Chapter 24 Anatomy and Physiolog

FIGURE 24.17 Liver

(a) Anterior view

FIGURE 24.11 Anatomy and Histology of the Stomach

(a) Cutaway section reveals the muscular layers and internal anatomy of the stomach. (b) A section of the stomach wall illustrates its histology, including several gastric pits and glands. (c) Photomicrograph of gastric glands.

FIGURE 24.16 Anatomy and Histology of the Duodenum

(a) Ducts from the liver and pancreas empty into the duodenum. (b) Wall of the duodenum, showing the circular folds. (c) The villi on a circular fold. (d) A single villus, showing the lacteal and capillary network. (e) Transmission electron micrograph of microvilli on the surface of a villus.

FIGURE 24.7 Teeth

(a) Permanent teeth.

FIGURE 24.5 Peritoneum and Mesenteries

(a) Sagittal section through the trunk, showing the peritoneum and mesenteries associated with some abdominal organs.

FIGURE 24.26 Histology of the Large Intestine

(a) Section of the transverse colon cut open to show the inner surface. (b) Enlargement of the inner surface, showing openings of the crypts. (c) Higher magnification of a single crypt. (d) Photomicrograph showing the histology of the large intestine wall.

FIGURE 24.22 Anatomy and Histology of the Duodenum and Pancreas

(a) The head of the pancreas lies within the duodenal curvature, with the pancreatic duct emptying into the duodenum. (b) Histology of the pancreas, showing both acing cells and the pancreatic duct system.

FIGURE 24.25 Large Intestine

(a) The large intestine consists of the cecum, colon, rectum, and anal canal. The tenure coli and mental appendages are along the length of the colon.

FIGURE 24.9 Salivary Glands

(a) The large salivary glands are the parotid glands, the submandibular glands and the sublingual glands.

FIGURE 24.9 Salivary Glands

(b) An idealized schematic illustrates the histology of the large salivary glands. The figure is representative of all the glands and does not depict any specific salivary gland.

FIGURE 24.7 Teeth

(b) Deciduous teeth. Dental professionals have developed a "universal" numbering and lettering system for convenience in identifying individual teeth.

FIGURE 24.17 Liver

(b) Inferior view

FIGURE 24.5 Peritoneum and Mesenteries

(b) Photograph of the abdomen of a cadaver, with the greater momentum in place.

FIGURE 24.25 Large Intestine

(b) Radiograph of the large intestine following a barium enema.

FIGURE 24.5 Peritoneum and Mesenteries

(c) Photograph of the abdomen of a cadaver, with the greater momentum removed to reveal the underlying viscera.

FIGURE 24.9 Salivary Glands

(c) Photomicrograph of the parotid gland.

FIGURE 24.17 Liver

(c) Superior View

Describe the connective tissue capsule and visceral peritoneum that cover the liver.

-A connective tissue capsule and visceral peritoneum cover the liver, except for the bare area, a small on the diaphragmatic surface that lacks a visceral peritoneum and is surrounded by the coronary ligament. -At the porta, the connective tissue capsule sends a branching network of septa (walls) into the substance of the liver to provide its main support. -Vessels, nerves, and ducts follow the connective tissue branches throughout the liver.

What is the terminal sulcus?

-A groove called the terminal sulcus divides the tongue into two parts. -The part anterior to the terminal sulcus accounts for about two-thirds of the surface area and is covered by papillae, some of which contain taste buds. -The posterior one-third of the tongue is devoid of papillae and has only a few scatter taste buds. -Instead, it has a few small glands and a large amount of lymphatic tissue, which form the lingual tonsil. -Moist stratified squamous epithelium covers the tongue.

What is a major function of hydrochloric acid?

-A major function of hydrochloric acid is to kill bacteria that are ingested with essentially everything humans put into their mouths. -However, some pathogenic bacteria have an outer coat that resists stomach acids and one type of bacteria (H. pyloric) is normally present in many human stomachs.

Describe what meals leave and enter the stomach the quickest and the slowest.

-A meal of polysaccharide carbohydrates (starch and glycogen) has the fastest clearance time from the stomach, typically 1 hour. -For comparison, a meal heavy with dietary fats and proteins takes up to 6 hours to clear from the stomach. -A major reason for this difference is that a fatty meal increases the release of cholecystokinin, which is a major inhibitor of stomach emptying.

Describe the digestion of carbohydrates.

-A minor amount of carbohydrate digestion begins int he oral cavity with the partial digestion of starches by salivary amylase. -Digestion continues in the stomach until the food is well mixed with acid, which inactivate salivary amylase. -Carbohydrate digestion is resumed in the small intestine by pancreatic amylase. -Pancreatic lipase in the small intestine is responsible for the majority of carbohydrate digestion. -However, many of the digested carbohydrates at this point are disaccharides, which can not be absorbed. -The final step in carbohydrate digestion is performed by a series of disaccharidases that are bound to the microvilli of the intestinal epithelium. -These enzymes digest disaccharides into three monosaccharides: (1) glucose, (2) galactose, and (3) fructose. -The major monosaccharide is glucose.

Describe hormones secreted by the digestive system.

-A number of hormones, such as gastrin and secretin, are secreted by endocrine cells in the digestive system and are carried through the blood to target organs of the digestive system or to target tissues in other systems. -These hormones help regulate many digestive tract functions, as well as the secretions of associated glands, such as the liver and pancreas.

What can cause parotid glands to become inflamed?

-A viral infection can cause the parotid glands to become inflamed and swollen, making the cheeks quite large. -Before the measles/mumps/rubella (MMR) vaccination program was begun in the United States, mumps was a common childhood diseases caused by the mumps virus. -Now mumps is very rare in the United States. -The virus causing mumps can also infect other tissues, including the testes, which can result in sterility in an adult male.

Describe how eating can cause production of cholesterol.

-About 15% of the cholesterol in the body is ingested int eh food we eat. -Eating foods containing saturated fatty acids can raise plasma cholesterol levels by stimulating LDL production and inhibiting LDL receptor production. -Conversely, ingestion unsaturated fatty acids lowers plasma cholesterol. -The remaining 85% is manufactured in body cells, mostly in the liver and intestinal mucosa. -Most of the cholesterol and other lipids taken into or manufactured in the liver leave the liver in the form of VLDL. -Most of the triglycerides are removed from VLDL to be stored in adipose tissue, as a result, VLDL becomes LDL.

Describe water in the digestive tract.

-About 9 L of water enter the digestive tract each day as a combination of ingested and secreted fluids. -Of this 9 L, about 92% is absorbed in the small intestine, and another 6-7% is absorbed in the large intestine. -Water moves in either direction across the wall of the small intestine by osmosis. -Osmotic gradients across the epithelium determine the direction of this diffusion. -When the chyme is dilute, water is absorbed by osmosis across the intestinal wall into the blood. -When the chyme is very concentrated and contains very little water, water moves by osmosis into the lumen of the small intestine. -As nutrients are absorbed in the small intestine, its osmotic pressure decreases; as a consequence, water moves form the small intestine into the surrounding extracellular fluid. -Water in the extracellular fluid can then enter the blood. -Because of the osmotic gradient produce as nutrients are absorbed in the small intestine, nearly all the water that enters the small intestine by way of the oral cavity, stomach, or intestinal secretions is reabsorbed.

What is absorption?

-Absorption is the means by which molecules are moved out of the digestive tract into the blood for distribution throughout the body. -Nearly all absorption of nutrients occur in the duodenum and jejunum of the small intestine. -Some absorption also occurs int eh ileum. -A few chemical, such as nitroglycerin, can be absorbed through the thin mucosa of the oral cavity below the tongue. -Some small molecules (e.g., alcohol and aspirin) can diffuse through the stomach epithelium into the blood. -Some molecules can be absorbed by diffusion, whereas others must be transported across the intestinal wall. -Transport requires transport proteins, which work by faicitated diffusion, active-transport, or secondary-transport mechanisms, such as symport and antipode. -The epithelial cells that form the intestinal wall have two distinct sides with different transport proteins on each side. -The side that faces the digestive tract lumen is called the apical membrane, and the side that faces the blood vessels is called the basolateral membrane. -The transport proteins in these membranes are responsible for the one-way movement of molecules from the digestive tract to the rest of the body.

Describe the major salivary glands.

-All of the major large salivary glands are compound acing glands, which are branching glands with clusters of acini resembling grapes. -They produce thin serous secretions or thicker mucous secretions.

Does the ENS work together with the CNS?

-Although the ENS can control the activities of the digestive tract independent of the CNS, the two systems normally work together. -Autonomic innervation from the CNS can increase or decrease ENS activity.

What is amino acid transport stimulated by?

-Amino acids are actively transported into the various cells of the body. -This transport is stimulate by growth hormone and insulin. -Most amino acids serve as building blocks to form new proteins, but some amino acids may be used for energy.

What is an effective rehydration strategy?

-An effective rehydration strategy is to drink water containing sodium and glucose. -As sodium and glucose are absorbed by symport across the intestinal epithelium, water follows by osmosis. -As an added value, this strategy also replaces ions and provides an immediate energy source. -Most sports drinks contain sodium and glucose, which efficiently rehydrate the athlete. -The same principle is used in oral rehydration therapy for severe diarrhea. -This simple, cheap treatment is especially valuable in Third World countries where people often die form diarrhea caused by intestinal infections.

What is another age-related complication/

-Another age-related complication in the digestive system involves the way medications and other chemicals are absorbed from the digestive tract. -The decreased mucous covering and the thinned connective tissue layers allow chemicals to pass more readily for the digestive tract into the blood. -However, a decline in the blood supply to the digestive tract hinders the absorption of such chemicals. -Drugs administered to treat cancer, which occurs in many elderly people, may irritate the mucosa of the digestive tract, resulting in nausea and loss of appetite.

What is another major ENS neurotransmitter?

-Another major ENS neurotransmitter is serotonin, which stimulates digestive tract motility. -In addition to neural release, serotonin is also produce by endocrine cells within the digestive tract wall. -Over 95% of the serotonin in the body is found in the digestive tract, so drugs that increase serotonin levels and function, such as antidepressants and chemotherapeutics used for cancer treatment, can also effect digestive tract activity. -An unintended consequence of many cancer therapies is nausea, due to increased serotonin release from endocrine cells in the digestive tract. -Serotonin binds to a subset of serotonin receptors on sensory terminal of the vagus nerve, which stimulates the voting center in the brain. -This results in the nausea and vomiting associated with chemotherapy and radiotherapy. -Serotonin receptor blockers, such as ondansetron, are commonly used to alleviate nausea.

What is appendicitis?

