different regions or parts of the body. For example, the term brachium refers to the arm and the term femoral refers to the thigh. Figure 22-9 illustrates many of the common anatomical terms that describe body parts.

The Study of the Body

Anatomy is the scientific term for the study of body structure. For example, the heart may be described as a hollow, cone-shaped organ that is an average of 14 centimeters long and 9 centimeters wide. Understanding anatomy allows us to understand the normal position of body structures and how to describe these positions precisely and correctly. Physiology is the term for the study of the function of the body's organs. For example, the physiology of the heart can be described by saying that the heart pumps blood into blood vessels to transport nutrients throughout the body. Anatomy and physiology are commonly studied together because they are intimately related. For example, the anatomy of the heart (a hollow, muscular organ) allows it to do its function (pump blood into tubular blood vessels). If the heart was not hollow, it could not allow blood to flow into it. If the heart was not muscular, it could not pump blood. Knowledge of anatomy and physiology will help you grasp the meaning of diagnostic and procedural codes, and help you understand the clinical procedures you will perform and assist with as a medical assistant. Understanding anatomy and physiology can also make it easier to see how and why certain diseases develop. Diseases develop in the body when homeostasis—the relative consistency of the body's internal environment—is not maintained. Body conditions that must remain within a stable range include body temperature, blood pressure, and the concentration of various chemicals within the blood. Individual cells must also maintain homeostasis. For example, if chemicals within a cell change the deoxyribonucleic acid (DNA) or genetic makeup of the cell, that cell can become cancerous.

Body Cavities and Abdominal Regions

Body cavities house and protect the internal organs. The largest body cavities are the dorsal cavity and the ventral cavity. The dorsal cavity is divided into the cranial cavity (which houses the brain) and the spinal cavity (which contains the spinal cord). The ventral cavity is divided into the thoracic cavity and the abdominopelvic cavity. The muscle called the diaphragm separates the thoracic and abdominopelvic cavities. The thoracic cavity contains the: Lungs Heart Page 482 Esophagus Trachea The abdominopelvic cavity is divided into a superior abdominal cavity and an inferior pelvic cavity. It contains the: Stomach Small and large intestines Gallbladder Liver Spleen Kidneys Pancreas The bladder and internal reproductive organs are located in the pelvic cavity, which is depicted in Figure 22-10. The abdominal area is further divided into nine regions or four quadrants, which are illustrated in Figure

Cell Division

Cells can become damaged, diseased, or worn out, and replacements must be made. Also, new cells are needed for normal growth. Cells reproduce by cell division, a process that involves splitting the nucleus, through mitosis or meiosis, and splitting the cytoplasm, called cytokinesis. A cell that carries out its normal daily functions and is not dividing is said to be in interphase. For example, if a liver cell is in interphase, it is making liver enzymes, detoxifying blood, and processing nutrients. During interphase, a cell prepares for cell division by duplicating its DNA and cytoplasmic organelles. For most body cells, each daughter cell will have an exact copy of the DNA and organelles in the original mother cell. Sometimes when the DNA is duplicated, errors called mutations occur. These mutations will be passed on to the descendants (daughter cells) of that cell and may or may not affect the cells in harmful ways. Mutations can be simple changes in the DNA sequence or complete deletions of a gene or part of a gene. Think about this book as a gene; a simple mutation might be a mispelled word or the loss of a sentence, a chapter, or even the whole book. Mitosis Following interphase, a cell may enter mitosis—a part of cell division in which the nucleus divides. During this process, the cell membrane constricts to divide the cell's cytoplasm. This causes the organelles of the original cell to be distributed almost evenly into the two new cells. The stages of mitosis are listed here and pictured in Figure 22-14. Prophase occurs when the centrioles that have replicated just prior to the onset of mitosis move to opposite ends of the cell. As they separate, they create spindle fibers between them. During metaphase, the chromosomes line up in the middle of the cell between the centrioles on these spindle fibers. During anaphase, the centromeres divide, pulling the chromatids (now chromosomes) toward the centrioles at opposite sides of the cell. The final stage is called telophase. As the chromosomes reach the centrioles, each with its complete set, cytokinesis or division of the cytoplasm takes place and mitosis is complete. Remember, during mitosis, the nucleus makes a complete copy of all 23 of its chromosome pairs (46 chromosomes altogether). As the cell divides, each new cell receives a complete set of chromosome pairs. The resulting cells are identical to each other. Meiosis Meiosis is reproductive cell division. It takes place only in the reproductive organs when the male and female sex cells are formed. During meiosis, the nucleus copies all 23 chromosome pairs, but two divisions take place. The four cells that are formed each contain only one of each chromosome pair, for a total of 23 chromosomes. This type of cell division must occur so that when the sex cells combine during fertilization, the resulting cell contains the usual number of chromosomes (46).