-Appendicitis is an inflammation of the vermiform appendix that usually occurs because of an obstruction of the appendix. -Secretions from the appendix cannot pass the obstruction and accumulate, resulting in enlargement and pain. -Bacteria in the area can cause infection of the appendix. -Symptoms include sudden abdominal pain, particularly in the right-lower portion the abdomen; slight fever; loss of appetite; constipation of diarrhea; nausea; and vomiting. -In the right-lower quadrant of the abdomen, about one-third the distance along a line from the right anterior superior iliac spine to the umbilicus, is an area called the McBurney point. -This area of the body surface becomes very tender in patients with acute appendicitis because of pain referred form the inflamed appendix. -Each year, 500,000 people in the United States experience appendicitis. -The usual treatment is surgical removal of the appendix, called an appendectomy. -If the appendix bursts, the infection can spread throughout the peritoneal cavity, causing peritonitis, with life-threatening results.

Describe gastric secretions.

-Approximately 2-3 L of gastric secretions (gastric juice) are produced each day. -The amount and type of food entering the stomach and small intestine dramatically affect the quantity of gastric secretions, but up to 700 mL are secreted as a result of a typical meal. -Both nervous and hormonal mechanisms regulate gastric secretions. -The neural mechanism involve reflexes integrated within the medulla oblongata and local reflexes integrated within the ENS. -In addition, higher brain centers influence the reflexes. -The chemical messengers that regulate stomach secretions include the hormones gastrin, secretin, and cholecystokinin as well as the paracrine chemical messenger histamine.

What happens during stomach filling?

-As food enters the stomach, the rugae flatten and the stomach volume increases up to 20-fold. -This expansion allows the stomach to accommodate a large amount of food with very little increased pressure, until the stomach nears maximum capacity. -Relaxation of the rugae is mediated by a reflex integrated within the medulla oblongata that inhibits muscle tone and pressure is further minimized by the ability of smooth muscle to stretch without an increase in tension.

How are basic, acidic, and neutral amino acids transported into the epithelial cells?

-Basic, acidic, and neutral amino acids are transported into the epithelial cells by separate carrier molecules. -Acidic and most neutral amino acids enter by symport with a Na+ gradient, similar to the mechanism used for glucose transport. -Basic amino acids enter the epithelial cells by a H+ symport mechanism analogous to Na+ symport. -The total amount of each amino acid that enters the intestinal epithelial cells as dipeptides or tripe tides is considerably more than the amount that enters as single amino acids. -Once inside the cells, dipeptidases and tripeptidases split the dipeptides and tripeptides into their component amino acids. -Individual amino acids then leave the epithelial cells and enter the hepatic portal system, which transports them to the liver. -The amino acids may be modified in the liver or released in to the bloodstream and distributed throughout the body.

Describe lipoproteins.

-Because lipids are either insoluble or only slightly soluble in water, they are transported through the blood in combination with proteins, which are water-soluble. -Lipids combined with proteins are called lipoproteins and are categorized as high- or low-density. -Density describes the compactness of a substance and is the ratio of mass to volume. -Lipids are less dense than water and tend to float in water. -Proteins, which are denser than water, tend to sink in water. -A lipoprotein with a high lipid content has a very low density, whereas a lipoprotein with a high protein content has a relatively high density. -Chylomicrons, which are made up of 99% lipid and only 1% protein, are lipoproteins with an extremely low density. -The other major transport lipoproteins are very low-density lipoprotein (VLDL), which is 92% lipid and 8% protein; low-density lipoprotein (LDL), which is 75% lipid and 25% protein; and high-density lipoprotein (HDL), which is 55% lipid and 45% protein.

Describe the submucosa.

-Beneath the mucosa lies the submucosa, a thick connective tissue layer. -This tunic contains nerves, blood vessels, lymphatic vessels, and small glands. -A network of nerve cells int eh submucosa forms the submucosal plexus, or Meissner plexus, consisting of axons, many scattered neuron cell bodies, and glial cells. -Axons from the submucosal plexus extend to cells in epithelial intestinal glands, stimulating their secretion. -The esophagus and stomach lack a submucosal plexus, but the plexus is extensive through the rest of the digestive tract.

What is the myenteric plexus?

-Between the two muscle layers is the myenteric plexus, or Auerbach plexus. -The myenteric plexus controls the motility of the intestinal tract. -This function is in contrast with the submucosal plexus, which controls secretions. -Both the myenteric plexus and the submucosal plexus consist of axons, many scattered neuron cell bodies, and glial cells, although the myenteric plexus is much more extensive than the submucosal plexus.

Explain how bale secretion is a positive-feedback system.

-Bile salts increase bile secretion through a positive-feedback system. -Over 9-% of bile salts are reabsorbed in the ileum and carried in the blood by the hepatic portal circulation. -Upon their return to the liver, the bile salts stimulate further bile secretion and are once again secreted into the bile. This recycling process reduces the loss of bile salts in the feces. -Bile secretion into the duodenum continues until the duodenum empties.

Explain blood flow through the liver.

-Blood from the digestive tract first flows into the liver. -Nutrient-rich, deoxygenated blood from the viscera enters the hepatic sinusoids from branches of the hepatic arteries. -Mixed blood in the hepatic sinusoids flows to the central vein, where it exits the lobule and then exits the liver through the hepatic veins. -Bile produced by the hepatocytes flows through the bile canaliculi toward the hepatic triad and exits the liver through the hepatic ducts. -Blood, therefore, flows form the triad toward the center of each lobule, whereas big flows away from the center of the lobule toward the triad.

What control secretion of pancreatic juices?

-Both hormonal and neural mechanisms control secretin of pancreatic juice. -The hormones secretin and cholecystokinin are related from the duodenum in response to specific stimuli. -Each hormone stimulates secretion of a specific type of pancreatic juice. -Secretin stimulates secretion of the bicarbonate-rich aqueous juice. -An acidic chyme in the duodenum stimulates the release of secretin.

Describe how production of cholesterol by LDLs are regulated.

-Cells not only take in cholesterol and other lipids from LDLs but also make their own cholesterol. -When the combined intake and manufacture of cholesterol exceeds a cell's needs, a negative-feedback system reduces the amount of LDL receptors and cholesterol manufactured by the cell. -Excess lipids are also packaged into HDLs by the cells. -These are transported back to the liver for recycling or excretion of bile.

What do chief cells secrete?

-Chief cells within the gastric glands secrete pepsinogen. -Pepsiongen is packaged in zymogen granules, which are released by exocytosis when pepsinogen secretion is stimulated. -Zymogen is the term for an inactive enzyme. -Once pepsinogen enters the lumen of the stomach, hydrochloric acid and previously formed pepsin molecules convert it to pepsin. -Pepsin exhibits optimal enzymatic activity at a pH of 3 or less. -Pepsin catalyzes the cleavage of some covalent bonds in proteins, thus breaking them into smaller peptide chains. -Chief cells also secrete the enzyme gastric lipase, which can digest lipids even in an acidic environment.

What does cholecystokinin stimulate?

-Cholecystokinin stimulates the secretion of the enzyme-rich pancreatic juice. -Recall that cholecystokinin also stimulates the release of bile form the gallbladder, which aids in the digestion of lipids. -The major stimulus for the release of cholecystokinin is the presence of fatty acid and other lipids in the duodenum.

Describe how control of the digestive system from the CNS occurs.

-Control of the digestive system by the CNS occurs when reflexes are activated by stimuli originating either in the digestive tract or in the CNS. -From within the digestive system, action potentials are carried by sensory neurons in the vagus and sympathetic nerves to the CNS, where the reflexes are integrated. -Reflexes within the CNS can be activated by the sight, smell, or taste of food. -An example is increased salivation and pancreatic secretions when food is seen or smelled. -All of these reflexes influence activity in parasympathetic neurons of the CNS. -Parasympathetic neurons extend to the digestive tract through the vagus nerves to control responses or alter the activity of the ENS and local reflexes. -Some sympathetic neurons inhibit muscle contraction and secretion in the digestive system and decrease blood flow to the digestive system.

What is cystic fibrosis?

-Cystic fibrosis is a hereditary disorder that occurs in 1 of every 2000 births and affects 33,000 people in the United States; it is the most common lethal genetic disorder Amon Caucasians. -The most critical effects of the disease, accounting for 90% of the dates, are on the respiratory system. -Several other problems occur, however, in affected people. -Because the disease is a disorder in a Cl-transport channel protein -which affects chloride transport and, as a result, the movement of water-all exocrine glands are affected. -The buildup of thick mucus in the pancreatic and hepatic ducts causes blockages of the ducts, so that bile salts and pancreatic digestive enzymes are prevented from reaching the duodenum. -As a result, digestion is reduced, and fat-soluble vitamins are poorly absorbed due to the lack of bile to form micelles. -The person suffers from vitamin A, D, E, and K deficiencies, which result in conditions such as night blindness, skin disorders, rickets, and excessive bleeding. -Therapy includes administering the missing vitamins to the person and reducing dietary fat intake.

How is defecation initiated?

-Defecation can be initiated by voluntary actions that stimulate a defecation reflex. This "straining" includes a large inspiration of air, followed by closure of the larynx and forceful contraction of the abdominal muscles. -As a consequence, the pressure in the abdominal cavity increases and forces feces into the rectum. -Stretch of the rectum initiates defecation reflex, and input from the brain overrides the reflexive contraction of the external anal sphincter stimulated by increased abdominal pressure. -The increased abdominal pressure also helps push feces through the rectum.

What is digestion?

-Digestion is the breakdown of food to molecules small enough to be absorbed into the blood. -Mechanical digestion breaks large food particles into smaller ones. -Chemical digestion is the breaking of covalent chemical bonds in organic molecules by digestive enzymes. -Carbohydrates break down into monosaccharides, lipids break down into fatty acids and monoglycerides, and proteins break down into amino acids. -However, some molecules (e.g., vitamins, minerals, and water) are not broken down. -A relatively small amount of digestion begins in the oral cavity and some occurs in the stomach, but the vast majority of digestion occurs the proximal end of the small intestine, especially in the duodenum.

What happens during defecation?

-During defecation, the contractions that move feces toward the anus must be coordinated with the relaxation of the internal and external anal sphincters. -The movement of feces from the colon into the rectum distends the rectal wall, which stimulates the defecation reflex.

Describe the components of teeth.

-Each tooth consists of (1) a crown, (2) a neck, and (3) a root. -The crown is the entire enamel-covered part of the tooth. -It is also called the anatomical crown to distinguish it from the clinical crown, which only includes the part of the tooth exposed in the oral cavity. -The crown can have one or more cusps (points). -The neck is the small region between the crown and root. -The root is the largest region of the tooth. -It anchors the tooth in the jawbone.

What does enzyme-rich pancreatic juice also contain?