Cell Characteristics

Chemicals react to form the complex substances that make up cells, the basic unit of life. The human body is composed of millions of cells. There are many kinds of cells, and each type has a specific function. Most cells have three main parts: cell membrane, cytoplasm (liquid matrix containing each cell's organelles), and the nucleus. Figure 22-13 shows the structure of a composite cell. Cell Membrane The cell membrane is the outer limit of a cell. It is very thin and selectively permeable, which means that it allows some substances to pass through it while preventing other substances from passing through. Think of a fence and gate at an amusement park; people who have a ticket can enter through the gate, those who do not have a ticket are kept behind the fence. The cell membrane is composed of two layers of phospholipids, different types of proteins, cholesterol, and a few carbohydrates. Cytoplasm and Its Organelles The cytoplasm is the "inside" of the cell. Mostly made up of water, proteins, ions, and nutrients, the cytoplasm houses organelles that perform many cell functions, and therefore body system functions. These organelles, described below, include: cilia, the flagellum, ribosomes, the endoplasmic reticulum, mitochondria, the Golgi apparatus, lysosomes, and centrioles. Page 486 Many cells contain hair-like projections on the outside of the cell membrane called cilia. Cilia assist with propelling matter throughout the body tracts, including the respiratory system. Cells with cilia are often found in the mucous membranes. A flagellum is a tail-like structure found on the human sperm cell and provides its "swimming" type of locomotion. Ribosomes, in conjunction with RNA molecules, are responsible for protein synthesis. Amino acids are connected together to form proteins through a specialized process involving different types of RNA molecules. The ribosome supports the protein chain as it is formed. The endoplasmic reticulum comes in two forms—smooth and rough. The rough endoplasmic reticulum is named for the presence of ribosomes on its surface, which give it a bumpy or rough appearance. Both types of endoplasmic reticulum form networks or passageways to transport substances throughout the cytoplasm. Mitochondria—the centers for cell respiration— provide energy for the cell. There may be only one mitochondrion in a cell or many, depending on how much energy each cell type requires. Page 487 The cell's Golgi apparatus is known to process and sort proteins from the ribosome and to synthesize or produce carbohydrates. It is also thought to prepare and store secretions for discharge from the cell. The organelles known as lysosomes perform the cell's digestive function. The centrioles—two cylindrical organelles near the nucleus—are essential to cell division as they equally distribute chromosomes to the resultant "daughter" cells. Nucleus The nucleus of a cell is typically round and located near the center of a cell. It is enclosed by a nuclear membrane that contains nuclear pores, which allow larger substances to move into and out of the nucleus. It contains chromosomes, which are thread-like structures made up of DNA.