-Enzyme-rich pancreatic juice also contains other enzymes that digest carbohydrate, lipids, and nucleic acids. -The enzyme pancreatic amylase continues the polysaccharide digestion initiated in the oral cavity. -The lipid-digesting enzyme pancreatic lips breaks down lipids into monoglycerides and free fatty acids. -In addition to lipase, the pancreas also secretes the enzyme cholesterol esterase, which digests cholesterol esters (dietary form of cholesterol) into cholesterol and free fatty acid. -Deoxyribonucleases and ribonuclease are enzymes that degrade DNA and RNA, respectively.

Describe enzymes of the intestinal mucosa.

-Enzymes of the intestinal mucosa are not actually secreted, but are bound to the membranes of the absorptive cell microvilli. -These surface-bound enzymes include disaccharidases, which break down disaccharide to monosaccharide, and peptidases, which hydrolyze the peptide bonds between small Amin acid chains. -Although these enzymes are not secreted into the intestine, they influence the digestive process significantly, and the large surface area of the intestinal epithelium brings these enzymes into contact with the intestinal contents. -Small molecules, which are breakdown products of digestion, are absorbed through the microvilli and enter the circulatory or lympatic system.

Describe exocrine secretions.

-Exocrine secretions flow from the pancreas to the small intestine via a series of ducts. -Secretions from the acini first flow into small intercalated ducts, then into interlobular ducts, which leave the lobules to join interlobular ducts between the lobules. -The interlobular ducts attach to the main pancreatic duct which joins the common bile duct at the hepatopancreatic ampulla, or Vater's ampulla. -The hepatopancreatic ampulla empties into the duodenum at the major duodenal papilla. -A smooth muscle sphincter, the hepatopancreatic ampulla sphincter, or sphincter of Oddi, regulates the opening of the ampulla. -In most people, an accessory pancreatic duct opens at the minor duodenal papilla. -The ducts are lined with simple cuboidal epithelium, and the epithelial cells of the acini are pyramid-shaped. -A smooth muscle sphincter surrounds the pancreatic duct where it enters the hepatopancreatic ampulla.

Describe mastication.

-Food taken into the mouth is masticated, or chewed, by the teeth. -The anterior teeth (the incisors and the canines) primarily cut and tear food, whereas the premolars and molars primarily crush and grind it. -Mastication breaks large food particles into smaller ones, creating a much larger total surface area. -Because digestive enzymes digest food molecules only at the surface of the particles, mastication increases the efficiency of digestion.

What muscle move the mandible during mastication?

-Four pairs of muscles move the mandible during mastication: (1) temporals, (2) master, (3) medial pterygoid, and (4) lateral pterygoid muscles. -All four close the jaw, while the lateral pterygoid muscle opens it. -The medial and lateral pterygoids and the master muscles accomplish protraction and lateral and medial excursion of the jaw. -The temporals retracts the jaw. -All these movements are involved in tearing, crushing, and grinding food.

What are gallstones?

-Gallstones are insoluble aggregates formed in the gallbladder. -They are often caused by precipitation of excess cholesterol, which can be the result of a high-cholesterol diet or other causes. -Cholesterol is not soluble in water and is ordinarily kept in solution by bile salts. -Occasionally, a gallstone passes out of the gallbladder and enters the cystic duct, blocking the release of bile. -This condition interferes with normal digestion, and often the gallstone must be surgically removed. -If the gallstone moves far enough down the duct, it can also block the pancreatic duct, resulting in pancreatitis.

What increases with advancing age?

-Gastroesophageal reflux disorder increases with advancing age. -It is probably the main reason that elderly people take antacids, H2 antagonists, and proton pump inhibitors. -Disorders that are not necessarily age-induced, such as hiatal hernia and irregular or inadequate esophageal motility, can be worsened by the effects of agin because of general decreased motility in the digestive tract.

Explain hepatic cords.

-Hepatic cords are strings of cells that radiate out from the central vein of each lobule like the spokes of a wheel. -The hepatic cords are composed of hepatocytes, the functional cells of the liver. -Hepatocytes take up nutrients from the portal blood. -The nutrients are stored, detoxified, or used to synthesize new compounds before being released into the hepatic sinusoids or into the bile canaliculi.

Describe hepatic phagocytic cells.

-Hepatic phagocytic cells (Kupffer cells), which lie along the sinusoid walls of the liver, phagocytize "worn-out" and dying red and white blood cells, some bacteria, and other debris that enters the liver through the blood vessels.

What are hepatic sinusoids?

-Hepatic sinusoids are blood channels in the spaces between the hepatic cords. -The sinusoids are lined with a very thin, irregular squamous endothelium consisting of two cell populations: (1) extermely thin, sparse endothelial cells and (2) hepatic phagocytic cells called Kupffer cells.

What other substances do hepatocytes transform?

-Hepatocytes also transform substances that cannot be used by most cells into more readily usable substances. -For example, they combine ingested dietary fats with choline and phosphorus in the liver to produce phospholipids, which are essential components of plasma membranes. -In addition, vitamin D is hydroxylated in the liver hepatocytes. -The hydroxylated form of vitamin D, which is the major circulating form of vitamin D, is transported through the blood to the kidneys, where it is again hydroxylated. -The double-hydroxylated vitamin D is the active form of the vitamin, which functions in calcium maintenance.

Describe how the liver stores nutrients.

-Hepatocytes can remove sugar from the blood and store it in the form of glycogen. -They can also store lipids, vitamins (A, B12, D, E, and K), copper, and iron. -This storage function is usually short-term, and the amount of stored material in the hepatocytes-hence their size-fluctuates during the day.

What do hepatocytes do?

-Hepatocytes help maintain blood glucose levels within very narrow limits. -If a large amount of sugar enters the general circulation after a meal, the blood osmolality will increases, resulting in hyperglycemia. -Under normal conditions, this is prevented because the blood from the small intestine passes through the hepatic portal vein to the liver, where hepatocytes remove glucose and other substances from the blood, store them, and then secrete them back into the circulation when needed.

What happens if the stomach empties too fast?

-If the stomach empties too fast, the efficiency of digestion and absorption is reduced, and acidic gastric contents dumped into the duodenum may damage its lining. -However, if the rate of emptying is too slow, then the slow delivery to the small intestine will reduce the rate at which nutrients are digested and absorbed. -In addition, the highly acidic contents of the stomach may damage the stomach wall. -To prevent these two extremes, stomach emptying is regulated.

Explain what intrinsic factor is.

-In addition to hydrochloric acid, parietal cells secrete intrinsic factor. -Intrinsic factor is a glycoprotein that binds with vitamin B12, making the vitamin more readily absorbed in the ileum of the small intestine. -Vitamin B12 is important in deoxyribonucleic acid (DNA) synthesis, which is especially important for continual red blood cell production. -A lack of vitamin B12 absorption leads to pernicious anemia. -Deficiency of vitamin B12 also leads to neurological symptoms, including peripheral neuropathy since vitamin B12 is required for maintaining myelin in the peripheral nervous system.

What other chemicals are released in the digestive system?

-In addition to the hormones produced by the digestive system that enter the blood, other paracrine chemicals, such as histamine, are released locally within the digestive tract, where they influence the activity of nearby cells. -These localized chemical regulators help local reflexes within the ENS control local digestive tract environments, such as pH levels.

Describe blood vessels in the fetus.

-In the fetus, special blood vessels bypass the liver sinusoids. -The remnants of fetal blood vessels can be seen in the adult as the round ligament (ligaments tires) and the ligaments venosum.

How are carbohydrates digested?

-Ingested carbohydrates consists primarily of polysaccharides, such as starches; disaccharides, such as sucrose (table sugar) and lactose (milk sugar); and monosaccharides, such as glucose and fructose (the sugar found in many fruits). -During digestion, polysaccharides break down first into smaller chains and some disaccharides and monosaccharides. -Disaccharides then break down into monosaccharides.

Explain how food is mixed within the stomach.

-Ingested food is thoroughly mixed with stomach gland secretions to form chyme. -This mixing is accomplished by gentle mixing waves, which are contractions that occur about every 20 seconds. -They proceed from the body of the stomach toward the pyloric sphincter. -Peristaltic waves occur less frequently, are significantly more powerful than mixing waves, and force the chyme near the periphery of the stomach toward the pyloric sphincter. -The more solid material near the center of the stomach is pushed superiorly toward the cardiac part for further digestion. -Roughly 80% of the contractions are mixing waves, and 20% are peristaltic waves. -The back-and-forth movement of the chyme effectively mixes the ingested food with gastric juice.

Explain ions in the digestive tract.

-Ions are predominantly absorbed by active transport in the small intestine. -Active transport mechanisms drive the absorption of (1) Na+, (2) K+, (3) Ca2+, (4) Mg2+, (5) PO4-. -Chloride ions move passively through the intestinal wall of the duodenum and the jejunum following the positively charged Na+, but Cl- is actively transported from the ileum. -Although Ca2+ is actively transported along the entire length of the small intestine, vitamin D is required for that transport process. -The absorption of Ca2+ is under hormonal control, as are its excretion and storage. -Parathyroid hormones, calcitonin, and vitamin D all play a role in regulating blood levels of Ca2+.

Explain the difference between LDLs and HDLs.

-LDL is commonly considered "bad" because, when in excess, it deposits cholesterol in arterial walls. -On the other hand, HDL is considered "good" because it transport cholesterol from the tissues via blood to the liver for removal from the body in the bile. -A high HDL/LDL ratio in the blood is related to a lower risk for heart disease. -Low HDL levels are linked to obesity, and weight reduction increases HDL levels. -Aerobic exercise can decrease LDL levels and increase HDL levels.

Describe LDL receptors.

-LDL is delivered to cells of various tissues through the blood. -Cells have LDL receptors in "pits" on their surfaces, which bind the LDL. -Once LDL is bound to the receptors, the pits on the cell surface become endocytotic vesicles, and the cell takes in LDL by receptor-mediated endocytosis. -For example, each fibroblast has 20,000-50,000 LDL receptors on the surface. -However, those receptors are confined to cell surface pits, which occupy only 2% of the cell surface. -Once inside the cell, the endocytotic vesicle combines with a lysosome, and the LDL components are separated for use in the cell.

What are lipase enzymes?

-Lipase enzymes digest lipid molecules. -The primary products of lipase digestion are free fatty acids and monoglycerides. -There are three lipase released into the digestive tract: (1) pancreatic lipase, (2) lingual lipase, and (3) gastric lipase. -The vast majority of lipase is pancreatic lipase, which is secreted by the pancreas and digests lipids in the small intestine. -A minor amount of lingual lipase is secreted in the oral cavity and swallowed with food. -It digests a small amount (<10%) of lipid in the stomach. -The stomach also produces very small amounts of gastric lipase. -Lingual and gastric lipase are most important in neonatal infants, while pancreatic lipase is the major enzyme in adults.

What are lipids?

-Lipids are molecules that are insoluble or only slightly soluble in water. -They include triglycerides, phospholipids, cholesterol, steroids, and fat-soluble vitamins. -Triglycerides are the most common type of lipid and are often referred to as fats. -They consists of three fatty acids bound to glycerol.

What is especially important in regulating the motility of the small intestine.

-Local mechanical and chemical stimuli are especially important in regulating the motility of the small intestine. -Smooth muscle contraction increases in response to distension of the intestinal wall. -Solutions that are either hypertonic or hypotonic, solutions with a low pH, and certain products of digestion, such as amino acids and peptides, also stimulates contractions of the small intestine. -Local reflexes, which are integrated within the ENS of the small intestine, mediate the intestine's parasympathetic nerve fibers may also increase the intestine's motility, but the parasympathetic influences in the intestine are not as important as those in the stomach.

Describe how the liver detoxifies chemicals.

-Many ingested substances are harmful to body cells. -In addition, the body itself produces many by-products of metabolism that, if accumulated, are toxic. -The liver forms a major line of defense by altering the structure of many of these harmful substances to make them less toxic or to make their elimination easier. -Ammonia, for example, a by-product of amino acid metabolism, is toxic and not readily removed from the blood by the kidneys. -Hepatocytes remove ammonia form the blood and convert it to urea, which is less toxic than ammonia. -Urea is then secreted into the blood and eliminated by the kidneys in the urine. -The liver hepatocytes also remove other substances from the blood and excrete them into the bile.

What are mesenteries?

-Many organs within the abdominal cavity are held in place by connective tissue sheets called mesenteries. -The mesenteries consist of two layers of serous membranes with a thin layer of loose connective tissue between them. -They provide a route by which vessels and nerves can pass form the abdominal wall to the organs. -Although mesentery is a general term referring to the serous membranes attached to the abdominal organs, it is also applied specifically to the mesentery associated with the small intestine, sometimes called the mesentery proper. -The mesenteries of parts of the colon are the transverse mesocolon, which extends from the transverse colon to the posterior body wall, and the sigmoid mesocolon, which extends from the sigmoid colon to the posterior body wall. -The vermiform appendix has its own little mesentery, called the mesoappendix.

How are mass movement coordinated?

-Mass movements are coordinated by two reflexes in the ENS: (1) gastrocolic and (2) duodenocolic. -Gastrocolic reflexes are initiated by distension in the duodenum. -The gastrocolic and duodenocolic reflexes promote peristalsis of the small and large intestines, including mass movements. -These reflexes are mediated by parasympathetic reflexes, local reflexes, and hormones, such as cholecystokinin and gastrin. -The though or smell of food, distension of the stomach, and the movement of chyme into the duodenum can stimulate them.

Describe mixing contractions.

-Mixing contractions blend food with digestive fluids in the stomach and small intestine. -These contractions aid with mechanical digestion.

Describe how the ENS controls the digestive system.

-Most of the nervous regulation of the digestive tract is under local control by the enteric nervous system. -The enteric nervous system is an extensive network fo the submucosal and myenteric plexuses within the walls of the digestive tract. -This network of neurons and associated glial cells is a division of the autonomic nervous system. -The ENS contains more neurons than the spinal cord. -In addition to local reflexes within the ENS, there is also control mediated by autonomic innervation from the CNS. -This innervation is largely by the parasympathetic division of the ANS through the vagus nerves and to a lesser extent by sympathetic nerves.

Describe movement in the small intestine.

-Movement in the small intestine involves mixing of the chyme and slow propulsion down the tract. -Segmental contractions mix the intestinal contents, and peristaltic contractions primarily propel the intestinal contents along the digestive tract. -The peristaltic contractions are generally propagated for only short distances, but a few may proceed the entire length of the intestine. -Frequently, intestinal peristaltic contractions are continuations of peristaltic contractions that being in the stomach. -These contractions both mix and propel substances through the small intestine as the wave of contraction proceeds. -The contractions move at a rate of about 1 cm/min. -It usually takes 3-5 hours for chyme to move form the pyloric region to the ileocecal junction.

What stimulates the secretion and release of bile?

-Neural and hormonal signals stimulate the secretion and release of bile. -Parasympathetic stimulation through the vagus nerve increases bile secretion form the liver. -Two hormones released from the duodenum increase bile in the digestive tract: (1) secretin and (2) cholecystokinin. -Secretin stimulates bile secretion form the liver, primarily by increasing the water and bicarbonate ion content of bile. -Cholecystokinin stimulates gallbladder contractions to release bile into the duodenum.

Describe the microorganisms of the colon.

-Numerous microorganisms inhabit the colon. -They reproduced rapidly and ultimately constitute about 30% of the dry weight of the feces. -An important function of colonic bacteria is to synthesize vitamin K, which is passively absorbed in the colon. -Acids are secreted by colonic bacteria as metabolic by-products. -An antiporter exchanges HC3- for Cl- in epithelial cells of the colon in response to acid produced by colic bacteria. -Another anti porter exchanges Na+ for H+. Movement of Na+ into the epithelial cells via this exchanger and other Na+ channels is drive by the Na+ gradient established by the Na+ - K+ pump. -Water leaves the lumen of the colon through osmosis as Na+ and Cl- move into the epithelial cells. -Colonic bacteria also break down a small amount of cellulose to glucose. -However, the glucose cannot be absorbed in the large intestine. -Bacterial actions int eh colon produces gasses called flatus. -The amount of flatus depends partly on the bacterial population in the colon and partly on the type of food consumed. -For example, beans, which contain certain complex carbohydrates, are well known for their flatus-producing effect.

Describe what happens once food enters the stomach.

-Once food enters the stomach, it is mixed with stomach secretions to form a semifluid material called chyme. -The primary function of the stomach is to stored and mix the chyme. -Although some digestion and absorption occur in the stomach, they are not its major functions.

What happens once lipids are digested?

-Once lipids are digested in the intestine, bile salts aggregate around the small droplets. -Lipid droplets surrounded by bile slats are called micelles. -the hydrophobic ends of the bile salts are directed toward the free fatty acids, cholesterol, and monoglycerides at the center of the micelle; the hydrophilic ends are directed outward toward the environment. -When a micelle comes in contact with the epithelial cells of the small intestine, the lipid contents of the micelle pass by simple diffusion through the plasma membrane of the epithelial cells. -The bile salts are not absorbed until they reach the epithelium of the distal ileum.

What happens once digestive products have been absorbed?

-Once the digestive products have been absorbed, they are transported to other parts of the body by two routes. -Water, ions, and water-soluble digestion products, such as glucose and amino acids, enter the hepatic portal system and travel to the liver. -The products of lipid metabolism are coated with proteins and transported into lymphatic capillaries called lacteals. -The lacteals are connected by lymphatic vessels to the thoracic duct, which empties into the left subclavian vein. -The protein-coated lipid products then travel in the blood to adipose tissue or to the liver.

What extends from the upper and bottom lip?

-One or more labial frenula, which are mucosal folds, extend from the alveolar process of the maxilla to the upper lip and from the alveolar process of the mandible to the lower lip.

Describe retroperitoneal.

-Other abdominal organs that have no mesenteries are referred to as retroperitoneal. -The retroperitoneal organs lie along the abdominal wall and include the duodenum, pancreas, ascending colon, descending colon, rectum, kidneys, adrenal glands, and urinary bladder.

Describe the neurotransmitters associated with the ENS.

-Over 30 neurotransmitters are associated with the ENS. -Two major ENS neurotransmitters are acetylcholine and norepinephrine. -In general, acetylcholine stimulates and norepinephrine inhibits digestive tract motility and secretions.

What can pancreatic lipase do?

-Pancreatic lipase alone cannot efficiently digest lipids. -A key step in lipid digestion is emulsification, by which bile salts transform large lipid droplets into much smaller droplets. -Bile slats mix with lipids and act as detergents to disrupt lipid droplets. -By decreasing the droplet size, emulsification increases the surface area of the lipid exposed to lipase and other digestive enzymes. -This is necessary because lipase is water-soluble and can digest lipids only at the surface of droplets. -The bile salts are secreted by the lier and stored in the gallbladder until needed in the duodenum. -Lingual and gastric lipase, which work in the acidic environment of the stomach, do not require bile salts.

What also stimulates the secretion of enzyme-rich pancreatic juices.

-Parasympathetic stimulation through the vagus (X) nerves also stimulates the secretion of enzyme-rich pancreatic juices. -Sympathetic impulses inhibit secretion. -The effect of vagal stimulation of pancreatic juice secretion is greatest during the cephalic and gastric phases of stomach secretion.

Describe how hydrochloric acid is produced.

-Parietal cells in the gastric glands of the pyloric region secrete a concentrated solution of hydrochloric acid. -Hydrochloric acid produces the low pH of the stomach's contents, which is normally between 1 and 3. -The key player in the formation of gastric acid is a H+ -K+ exchange pump that is commonly called the proton pump. -The proton pump actively transports hydrogen ions across the mucosal surface of the parietal cell into the lumen of the stomach. -The process begins with H+ derived from CO2 and water, which enter the parietal cell from its serial surface (the side opposite the lumen of the gastric pit). -Inside the cell the enzyme carbonic anhydrase catalyzes the reaction between CO2 and water to form carbonic acid. -Some of the carbonic acid molecules then dissociate to form H+ and HCO3- (bicarbonate). -Drugs that block the proton pump are used to lower gastric acid levels. -The pump moves H+ by active transport against a steep concentration gradient, and Cl- diffuses from the cell through ion channels int he plasma membrane. -Diffusion of Cl- into the gastric gland duct balances the positively charged H+ to reduce the amount of energy needed to transport the H+ against both a concentration gradient and an electrical gradient.

What are peristaltic waves.

-Peristaltic waves are muscular contractions consisting of a wave of relaxation of the circular muscles in front of the bolus, followed by a wave of strong contraction of the circular muscles in front of the bolus, followed by a wave of strong contraction of the circular muscles behind the bolus, which force the bolus along the digestive tract. -Each peristaltic wave travels the length of the esophagus in about 10 seconds. -Peristaltic waves in the small and large intestines usually travel only short distances.

Describe peritonitis.

-Peritonitis is a potentially life-threatening inflammation of the peritoneal membranes. -The inflammation can result from chemical irritation by substances, such as bile, that have escape from a damaged digestive tract or from infection originating in the digestive tract, as when the appendix ruptures. -The main symptoms of peritonitis are acute abdominal pain and tenderness that are worsened by movement. -An accumulation of excess serous fluid in the peritoneal cavity, called ascites, can occur in peritonitis. -Ascites can also accompany starvation, alcoholism, or liver cancer.

Describe digestion of proteins.