Genetic Techniques

DNA is the primary component of genes and is found in the nucleus of most cells within the body. A gene is a segment of DNA that determines a body trait. Genetic techniques involve using or manipulating genes. The chemical structure of every person's DNA is the same. The unique sequence of the nucleotides (groups of molecules that form the basic unit of DNA) determines an individual's characteristics. As an illustration, take the statement "my cat has blue eyes." Think of each letter as one nucleotide. If you change the letter "c" in the statement to an "r" you have an entirely different statement; "my rat has blue eyes." Many genetic differences—excessively large muscles in sheep for instance—are caused by changes in just a few nucleotides. One DNA molecule contains hundreds or thousands of genes. Each gene occupies a particular location on the DNA molecule, making it possible to compare the same gene in a number of different samples. Two widely used genetic techniques in the clinical setting are the polymerase chain reaction (PCR) and DNA fingerprinting. Page 489 Polymerase Chain Reaction The polymerase chain reaction (PCR) is a quick, easy method for making millions of copies of any fragment of DNA. This technique has been revolutionary in the study of genetics and has very quickly become a necessary tool for improving human health. PCR can produce millions of gene copies from tiny amounts of DNA, even from just one cell. This method is especially useful for detecting disease-causing organisms that are impossible to culture, such as many kinds of bacteria, fungi, and viruses. For example, it can detect the AIDS virus sooner than other tests—during the first few weeks after infection. PCR is also more accurate than standard tests. The technique can detect bacterial DNA in children's middle ear fluid, which indicates an infection, even when culture methods fail to detect bacteria. Other diseases diagnosed through PCR include Lyme disease, stomach ulcers, viral meningitis, hepatitis, tuberculosis, and many sexually transmitted infections (STIs), including herpes and chlamydia. PCR is also leading to new kinds of genetic testing because it can easily distinguish among the tiny variations in DNA that all people possess. This testing can diagnose people who have inherited disorders or who carry mutations that could be passed to their children. PCR is also used in tests that determine who may develop common disorders such as heart disease and various types of cancer. This knowledge helps individuals take steps to prevent those diseases. DNA Fingerprinting A DNA "fingerprint" refers to the unique sequences of nucleotides in a person's DNA and is the same for every cell, tissue, and organ of that person. Consequently, DNA fingerprinting is a reliable method for identifying and distinguishing among human beings to establish paternity and identify suspects in criminal cases (see Figure 22-15). It is also used to diagnose genetic disorders such as cystic fibrosis, hemophilia, Huntington's disease, familial Alzheimer's, sickle cell anemia, thalassemia, and many others. Detecting genetic diseases early, or in utero, allows patients and medical staff to prepare for proper treatment. Researchers also use this information to identify DNA patterns associated with genetic diseases.

Heredity and Common Genetic Disorders

Heredity is the transfer of genetic traits from parent to child. When a sperm cell and an ovum (egg) unite, a cell called a zygote forms. The zygote has 46 chromosomes, or 23 chromosomal pairs. One half of each pair comes from the sperm, and the other half from the ovum. The first 22 pairs, which are the same size and shape, are called homologous chromosomes, also known as autosomes. The 23rd pair are called sex chromosomes. If the sex chromosomes are an X chromosome and a Y chromosome, the child is a male. If the sex chromosomes are both X chromosomes, the child is a female. Although the sex chromosomes determine a child's gender, they also determine other body traits. However, the autosomes determine most body traits such as eye color or freckles. Each chromosome possesses many genes. Homologous chromosomes carry the same genes that code for a particular trait, but the genes may be of different forms, which are called alleles. Many times only one allele is actually expressed as a trait even if another allele is present. The allele that is always expressed over the other is a dominant allele. The one that is not expressed is recessive. The only way a recessive allele can be expressed is if no dominant allele is present. Detached earlobes are an example of a trait determined by a dominant allele. If a child inherits a dominant allele for this trait from one parent but inherits the recessive allele from the other parent, the child will have detached earlobes. If the child inherits recessive alleles from both parents, then he or she will have attached earlobes. See Figure 22-16. Most traits in the body are determined by multiple alleles. For example, hair color, height, skin tone, eye color, and body build are each determined by many different genes. Complex inheritance is the term that describes inherited traits that are determined by multiple genes. It explains why different children within the same family can each have different characteristics. Sex-linked traits are carried on the sex chromosomes, X and Y. The Y chromosome is much smaller than the X chromosome and does not carry many genes. So, if the X chromosome carries a recessive allele, it is likely to be expressed because there is usually no corresponding allele on the Y chromosome. For example, the presence of a recessive allele that is always found on the X chromosome determines red—green color blindness. This disorder (like most sex-linked disorders) primarily affects males because the corresponding Y chromosome does not have any allele to prevent the expression of the recessive allele. Genetic influences are known to contribute to many thousands of different health conditions.

Chemistry of Life

Now that you have studied how the body is organized structurally, you need to learn about its chemical structure. Chemistry is the study of what matter is made of and how it changes. It is important to have a basic understanding of chemistry when studying anatomy and physiology because body structures and functions result from chemical processes that occur within body cells or fluids. As you learned earlier in the chapter, the chemical level is the lowest level of organization. The building blocks of every living organism are the same chemical elements that make up all matter, liquids, solids, and gases. When two or more atoms are chemically combined, a molecule is formed. A compound is formed when two or more atoms of different elements are combined. Molecules of oxygen (O2) and hydrogen (H2) are not compounds because they are made up of only one element. Water is an example of a molecule, which is composed of two hydrogen atoms and one oxygen atom. Water is a compound because its molecules are made up of atoms of two different elements—hydrogen and oxygen. Water is critical to both chemical and physical processes in human physiology, and it accounts for approximately two-thirds of a person's body weight. Metabolism is the overall chemical functioning of the body. The two processes of metabolism are anabolism and catabolism. In anabolism, small molecules combine to form larger ones (for example, when amino acids combine to form proteins). In catabolism, larger molecules are broken down into smaller ones (for example, when stored glycogen is converted to glucose molecules for energy). Electrolytes When put into water, some substances release ions, which are either positively or negatively charged particles. These substances are called electrolytes. For example, when you put NaCl in water, it releases two electrolytes: the sodium ion (Na) and the chloride ion (Cl). Electrolytes are critical because the movements of ions into and out of body structures regulate or trigger many physiologic states and activities in the body. For example, electrolytes are essential to fluid balance, muscle contraction, and nerve impulse conduction. Exercising makes you sweat, causing fluid and electrolyte loss. Drinking a sports drink after exercising helps you maintain fluid balance because sports drinks contain water and electolytes such as sodium and potassium. Page 484 Acids and Bases Acids are substances that release hydrogen ions (H+) in water. Many acids, such as lemon juice and vinegar, have a sour taste. Bases are are substances that release hydroxyl ions (OH-) in water. A basic substance may also be referred to as an alkali. Many basic substances are slippery and bitter to the taste. Laundry detergents, bleach, dish soaps, and many other household cleaning agents are examples of basic substances. Testing Acids and Bases In the clinical setting, litmus paper, liquid pH indicator test kits, or a pH meter is often used to determine if a substance is acidic or basic. An acidic substance will turn blue litmus paper red, and a basic substance will turn red litmus paper blue. The pH scale runs from 0 to 14. If a solution has a pH of 7, the solution is neutral, which means it is neither acidic nor basic. If a solution has a pH less than 7, the solution is acidic. If a solution has a pH greater than 7, it is basic, or alkaline. The more acidic a solution is, the higher the concentration of hydrogen ions it contains. The pH values of some common substances are shown in Figure 22-12. Biochemistry Biochemistry is the study of matter and chemical reactions in the body. Matter can be divided into two large categories: organic and inorganic matter. Organic matter contains carbon and hydrogen, and its molecules tend to be large. Inorganic matter generally does not contain carbon and hydrogen; these molecules tend to be small. Examples of inorganic substances are water, oxygen, carbon dioxide, and salts such as sodium chloride. Water is the most abundant inorganic compound in the body. The four major classes of organic matter in the body are carbohydrates, lipids, proteins, and nucleic acids. These are outlined in the following sections. Carbohydrates Body cells depend on carbohydrate molecules to make energy. The carbohydrate most commonly used by the body cells is glucose. A type of carbohydrate commonly found in potatoes, pastas, and breads is starch, which is broken down into glucose when needed. Lipids Lipids are fats. Three types of lipids are found in the body: triglycerides, phospholipids, and steroids. Triglycerides store energy for cells, and phospholipids primarily make cell membranes. Butter and oils are composed of triglycerides, and the body stores these molecules in adipose tissue (fat). Steroids are very large lipid molecules that make cell membranes and some hormones. Cholesterol is an example of an essential steroid for body cells. Proteins Proteins have many functions in the body. Many proteins act as structural materials for the building of solid body parts. Other proteins act as hormones, enzymes, receptors, and antibodies. Nucleic Acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are two examples of nucleic acids. DNA contains the genetic information of cells, and RNA makes proteins. Nucleic acids are made up of nucleotides, which are discussed later in this chapter.