-Proteins are taken into the body from a number of dietary sources. -Digestion of proteins begins in the stomach. -Pepsin secreted by the stomach catalyzes the cleavage of covalent bonds in proteins to produce smaller polypeptide chains. -Pepsin digests as much as 10-20% of the total ingested protein. -Once the proteins and polypeptide chains leave the stomach, pancreatic proteases, continue the digestive process to produce small peptide chains. -There are three major pancreatic proteases: (1) trypsin, (2) chymotrypsin, and (3) carboxypeptidase. -The protein digestion products are finally broken down into tripeptides, dipeptides, and amino acids by peptidases bound to the microvilli of the small intestine. -Each peptidase is specific for a certain peptide chain length or for a certain amino acid sequence.

What is saliva?

-Saliva is a combination of serous and mucous secretions from the various salivary glands.

What are the roles of saliva?

-Saliva is composed of fluid and proteins and has three main roles: (1) it helps keep the oral cavity moist, which is needed for normal speech and for the suspension of food molecules in solution so they can be tasted; (2) it has protective functions; and (3) it begins the process of digestion.

What is salivary gland secretion stimulated by?

-Salivary gland secretion is stimulated by both the parasympathetic and the sympathetic nervous systems, but the parasympathetic system is more important. -Salivary nuclei in the brainstem increase salivary secretions by sending action potentials through parasympathetic fibers of the facial (VII) and glossopharyngeal (IX) cranial nerves in response to a variety of stimuli, such as tactile stimulation in the oral cavity or certain tastes, especially sour. -Higher centers of the brain also affect salivary gland activity. -Odors that trigger thoughts of food or the sensation of hunger can increase saliva secretion as well.

Describe how substances are moved through the large intestine.

-Segmetnal mixing movements occur in the colon much less often than in the small intestine. -Peristaltic waves are largely responsible for moving chyme along the ascending colon. -At widely spaced intervals (normally three or four times each day), large parts of the transverse and descending colon contents a considerable distance toward the anus. -Mass movement are very common after meals because they are initiated by the presence of food in the stomach or duodenum. -Mass movement are most common about 15 minutes after breakfast. -They usually persist for 10-30 minutes and then stop for perhaps half a day.

Describe some conditions that can affect the health of teeth.

-Several conditions can affect the health of the tooth and structures associated with it, such as the peridontal ligament and gingiva. -Dental caries, or tooth decay, is a breakdown enamel by bacterial acids on the tooth surface. -Because the enamel is nonliving and cannot repair itself, a dental filling is necessary to prevent further damage. -If the decay reaches the pulp cavity, with its rich supply of nerves, a toothache may result. -Sometimes, when decay has reached the pulp cavity, a dentist must perform a procedure called a root canal, which consists of removing the pulp from the tooth. -Peridontal disease is the inflammation and degradation of the periodontal ligaments, gingiva, and alveolar bone. -This disease is the most common cause of tooth loss in adults. -Gingivits is an inflammation of the gingiva, often caused by food deposited in gingival crevices and not promptly removed by brushing and flossing. -Gingivitis may eventually lead to periodontal disease. -Halitosis, or bad breath, often occurs with periodontal disease.

What do stomach secretions include?

-Stomach secretions include (1) hydrochloric acid, (2) intrinsic factor, (3) mucus, (4) digestive enzymes (pepsinogen and gastric lipase). -The functions of these gastric secretions are summarized in table 24.2

Discuss the phases of swallowing.

-Swallowing, or deglutition is divided into three phases: (1) voluntary, (2) pharyngeal, and (3) esophageal . -During the voluntary phase a bolus of food is formed in the mouth and pushed by the tongue against the hard palate, until it is forced toward the posterior part of the mouth and into the oropharynx. -The pharyngeal phase of swallowing is a reflex initiated by the stimulation of tactile receptors int eh area of the oropharynx. -Afferent action potentials travel through the trigeminal (V) and glossopharyngeal (IX) nerves to the swallowing center in the medulla oblongata. -There, they initiate action potentials in motor neurons, which pass through the trigeminal (V), glossopharyngeal (IX), vagus (X), and accessory (XI) nerves to the soft palate and pharynx. -This phase of swallowing begins with the elevation of the soft palate, which closes the passage between the nasopharynx and oropharynx. -The pharynx elevates to receive the bolus of food form the mouth and moves the bolus down the pharynx into the esophagus. -The superior, middle, and inferior pharyngeal constrictor muscles contract in succession, forcing the food through the pharynx. -At the same time, the upper esophageal sphincter relaxes, the elevated pharynx opens the esophagus, and food is pushed into the esophagus. -This phase of swallowing is unconscious and is controlled automatically, even though the muscle involved are skeletal. -The pharyngeal phase of swallowing lasts about 1-2 seconds. -During the pharyngeal phase, the vestibular folds and vocal cords close, and the epiglottis is tipped posteriorly, so that the epiglottic cartilage covers the opening into the larynx, and the larynx is elevated. -These movement prevent food from passing into the larynx. -The esophageal phase of swallowing, which takes about 5-8 seconds, is responsible for moving food from the pharynx to the stomach. -Muscular contractions in the wall of the esophagus occur in peristaltic waves. -Gravity helps move liquids and watery food through the esophagus. -However, the peristaltic contractions in the esophagus are forceful enough to allow a person to swallow even while doing a headstand or floating in the zero-gravity environment of space. -As the peristaltic waves and the food bolus approach the stomach, the lower esophageal sphincter in the esophagus relaxes. -This sphincter is not anatomically distinct from the rest of the esophagus, but it can be identified physiologically because it remains tonically constricted to prevent the reflux of stomach contents int the lower part of the esophagus. -The presence of food in the esophagus stimulates the myenteric plexus, which controls the peristaltic waves. -Food in the esophagus also stimulates tactile receptors, which send afferent impulses to the medulla oblongata through the vagus nerves. -Motor impulses, in turn, pass along the vagal efferent fibers to the skeletal and smooth muscles within the esophagus, thereby stimulating their contractions and reinforcing the peristaltic contractions.

How does the ENS function?

-The ENS functions through local reflexes to control activities within specific, short regions of the digestive tract. -The ENS is capable of controlling the complex peristaltic and mixing movements, as well as blood flow to the digestive tract, without any outside influences. -The importance of the ENS is highlighted by the poor intestinal motility observed in patients with Hirshprung disease, or megacolon, who lack a subset of enteric neurons.

Describe stomach emptying.

-The amount of time food remains in the stomach depends on a number of factors, including the type of volume of food. -Liquids rapidly exit the stomach within minutes and are fully gone 1 1/2-21/2 hours after ingestion. -After a typical meal, the stomach is usually empty within 3-4 hours. -The pyloric sphincter normally remains partially closed because of mild tonic contraction. -Each peristaltic contraction is strong enough to force a small amount of chyme through the pyloric opening and into the duodenum. -The peristaltic contractions responsible for moving chyme through the partially closed pyloric opening are called the pyloric pump. -In general, increased motility leads to increased emptying. -In an empty stomach, peristaltic contractions that approach tetanic contractions can occur for about 2-3 minutes. -The contractions are increased by low blood glucose levels and are strong enough to created uncomfortable sensations called hunger pangs. -Hunger pangs usually begin 12-23 hours after a meal, in less time for some people. -If nothing is ingested, hunger pangs reach their maximum intensity within 3-4 days and then become progressively weaker.

Describe the aqueous pancreatic juice.

-The aqueous pancreatic juice is bicarbonate-rich. -Bicarbonate ions (HCO3-) are a major part of the aqueous pancreatic juice. -They are actively secreted by columnar epithelial cells that line the smaller ducts of the pancreas. -The aqueous juice contains Na+ and K+ in about the same concentration found in extracellular fluid, and water follows passively to make the pancreatic juice isotonic. -The HCO3- ions of the aqueous pancreatic juice neutralize the acidic chyme that enters the small intestine from the stomach. -The increased pH caused by pancreatic secretions in the duodenum stops Pepsi digestion. -Importantly, the neutralized pH is required for the functions of pancreatic and brush-border enzymes. -The neutralized pH also prevents damage to the duodenum by the acid from the stomach.

What is the bile canaliculus?

-The bile canaliculus is a cleft like lumen that lies between the cells within each cord.

What is the cecum?

-The cecum is the proximal end of the large intestine, where it meets the small intestine at the ileocecal junction. -The cecum extends inferiorly about 6 cm past the ileocecal junction in the form of a blind sac. The vermiform appendix is a smaller, blind tube about 9 cm long attached to the cecum. -The walls of the appendix contain many lymphatic nodules which contribute to immune functions.

Describe the cellular mechanism responsible for the secretion of HCO3-.

-The cellular mechanism responsible for the secretion of HCO3- is diagrammed in figure 24.23. -The enzyme carbonic anhydrase in duct epithelial cells forms carbonic acid, which dissociates into H+ and HCO3-. -The H+ are exchanged for Na+, which are removed from the cell by the Na+ -K+ pump. -The Na+ - K+ pump is an ATPase that moves Na+ out and K+ into the cell. -It generates a Na+ gradient that is important in many cellular processes, including intestinal nutrient and water absorption describe later in this chapter. -The HCO3- are then transported into the intercalated duct lumen in exchange for Cl-. -The result is a bicarbonate-rich aqueous pancreatic juice. -The production of HCO3- in the pancreas is similar to the production of H+ in gastric gland. -Both processes require carbonic anhydrase and the exchange of HCO3- and Cl-. -The major difference is that an alkaline solution is produced in the pancreas, whereas an acidic solution is produced in the stomach.

Describe the cheeks.

-The cheeks form the lateral walls of the oral cavity. -They consist of an interior lining of moist stratified squamous epithelium and an exterior covering of skin. -The substance of the cheek includes the buccinator muscle, which flattens the cheek against the teeth, and the buccal fat pad, which rounds out the profile on the side of the face.

What is the cholesterol in LDL critical for?

-The cholesterol in LDL is critical for the production of steroid hormones and bile salts int eh liver. -It is also an important component of plasma membranes. -Abnormally low cholesterol levels may lead to weakened blood vessel walls and an increased risk for cerebral hemorrhage.

what is the colon?

-The colon about 1-5-1.8 m long, consists of four parts: (1) ascending colon, (2) transverse colon, (3) descending colon, and (4) sigmoid colon. -The ascending colon extends superiorly from the cecum and ends at the right colic flexure (hepatic flexure) near the right inferior margin of the liver. -The transverse colon extends from the right colic flexure to the left colic flexure (splenic flexure), and the descending colon extends from the left colic flexure to the superior opening of the true pelvis, where it becomes the sigmoid colon. -The sigmoid colon forms an S-shaped tube that extends into the pelvis and ends at the rectum.

Describe the coronary and falciform ligaments.

-The coronary ligament attaches the liver to the diaphragm. -Unlike other mesenteries, the coronary ligament has a wide space in the center, the bare area of the liver, where no peritoneum exists. -The falciform ligament attaches the liver to the anterior abdominal wall.

What does the defecation reflex consists of?