Body Organs and Systems

Organs are structures formed by the organization of two or more different tissue types that work together to carry out specific functions. For example, the heart is made of cardiac muscle tissue, connective tissue, and epithelial tissue. These tissues work together to carry out the heart's function—to effectively pump blood into blood vessels. Organ systems are formed when organs join together to carry out vital functions. For example, the heart and blood vessels unite to form the cardiovascular system. The organs of the cardiovascular system circulate blood throughout the body to ensure that all body cells receive enough nutrients. See Figure 22-6 for a summary of the human body's organ systems, their general functions, and the organs within each.

Organization of the Body

The body's structure can be divided into different levels of organization. The chemical level is the simplest level. It refers to the billions of atoms and molecules in the body. Atoms are the simplest units of all matter, and many are essential to life. Matter is anything that takes up space and has weight. The four most common atoms in the human body are carbon, hydrogen, oxygen, and nitrogen. Molecules are made up of atoms that bond together. Proteins and carbohydrates are examples of molecules that consist of hundreds of atoms. Molecules join together to form organelles, which can be thought of as cell parts. Organelles combine to form cells such as leukocytes (white blood cells), erythrocytes (red blood cells), neurons (nerve cells), and adipocytes (fat cells). Cells are considered to be the smallest living units in the body. When the same types of cells organize together, they form tissues. Two or more tissue types combine to form organs, and organs arrange to form organ systems. Finally, organ systems combine to form an organism. Figure 22-1 illustrates the organization of the body's organ systems.

Movement Through Cell Membranes

The selectively permeable cell membrane controls what moves into and out of a cell. Selective permeability means there is some selection or choice in what substances cross the membrane. Some substances like oxygen and water move across the cell membrane without the use of energy. These movements are called passive mechanisms. Sometimes the cell has to use energy to move a substance across its membrane in movements known as active mechanisms. Large molecules like glucose and proteins move across the cell membrane by active transport. Diffusion Diffusion is the movement of a substance from an area of high concentration of that substance to an area of low concentration of the same substance; it can be described as the spreading out of a substance. Substances that easily diffuse across the cell membrane include gases such as oxygen and carbon dioxide. Osmosis Osmosis is the diffusion or movement of water across a semipermeable membrane, such as a cell membrane. A semipermeable membrane lets water and other solvent liquids through but nothing else. Remember, water will always try to diffuse or move toward the higher concentration of solutes (solids in solution). Filtration In filtration, some type of pressure, such as gravity or blood pressure, forces substances across a membrane that acts like a filter. Filtration separates substances in solutions. For example, you could separate sand from water by pouring the sand/water mixture through a filter. In the body, capillaries in the kidneys act as filters to separate components of blood. Active Transport In active transport, substances move across the cell membrane with the help of carrier molecules, from an area of low concentration of the substance to an area of high concentration of the substance. In other words, substances are gathered together, which is the opposite of diffusion. These molecules create channels in the cell membrane or otherwise change the membrane so large substances can pass through. Think of these carrier or transport molecules as tiny doormen opening the door for large substances to enter an already crowded room. Some substances that move across the cell membrane through active transport include sugars, amino acids, and potassium, calcium, and hydrogen ions.

Major Tissue Types

Tissues are groups of cells that have similar structures and functions. The four major tissue types in the body are epithelial, connective, muscle, and nervous tissue. These are explained more fully in the following paragraphs. Epithelial Tissue When you think of epithelial tissue, think of a covering, lining, or gland. Epithelial tissue covers the body and most organs in the body. It lines the body's tubes, such as blood vessels and the esophagus, and the body's hollow organs, such as the stomach and heart. This type of tissue also lines body cavities, such as the thoracic cavity and the abdominopelvic cavity. Glandular tissue is also classified as a type of epithelial tissue. It is composed of cells that make and secrete (give off) substances. If a gland secretes its product into a duct, as with a sweat or oil (sebaceous) gland, it is called an exocrine gland. If a gland secretes its product directly into surrounding tissue fluids or blood, it does not have ducts and it is called an endocrine gland. The pancreas and thyroid are considered endocrine glands because they release their hormones directly into the bloodstream. Epithelial tissues are avascular, which means they lack blood vessels. However, these tissues have a nerve supply and are very mitotic, meaning they divide constantly. In addition, the cells within epithelial tissues are packed together tightly. Epithelial tissues have many different functions, depending on their location in the body. For example, those covering the body protect against invading pathogens and toxins. Those that line the digestive tract secrete a variety of enzymes needed for digestion. They often possess microvilli, which allow the body to absorb nutrients. Epithelial tissues lining the respiratory tract have cilia and goblet cells. The goblet cells produce mucus that traps small particles that enter the respiratory tract. The cilia constantly push the mucus and trapped particles away from the lungs (see Figure 22-2). Epithelial cells within the kidneys act as filters that help to remove waste products from blood. Connective Tissue Connective tissues are the most abundant tissues in the body. The cells of connective tissues are not packed together tightly. Instead, a matrix separates the cells. Think of the matrix simply as the matter between the cells of connective tissue. It contains fibers, water, proteins, inorganic salts, and other substances. The components of the matrix vary, depending on the type of connective tissue. Connective tissues generally have a rich blood supply, except for cartilage and some dense connective tissues that contain a very poor blood supply. Many different cell types are located in connective tissues; the most common are fibroblasts, mast cells, and macrophages. Fibroblasts make fibers, and mast cells secrete substances like heparin and histamine that promote inflammation when tissue is damaged. Macrophages are cells that destroy unwanted material, like bacteria or toxins. The following sections discuss the body's different connective tissues in more detail. Blood This tissue is composed of red blood cells, white blood cells, platelets, and plasma. Plasma is the matrix of blood. Unlike other connective tissues, this matrix does not contain fibers. Blood transports substances throughout the body. Blood and its cell functions will be discussed in depth in a later chapter. Osseous (Bone) Tissue The matrix of osseous tissue contains mineral salts that make it a very hard tissue. Contrary to popular belief, bone tissue is a living tissue—it is metabolically active. Cartilage The matrix of cartilage is rigid, although it is not as hard as osseous tissue. Cartilage gives shape to structures such as the ears and nose. It also protects the ends of long bones and forms the discs between the vertebrae of the neck and spine. Dense Connective Tissue The matrix of dense connective tissue is packed with tough fibers that make it a soft but very strong tissue. Ligaments, tendons, and joint capsules have large amounts of this tissue type. Ligaments connect bones to bones, tendons connect muscles to bones, and joint capsules surround moveable joints in the body. Dense connective tissues also make up a large part of the skin's dermis. When skin is damaged, this tissue "fills in" the damaged space and forms a scar. Adipose (Fat) Tissue Within adipose tissue, unique cells—adipocytes—store fats. This tissue type stores energy for body cells, cushions body parts and organs, and insulates the body against excessive heat or cold (see Figure 22-3). Muscle Tissue Muscle is a specialized type of tissue that contracts and relaxes. The three types of muscle tissue are: skeletal, visceral (smooth), and cardiac. Skeletal Muscle Tissue As its name suggests, skeletal muscle tissue is attached to the skeleton. This type of tissue is voluntary because we can consciously control its movement. For example, we can consciously decide to contract the skeletal muscles attached to our arm bones and make them move. It is also referred to as striated because the cells of this muscle tissue type have striations or stripes in their cytoplasm (see Figure 22-4). Visceral Muscle Tissue This smooth muscle tissue is located in the walls of hollow organs (except the heart), the walls of blood vessels, and the dermis of skin. It is involuntary—we cannot consciously control its movement. For example, you do not consciously decide when the visceral muscle of your stomach contracts. This tissue is also called smooth muscle because its cells do not have striations in their cytoplasm. Cardiac Muscle Tissue This specialized muscle tissue is located in the wall of the heart. Like skeletal muscle tissue, cardiac muscle is striated. Like smooth muscle tissue, it is not under voluntary control; it is involuntary. Nervous Tissue Nervous tissue is located in the brain, spinal cord, and peripheral nerves. This tissue specializes in sending impulses or electrical messages to the neurons, muscles, and glands in the body (see Figure 22-5). Nervous tissue contains two types of cells: neurons and neuroglial cells. Neurons are the largest cells and they transmit impulses. Although neuroglial cells are smaller, they are more abundant and act as support cells for the neurons. They do not transmit impulses.