-The defecation reflex consists of local and parasympathetic reflexes. -Local reflexes cause weak contractions of the distal colon and rectum and relaxation of the internal anal sphincter. -Parasympathetic reflexes are responsible for most of the defecation reflex. -Action potentials produced in response to distension of the rectal wall travel along afferent nerve fibers to the defecation reflex center (S2-S4) in the conus medullar is of the spinal cord. -Then efferent acorn potentials are initiated that return through nerves to the colon and rectum, reinforcing peristaltic contractions and relaxation of the internal anal sphincter.

Describe the digestive functions of saliva.

-The digestive functions of saliva are relatively minor compared with digestion later in the tract. -The serous part of saliva contains a digestive enzyme called salivary amylase, which breaks the covalent bonds between glucose molecules in starch and other polysaccharides to produce the disaccharides maltose and isomaltose. -These sugars can give starches a sweet taste. -However, food spends very little time int eh mouth, so only about 3-5% of the total carbohydrates are digested there. -In addition, most starchy foods come from plants and are therefore covered by cellulose, making them inaccessible to salivary amylase. -Cooking and thoroughly chewing food destroy the cellulose covering and increase the efficiency of the digestive process. -In addition to carbohydrate digestion, there is a small amount of lipid digestion initiated by lingual lipase in saliva.

What is the digestive system?

-The digestive system consists of the digestive tract and accessory organs. -The digestive tract is a tube extending from the mouth to the anus. -It is also called the gastrointestinal tract and the alimentary canal. -The associated accessory organs include the salivary glands, liver, gallbladder, and pancreas. -These organs are primarily glands that secrete fluids into the digestive tract.

What is the digestive system controlled by?

-The digestive system is controlled by elaborate nervous and chemical mechanisms that regulate the movement, secretion, absorption, and elimination processes.

What does the digestive tract consists of?

-The digestive tract consists of four major tunics, or layers. -From the inside of the tube, going outward, the four tunics are (1) mucosa, (2) submucosa, (3) muscular, and (4) serosa or advent. -These four tunics are present in all areas of the digestive tract, from the esophagus to the anus.

Describe the duct system of the liver.

-The duct system that conveys bile from the liver also receives ducts from the gallbladder and pancreas before connecting the small intestine. -The right and left hepatic ducts first unite to form a single common hepatic duct. -The cystic duct from the gallbladder joins the common hepatic duct to form the common bile duct. -The common bile duct joins the pancreatic duct at hepatopancreatic ampulla, which is an enlargement where the two ducts merge. -The hepatopancreatic ampulla empties into the duodenum at the major duodenal papilla. -A smooth muscle sphincter surrounds the common bile duct where it enters the hepatopancreatic ampulla.

What is the duodenum?

-The duodenum is the shortest part of the small intestine. -The structure of the duodenum begins with a nearly 180-degree arc from where it leaves the stomach and curves around the head of the pancreas within the abdominal cavity. -This short, superior part ends in a sharp bend, where it joins the jejunum. -Within the duodenum, about two-thirds of the way down the descending part, are two small mounds: the major duodenal papilla and the minor duodenal papilla. -Ducts from the liver and/or pancreas at these papillae.

What happens to teeth as you age?

-The enamel on the surface of elderly people's teeth becomes thinner with age and may expose the underlying dentin. -In addition, the gingiva covering the tooth root recedes, exposing additional dentin. -Exposed dentin may become painful and change the person's eating habits. -Many elderly people also loose teeth, which can have a marked effect on eating habits unless they are fitted with artificial teeth. -The muscles of mastication tend to become weaker; as a result, older people tend to chew their food less before swallowing.

Describe the endocrine part of the pancreas.

-The endocrine part of the pancreas consists of pancreatic islets, or islet of Langerhans. -The islet cells produce the hormones insulin and glucagon, which are very important in controlling the blood vessels of nutrients, such as glucose and amino acids, and somatostatin, which regulates insulin and glucagon secretion and may inhibit growth hormone secretion.

What does enzyme-rich pancreatic juice contain?

-The enzyme-rich pancreatic juice contains enzymes that digest all major classes of food. -This enzyme-rich secretion is produced by the acing cells fo the pancreas. -Without the enzymes produced by the pancreas, lipids, proteins, and carbohydrates cannot be adequately digested.

Describe where the epithelial cells are produced.

-The epithelial cells are produced within tubular invaginations of the mucosa, called intestinal glands, or crypts of Lieberkuhn, at the base of the villi. -The absorptive and goblet cells migrate form the intestinal glands to cover the surface of the villi and are eventually shed from its tip. =The granular and endocrine cells remain in the bottom of the glands. -The submucosa of the duodenum contains coiled, tubular mucous glands called duodenal glands, or Brunner glands, which open into the base of the intestinal glands.

Explain the epithelium of the mucosa and the two cell types.

-The epithelium extends deep into the lamina propria in many places to form intestinal glands and crypts. -Two types of specialized cells in the mucosa are mechanoreceptors involved in peristaltic reflexes and chemoreceptors that detect the chemical composition of food.

What does the esophagus consist of?

-The esophagus has thick walls consisting of the four tunics common to the digestive tract: (1) mucosa, (2) submucosa, (3) muscularis, and (4) adventitia. -the muscularis has an outer longitudinal layer and an inner circular layer, as is true of most parts of the digestive tracts. -However, it differs from other regions by having skeletal muscle in the superior part of the esophagus and smooth muscle in the inferior part. -An upper esohpageal sphincter and a lower esophageal sphincter, at the upper and lower ends of the esophagus, respectively, regulate the movement of materials into and out of the esophagus. -The mucosal lignin of the esophagus is moist stratified squamous epithelium. -Numerous mucous glands in the submucosal layer produce a thick, lubricating mucus, which passes through ducts to the surface of the esophageal mucosa.

Describe the esophagus.

-The esophagus is the part of the digestive tract that extends between the pharynx and the stomach. -It is about 25 cm long and lies in the mediastinum, anterior to the vertebrae and posterior to the trachea. -It passes through the esophageal hiatus (opening) of the diaphragm and ends at the stomach. -The esophagus transports food from the pharynx to the stomach.

Describe the exocrine part of the pancreas.

-The exocrine part of the pancreas is a compound acing gland. -Acinar cells within the acini produce digestive enzymes. -Clusters of acini form lobules that are separated by thin septa.

Describe the exocrine secretions of the pancreas.

-The exocrine secretions of the pancreas, called pancreatic juice, have (1) an aqueous component and (2) an enzymatic compote. -Pancreatic juice is delivered to the small intestine through the pancreatic ducts, where it functions in digestion.

Describe watery feces.

-The feces that leave the digestive tract consists of water, solid substances, microorganisms, and sloughed-off epithelial cells. -An abnormally frequent discharge of watery feces is called diarrhea.

Describe the fourth layer of the digestive tract.

-The fourth layer of the digestive tract is either a serosa or an adventitia, depending on the structure of the layer. -Parts of the digestive tract located within the peritoneal activity have a serosa as the outermost layer. -This serosa, or serous membrane, is called the visceral peritoneum. -It consists of a thin layer of connective tissue and a simple squamous epithelium. -When the outer layer of the digestive tract is derived from adjacent connective tissue, the tunic is called the adventitia and consists of a connective tissue covering that blends with the surrounding connective tissue. -These areas include the esophagus and the retroperitoneal organs.

What is the function of the gallbladder?

-The function of the gallbladder is to store and concentrate bile. -The liver continually secretes bile, which flows to the gallbladder, where 40-70 mL of bile is stored. -While the bile is non the gallbladder, water and electrolytes are absorbed. -Thus, bile salts and pigments become as much as 5-10 times more concentrated than when secreted by the liver. -Bile is released from the gallbladder by contractions stimulated by cholecystokinin and, to a lesser degree, by vagal stimulation. -In this way, large amount of concentrated bile are dumped into the small intestine shortly after a meal.

Describe the gallbladder.

-The gallbladder is a saclike structure for bile storage. -It is on the inferior surface of the liver; it is about 8 cm long and 4 cm wide. -Three tunics form the gallbladder wall: (1) an inner mucosa folded into rugae which allow the gallbladder to expand; (2) a muscularis, which is a lyre of smooth muscle that allows the gallbladder to contract; and (3) an outer covering of series. -The cystic duct connects the gallbladder to the common bile duct.

What is gallbladder.

-The gallbladder is a small sac on the inferior surface of the liver that stores bile. -Bile can flow form the gallbladder through the cystic duct into the common bile duct, or it can flow back up the cystic duct into the gallbladder.

What are the five functions of the liver?

-The hepatocytes of the liver perform five important functions: (1) bile production for digestion and excretion, (2) storage of nutrients, (3) processing of nutrients, (4) detoxification, and (5) synthesis of new molecules.

Describe the ileocecal sphincter at the juncture between the ileum and the large intestine.

-The ileocecal sphincter at the juncture between the ileum and the large intestine remains mildly contracted most of the time, but peristaltic waves reaching it for the small intestine cause it to relax and allow the chyme to move form the small intestine into the cecum. -Cecal distension, however, initiates a local reflex that causes more intense constriction of the ileocecal sphincter. -Closure of the sphincter facilitates digestion and absorption in the small intestine by slowing the rate of chyme movement from the small intestine into the large intestine and prevents material form retuning to the ileum from the cecum.

Describe the enterogastric reflex.

-The inhibition of gastric secretion is also under nervous control. -The enterogastric reflex consists of a local reflex and a reflex integrated within he medulla oblongata that reduce gastric secretion. -Distension of the duodenal wall, the presence of irritating substances in the duodenum, reduced pH, and hypertonic or hypotonic solutions in the duodenum activate the enterogastric reflex.

What are the three layers of the mucosa?

-The innermost tunic, the mucosa, or mucous membrane, consists of three layers: 1. The inner mucous epithelium, which is moist stratified squamous epithelium in the mouth, oropharynx, esophagus, and anal canal and simple columnar epithelium in the remainder of the digestive tract; 2. a loose connective tissue called the lamina propria; and 3. a thin outer layer of smooth muscle called the muscular mucosa.

How does the internal and external anal sphincters prevent defecation?

-The internal and external anal sphincters prevents defecation. -Resting sphincter pressure results from tonic muscle contractions, mostly of the internal anal sphincter. -In response to increased abdominal pressure, reflexes mediated through the spinal cord cause contractions of the external anal sphincter. -Thus, the untimely expulsion of feces during coughing or exertion is avoided.

Describe the Jejunum and Ileum.

-The jejunum and ileum are similar in structure to the duodenum. -However, progressing from the duodenum through the ileum, there are gradual decreases in the diameter of the small intestine, the thickness of the intestinal wall, the number of circular folds, and the number of villi. -Lymphatic nodules called Peyer patches are numerous in the mucosa and submucosa of the ileum. -Peyer patches are numerous in the mucosa and submucosa of the ileum. -Peyer patches and other mucosa-associated lymphoid tissue int he digestive tract initiate immune responses against microorganisms that enter the mucosa form ingested food. -The site where the ileum connects to the cecum of the large intestine in the ileocecal junction. -The junction is a ring of smooth muscle, the ileocecal sphincter, and one-way ileocecal valve. -Together, the sphincter and valve allow intestinal contents to move from the ileum to the large intestine, but not in the opposite direction.

What is the large intestine?

-The large intestine is the portion of the digestive tract extending form the ileocecal junction to the anus. -It consists of four parts: (1) cecum, (2) colon, (3) rectum, and (4) anal canal. -Normally, 18-24 hours required for material to pass through the small intestine. -Thus, the movements of the colon are more sluggish than those of the small intestine. -While int eh colon, chyme is converted to feces. -The formation of feces involves the absorption of water and slats, secretion of mucus, and extensive action of microorganisms. -The colon stores the feces until they are eliminated by defecation. -About 1500 mL of chyme enter the cecum each day, but more than 90% of the volume is reabsorbed, so that only 80-150mL of feces are normally eliminated by defecation.

What are the largest salivary glands.

-The largest salivary glands, the parotid glands, are serous glands, which produce mostly watery saliva; they are located just anterior to the ear on each side of the head.

Describe the anal canal.

-The last 2-3 cm of the digestive tract is the anal can. -It begins at the inferior end of the rectum and ends at the anus (external digestive tract opening). -The smooth muscle layer of the anal can is even thicker than that of the rectum and forms the internal anal sphincter at its superior end. -Skeletal muscle forms the external anal sphincter at the inferior end of the canal. -The epithelium of the superior part of the anal can is simple columnar, and that of the inferior part is stratified squamous. -Rectal veins that supply the anal canal can become enlarged or inflamed, a condition known as hemorrhoids. -Hemorrhoids cause pain, itching, and bleeding around the anus. -They can usually be treated by changes in diet or medications.

Describe the lips.

-The libs, or labia, are muscular structures formed mostly by the orbiculares iris muscle and connective tissue. -The outer surfaces of the lips are covered by skin. -The keratinized stratified epithelium of the skin is thin at the margin of the lips and is not as highly keratinized as the epithelium of the surrounding skin; consequently, it is more transparent than the epithelium over the rest of the body. -The color from the underlying blood vessels shows through the relatively transparent epithelium, giving the lips a reddish-pink to dark red appearance, depending on the overlying pigment. -At the internal margin of the lips, the epithelium is continuous with the moist stratified squamous epithelium of the mucosa in the oral cavity. w

Why are the lips and cheeks important?

-The lips and cheeks are important in mastication and speech. -They help manipulate food within the oral cavity and hold it in place while the teeth crush or tear it. -They also help form words when we speak. -A large number of the muscles of facial expression are involved in moving the cheeks and lips.

Describe the synthesis of new molecules by the liver.

-The liver can produce its own new compounds, including plasma proteins such as albumins, fibrinogen, globulins, heparin, and clotting factors, which are related into the blood. In addition, the liver is the major site of cholesterol synthesis. -Cholesterol is used through the body.

How is the liver divided?

-The liver is divided into hepatic lobules. -Hepatic lobules are hexagon-shaped regions surrounded by connective tissue septa and defined by a portal triad at each corner and a central vein in the center of the lobule. -The portal triads are so named because three structures-the (1) hepatic portal vein, (2) hepatic artery, and (3) hepatic duct-are located in them. -Hepatic nerves and lymphatic vessels, often too small to be seen easily in light micrographs, are also located in these areas. -The central vein collects the blood as it leaves the lobule. -Central veins of the lobules unite to form hepatic veins, which exit the liver on its posterior and superior surfaces and empty into the inferior vena cava.

Describe the liver.

-The liver is the largest internal organ of the body, weighing about 1.36 kg (3 pounds). -It is in the right-upper quadrant of the abdomen, tucked against the inferior surface to he diaphragm. -The liver consists of two major lobes, (1) the right lobe and (2) the left lobe. -The right and left lobes are separated by a connective tissue septum, the falciform ligament. -Two minor lobes, (1) the caudate lobe and (2) quadrate lobe, can be seen form an inferior view, along with the aorta.

What does the liver produce and secrete?

-The liver produces and secretes about 600-1000 mL of bile each day (see table 24.2). -Bile is a complex solution that contains (1) bile salts, (2) bile pigments, (3) cholesterol, (4) lipids, (5) lipid-soluble hormones, and lecithin (a mixture of phospholipids). -Bile contains no digestive enzymes, but it plays a role in digestion because it neutralizes stomach acid and emulsifies lipids. -The relative alkaline pH of bile helps neutralize the acidic chyme entering the duodenum. -This is important because the pancreatic enzymes cannot function at the acidic pH of the chyme leaving the stomach. -Bile salts emulsify lipids, which is necessary for subsequent digestion by lipase. -Bile pigments are not required for any digestive function but rather are excretory products. -Bile pigments have greenish-yellow to brown colors and give feces their characteristic color. -One common bile pigment is bilirubin, which results from the breakdown of hemoglobin.

Describe the function the stomach low pH.

-The low pH of the stomach's contents has additional functions. -Stomach acid denatures many proteins, so that proteolytic enzymes can reach internal peptide bonds. -The acid environment provides the proper pH for the activation and function of pepsin. -The acid also stops carbohydrate digestion by inactivating salivary amylase.

Describe mucus production of the large intestine.

-The major secretion product of the large intestine is mucus. -Mucus is secreted from the numerous goblet cells scattered along int eh length of the mucosa of the colon. -In addition, there are numerous crypts lined almost entirely with goblet cells. -Little enzymatic activity is associated with secretions of the colon. -Mucus lubricates the wall of the colon and helps the fecal matter stick together. -Tactile stimuli and irritation of the colon wall trigger local enteric reflexes that increase mucous secretion. -Parasympathetic stimulation also increases the secretory rate of the goblet cells.

Describe the mastication reflex.

-The mastication reflex, or chewing reflex, is integrated in the medulla oblongata and controls the basic movements of chewing. -The presence of food in the mouth stimulates sensory receptors, which activate a reflex that relaxes the muscles of mastication. -As the mandible is lowered, the muscles stretch and activate a reflex that causes the muscles of mastication to contract. -Once the mouth is closed, the food again stimulates the muscles of mastication to relax, and the cycle repeats. -Descending pathways from the cerebrum strongly influence the mastication reflex, so that chewing can be consciously initiated or stopped. -The rate and intensity of chewing movements can also be influenced by the cerebrum.

Describe the omentum of the stomach.

-The mesentery connecting the lesser curvature of the stomach and the proximal end of the duodenum to the liver and diaphragm is called the lesser momentum, and the mesentery extending as a fold from the greater curvature and then to the transverse colon is called the greater momentum. -The greater momentum forms a long, double fold of mesentery that extends inferiorly from the stomach over the surface of the small intestine. -because of this folding, a cavity called the momentum bursa forms between the two layers of mesentery. -A large amount of adipose tissue accumulates in the greater momentum, and it is sometimes referred to as the "fatty apron." -The greater momentum has considerable mobility in the abdomen.

What is the moistening function of saliva aided by?

-The moistening function of saliva is aided by the large volume of serous saliva, 1-1.5 L/day, secreted primarily by the parotid and submandibular glands. -In addition, the mucous secretions of the submandibular and sublingual glands contain a large amount of mucin, a proteoglycan that gives a lubricating quality to the secretions of the salivary glands.

Describe how monosaccharides are taken up.

-The monosaccharides glucose and galactose are taken up into intestinal epithelial cells by symport, powered by a Na+ gradient. -The Na+ gradient is generated by the Na+ - K+ pump located on the basolateral membrane. -Diffusion of Na+ down its concentration gradient provides the energy to transport glucose or galactose across the plasma membrane. -In contrast to glucose and galactose, the monosaccharide fructose is taken up by facilitated diffusion. -Once inside the intestine epithelial cell, monosaccharides are transported by facilitated diffusion to the capillaries of the intestinal villi and carried by the hepatic portal system to the liver, where the non glucose monosaccharides are converted to glucose. -Glucose enters the cells through facilitated diffusion. -The rate of glucose transport into most types of cells is greatly influenced by insulin and may increase 10-fold in its presence.

Describe the four major cell types of the mucosa of the duodenum.

-The mucosa of the duodenum is simple columnar epithelium with four major cell types: (1) absorptive cells, (2) goblet cells, (3) granular cells, and (4) endocrine cells. -Absorptive cells are cells with microvilli that produce digestive enzymes and absorb digested food. -Goblet cells produce a protective mucus. -Granular cells, or Paneth cells, may help protect the intestinal epithelium from bacteria. -Endocrine cells produce regulatory hormones. -The hormones secretin and cholecystokinin stimulate hepatic and pancreatic secretions.

What does the mucosa of the small intestine produce?

-The mucosa of the small intestine produces secretions that contain primarily mucus, electrolytes, and water that lubricate and protect the intestinal wall. 1. Mucus is secreted from the duodenal glands, intestinal glands, and goblet cells. -It protects the wall of the intestine from the irritating effects of acidic chyme and from the digestive enzymes that enter the duodenum from the pancreas. -Secretions from the duodenal glands are stimulated by the vagus nerve, secretin, and chemical or tactile irritation of the duodenal mucosa. -Chemical and tactile irritation of the mucosa also stimulate goblet cells to produce mucus. 2. Secretion of electrolytes and water from the intestinal epithelium helps keep the chyme in a liquid form to facilitate the digestive process by pancreatic enzymes and brush border enzymes.

What does the mucosal lining of the large intestine consist of?

-The mucosal lining of the large intestine consists of simple columnar epithelium. -This epithelium is not formed into folds or villi like that of the small intestine but has numerous, straight, tubular glands called crypts. -The crypts, which are somewhat similar to the intestinal glands of the small intestine, are composed of three cell types. 1. absorptive. 2. goblet 3 granular. -The major difference is that, in the large intestine, goblet cells predominate, and the other two cell types are greatly reduced in number.

Name the epithelial cells of the stomach.

-The mucous lining of the stomach is simple columnar epithelium. -The epithelium forms numerous, tubelike invaginations called gastric pits. -Gastric pits are the openings for the gastric glands that secrete acid and other substances. -There are five types of epithelial cells of the stomach: surface mucous cells, mucous neck cells, parietal cells, chief cells, and endocrine cells. -All but the surface mucous cells are found in the gastric glands.

Describe the muscularis of the colon.

-The muscularis of the colon differs from the small intestine. -While the circular muscle layer of the colon is complete, the longitudinal muscle layer is incomplete. -Rather than completely enveloping the intestinal wall, the longitudinal layer forms three bands, called the tennis coli. -The tennis coli run the length of the colon. -Contractions of the tennis coli cause pouches called hausfrau to form along the length of the colon, giving it a puckered appearance. -Small, lipid-filled connective tissue pouches called mental appendages are attached to the outer surface of the colon along its length.