Understanding Medical Terminology

Unlike the English language in which word meanings often seem to have no rhyme or reason—like the overly used "whatever" and various difficult-to-translate modern slang phrasings—medical terminology often can be broken down into word parts that make the meaning concrete and easy to understand. All medical terms must have a word root that contains the base meaning for the term, and a suffix at the end of the term that alters the word root's meaning. In the term appendectomy, for example, the word root append refers to the "appendix" and is combined with the suffix -ectomy, which means "surgical removal." So, appendectomy means "surgical removal of the appendix." The word parts' meanings stay consistent, making it easier to learn new terms containing already understood word parts. Using your new knowledge, if you are told that the word root hyster means "uterus," you can easily see that hysterectomy means "surgical removal of the uterus." Page 480 In addition to word roots and suffixes, some terms also contain a prefix, which comes at the beginning of the term and, like the suffix, alters the term's meaning. Let's take the terms premenstrual and postmenstrual. In defining terms, the general rule is to start with the suffix, then add the prefix (if present), and finally the word root(s). For example: The suffix -al means "pertaining to" The prefix pre- means "before" The prefix post- means "after" The word root menstru refers to the menstrual period. Putting them together, premenstrual means "pertaining to before the menstrual period" and postmenstrual is "pertaining to after the menstrual period." For terms in which the suffix begins with a consonant, a combining vowel—often an "o"—is used between the word root and the suffix to ease pronunciation. An example of this would be the term tracheotomy. The word root trache (windpipe) is being joined to the suffix -tomy (to cut into). The letter "o" is inserted between the two to make pronunciation easier. Unlike prefixes and suffixes, combining vowels do not change the meaning of the term. Appendix I contains commonly used word roots, suffixes, and prefixes. Table 22-1 summarizes information on understanding medical terminology.

DOWN SYNDOME

also called Trisomy 21, is a disorder that causes intellectual disabilities and physical abnormalities. Causes. This disorder occurs when a person has three copies of chromosome 21 instead of two. This condition can be diagnosed through prenatal tests such as amniocentesis. The risk of having a child with Down syndrome increases with the mother's age. Signs and Symptoms. The signs of Down syndrome include a flat facial profile, protruding tongue, oblique slanting eyes, abundant neck skin, short broad hands, and poor muscle tone. Heart, digestive, hearing, and visual problems are also common in people with this condition. Learning difficulties are common in Down syndrome and can range from moderate to severe. Page 491 Treatment. There is no cure, but support programs and the treatment of health problems allow many patients with Down syndrome to live a relatively normal life.

PHENYLKETONURIA (PKU)

develops if a person cannot synthesize the enzyme that converts phenylalanine to tyrosine. Phenylalanine is an essential amino acid, but too much of it can be harmful, so the body regularly converts it to tyrosine. Causes. This condition is inherited as an autosomal recessive disorder. Signs and Symptoms. If phenylalanine builds up in the blood, it can lead to irreversible damage to organs, including the brain. Treatment. Phenylalanine is found in many proteins, so meats and other protein-rich foods must be avoided. Early detection of PKU is important to prevent developmental delays. There is no cure for PKU, but special diets allow a person to lead a normal life. Most newborns are tested for PKU, and prenatal diagnosis is also available.

KLINEFELTER'S SYNDROME

is a chromosomal abnormality that affects males. Causes. Males with this disorder have an extra X chromosome. Signs and Symptoms. Tall stature, pear-shaped fat distribution, small testes, sparse body hair, and infertility are the most common signs and symptoms. Thyroid problems, diabetes, and osteoporosis are also common in patients with this syndrome. Treatment. There is no cure, but treatments such as testosterone replacement therapy can decrease the risk of osteoporosis and produce more male characteristics.

ALBINISM

is a condition in which a person is born with little or no pigmentation in the skin, eyes, or hair. Albinism affects all races; in most cases there is no family history of it. Causes. At least six different genes are involved with pigment production. This condition develops when a person inherits one or more faulty genes that do not produce the usual amounts of a pigment. Signs and Symptoms. In addition to lack of pigment people with the condition experience visual problems and sun-sensitive skin. Treatment. Although there is no cure, treatments are available to help the symptoms. Prenatal testing for the condition is available.

HEMOPHILIA

is a group of inheritable blood disorders. Each condition may be mild to severe. Causes. In each type of hemophilia, an essential clotting factor is low or missing. Most types are X-linked recessive disorders; therefore, this disorder primarily affects males. Carriers of the gene can be identified with a blood test, and prenatal tests can diagnose the condition in the fetus. Signs and Symptoms. Symptoms include easy bruising, spontaneous bleeding, and prolonged bleeding. Repeated bleeding in the joints leads to arthritis and permanent joint damage. Treatment. Treatment includes injections of the missing clotting factors, often Factor VIII.

CYSTIC FIBROSIS

is a life-threatening disease that mainly affects the lungs and pancreas. This disease is one of the most common inherited life-threatening disorders among Caucasians in the United States. Causes. Inheritance is autosomal recessive, so if both parents are carriers, there is a 25% chance that each child born to them will develop cystic fibrosis. Signs and Symptoms. Patients with this disorder have increasing problems with breathing. Thick secretions eventually block passages in the airways, and these secretions may become infected. Treatment. There is no cure, but treatments are available to help patients live with the complications associated with this disorder. Newborn babies are commonly screened for the disease because the sooner treatment begins, the healthier the child can be. Parents are also commonly screened for the gene to determine the likelihood of having a child with cystic fibrosis.

FRAGILE X SYNDOME

is the most common inherited cause of learning disability. All races and ethnic groups seem to be affected equally by this syndrome. Causes. In this disorder, one of the genes on the X chromosome is defective and makes the chromosome susceptible to breakage. This sex-linked disorder affects boys more severely than girls. It is estimated that approximately 1 in 300 females is a carrier for this disorder. Signs and Symptoms. Mental impairment, learning disabilities, attention deficit disorder, a long face, large ears, and flat feet are some of the signs and symptoms. Fragile X syndrome can be easily diagnosed using prenatal tests such as amniocentesis. Treatment. There is no cure, but some treatments and support groups are available to patients with this disorder.