Describe the neural mechanisms that stimulate stomach secretion.

-The neural mechanisms that stimulate stomach secretions are also involved with increasing stomach motility. -The major stimulus for both motility and secretion is distension fo the stomach wall. -Increased stomach motility increases stomach emptying. -Conversely, the hormonal and neural mechanisms associated with the duodenum that decrease gastric secretions also decrease gastric motility and increase constriction of the pyloric sphincter. -The enterogastric reflex and the hormone cholecystokinin are major inhibitors of gastric motility. -The result is a decrease in the rate of stomach emptying.

Describe the muscularis.

-The next tunic is the muscular, a muscular layer. -The muscularis consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle. -Two exceptions are the upper esophagus, where the muscles are skeletal, and the stomach, which has three layers of smooth muscle.

Describe the two regions of the oral cavity.

-The oral cavity, or mouth, is divided into two regions: (1) The vestibule is the space between the lips or cheeks and the teeth and (2) the oral cavity proper lies medial to the teeth. -The oral cavity is lined with moist stratified squamous epithelium, which protects against abrasion.

How are elderly people affected by the aging of the digestive system?

-The overall decline in the defenses of the digestive tract leaves elderly people more susceptible to infections and toxic agents. -Elderly people are therefore more likely to develop ulcerations and cancers of the digestive tract. -Colorectal cancers, for example, are the second-leading cause of cancer deaths in the United States, with an estimated 135,000 new cases and 57,000 deaths each year.

Describe the anatomy of the pancreas.

-The pancreas is a complex organ composed of both endocrine and exocrine tissues that perform several functions. -The pancreas is located behind the stomach. -The head of the pancreas is nestled within the curvature of the duodenum. -The body and tail extend to the spleen.

Describe the pharynx.

-The pharynx, describe in detail in chapter 23, consists of three parts: (1) nasopharynx, (2) oropharynx, and (3) larygnopharynx. -Normally, only the oropharynx and laryngopharynx transmit food. -The oropharynx communicates with the nasopharynx superiorly, with the larynx and laryngopharynx inferiorly, and with the mouth anteriorly. -The larygopharynx extends from the oropharynx to the esophagus and is posterior to the larynx. -The epiglottis covers the opening of the larynx and keeps food and drink from entering the larynx. -The posterior walls of the oropharynx and laryngopharynx consist of three muscles: the superior, middle, and inferior pharyngeal constrictors, which are arranged like three stacked flowerpots, one inside the other. -The oropharynx and the laryngopharynx are lined with moist stratified squamous epithelium, and the nasopharynx is lined with ciliated pseudo stratified columnar epithelium.

Describe why the porta of the liver is important.

-The porta (gate) is on the inferior surface of the liver, where blood vessels and nerves enter and bile ducts and lymphatic vessels leave the liver. -Blood flows into the liver via the hepatic portal vein and the hepatic artery. -Bile flows out of the liver via two hepatic ducts, one each form the right and left lobes, exit the liver at the porta.

Describe how the livers processes nutrients.

-The processing, or interconversion, of nutrients is another important function of the liver. -Ingested nutrients are not always present in the proportion needed by the tissues. -In this case, the liver can convert some nutrients into others. -For example, if a person is on a diet that is excessively high in protein, an oversupply on amino acids and an undersupply of lipids and carbohydrates may be delivered to the liver. -The hepatocytes break down the amino acids and cycle many of them through metabolic pathways, so that they can be used to produce adenosine triphosphate, lipids, and glucose.

What is the rectum?

-The rectum is a straight, muscular tube that begins at the distal end of the sigmoid colon and ends at the anal canal. -The mucosal lining of the rectum is simple columnar epithelium, and the muscular tunic is relatively thick compared with the rest of the digestive tract.

Describe the regulation of stomach secretion.

-The regulation of stomach secretion is divided into three phases: cephalic, gastric, and intestinal. -The cephalic phase can be viewed as the "get started" phase, when stomach secretions are increased in anticipation of incoming food. -This is followed by the "go for it" gastric phase, when most of the stimulation of secretion occurs. -Finally, the intestinal phase is the "slow down" phase, during which stomach secretion decreases.

Describe the palate.

-The roof of the oral cavity is called the palate. -The palate separates the oral and nasal cavities and prevents food from passing into the nasal cavity during chewing and swallowing. -The palate consists of two parts. -The anterior, bony part is the hard palate. -The posterior, non bony part is the soft palate, which consists of skeletal muscle and connective tissue. -The uvula is a posterior projection from the soft palate. -The posterior boundary of the oral cavity is the faucet, which is the opening into the pharynx, or throat. -The palatine tonsils are in the lateral wall fo the faucet.

Explain the histology of the stomach.

-The serosa, or visceral peritoneum, is the outermost tunic of the stomach. -It consists of an outer layer of simple squamous epithelium and an inner layer of connective tissue. -The muscularis of the stomach consists of three layers: (1) an outer longitudinal layer, (2) a middle circular layer, and (3) an inner oblique layer. -The inner oblique layer is unique to the stomach wall. -This layer helps generate the strong stomach contractions that physically break down ingested food into smaller particles. -In some areas of the stomach, such as the fungus, the three layers blend with one another and cannot be separated. -Deep to the muscularis are the submucosa and the mucosa, which are thrown into large folds called rugae when the stomach is empty. -These folds allow the mucosa and submucosa to stretch, and the folds disappear as the stomach volume increases as it is filled.

What are the six major functions of the digestive system?

-The six major functions of the digestive system are (1) ingestion and mastication, (2) propulsion and mixing, (3) secretion, (4) digestion, (5) absorption, and (6) elimination.

What are the parts of the small intestine?

-The small intestine consists of three parts: (1) duodenum, (2) jejunum, and (3) ileum. -The entire small intestine is about 6 m long (range: 4.6 - 9 m). -The duodenum is about 25 cm long (duodenum means 12, suggesting that it is 12 inches long). -The jejunum, constituting about two-fifths of the total length of the small intestine, is about 2.5 m long. -The ileum, constituting three-fifths of the small intestine, is about 3.5 m long. -Two major accessory glands, the liver and the pancreas, are associated with the duodenum.

Describe what happens in the small intestines.

-The small intestine is where the greatest amount of digestion and absorption of nutrients and water occurs. -Within the small intestine, the duodenum and jejunum are the major sites of nutrient absorption, although some absorption occurs in the ileum. -Most of the water is absorbed by osmosis, along with the absorbed nutrients in the duodenum and jejunum. -Over 90% of the water is absorbed before the colon.

Describe the stomach.

-The stomach is an enlarged segment of the digestive tract that primarily functions as a storage and mixing chamber. -It is located in the left superior part of the abdomen. -Its shape and size vary from person to person, even within the same individual form time to time, depending on food content and body posture. -Nonetheless, several general anatomical features can be described.

Describe the anatomy of the stomach.

-The stomach is divided into four regions: (1) cardiac part, (2) fungus, (3) body, and (4) pyloric part. -The esophagus opens into the cardiac part of the stomach at the gastro-esophageal opening. -The lower esophageal sphincter, also called the cardiac sphincter, surrounds the opening. -Recall that, although this is an important structure in the normal function fo the stomach, it is a physiological constrictor only and cannot be seen anatomically. -The part of the stomach to the left of the cardiac part, the fundus, is actually superior to the cardiac opening. -The largest part of the stomach is the body, which turns to the right, creating a greater curvature and a lesser curvature. -The body narrows to from the funnel-shaped pyloric part of the stomach. -The wider part of the funnel, toward the body of the stomach, is the pyloric antrum. -The narrow part of the funnel is the pyloric canal. -The pyloric canal opens through the pyloric orifice into the small intestine. -The pyloric orifice is surrounded by the pyloric sphincter, or pylorus, a relatively thick ring of smooth muscle, which helps regulate the movement of gastric contents into the small intestine. -Hypertrophic pyloric stenosis is a common defect of the stomach in infants, in which the pyloric sphincter is greatly thickened and thus interferes with normal stomach emptying.

Describe sublingual glands.

-The sublingual glands, the smallest of the three large, paired salivary glands, are mixed glands containing some serous acini but consisting primarily of mucous acini. -They lie immediately below the mucous membrane in the floor fo the oral cavity. -These glands do not have single, well-defined ducts like those of the submandibular and parotid glands. -Instead, each sublingual gland opens into the floor of the oral cavity through 10-12 small ducts.

Describe the submandibular gland.

-The submandibular glands are mixed glands with more serous than mucous acini. -Each gland can be felt as a soft lump along the inferior border of the posterior half of the mandible. -A submandibular duct exits each gland, passes anteriorly deep to the mucous membrane on the floor of the oral cavity, and opens into the oral cavity beside eta frenulum of the tongue.

What do the surface of mucous cells and mucous neck cells secrete?

-The surface mucous cells and mucous neck cells secrete a vicous, alkaline mucus that covrers the surface of the epithelial cells, forming a layer 1-1.5 mm thick. -The thick layer of mucus lubricates and protects the epithelial cells of the stomach wall from the damaging effect of the acidic chyme and pepsin. -Irritation of the stomach mucosa stimulates the secretion of a greater volume of mucus.

Describe the modification of the surface of the duodenum/.

-The surface of the duodenum has three modifications that increase its area about 600-fold to allow for more efficient digestion and absorption of food. -All three of these modifications increase the surface area of the small intestine, which greatly enhances absorption.

Describe permanent teeth and milk teeth.

-The teeth of the adult mouth are called permanent teeth, or secondary teeth. -Most are replacements for deciduous teeth, or primary teeth, also called milk teeth, which are lost during childhood. -The deciduous teeth erupt (the crowns appear within the oral cavity) between about 6 months and 24 month of age. -The permanent teeth begin replacing the deciduous teeth at about 5 years, and the process is completed by about 11 years.

Describe teeth.

-The teeth play an important role in mastication and assist in speech. -Adults normally have 32 teeth, which are distributed in two dental arches: the maxillary arch and the mandibular arch. -The teeth in the right and left halves of each dental arch are roughly mirror images of each other. -As a result, the teeth are apportioned into four quadrants: right-upper, left-upper, right-lower, and left-lower. -The teeth in each quadrant include on central and one lateral incisor; one canine; first and second premolars; and first, second, and third molars. -The third molars are often called wisdom teeth because they usually appear in the late teens or early twenties, when a person is old enough to have acquired some wisdom. -In people with small dental arches, the third molars may not have room to erupt into the oral cavity and remains embedded within the jaw. -Embedded wisdom teeth are referred to as impacted and may cause pain or irritation. -Usually, the impacted wisdom teeth are surgically removed.

What are the three major proteolytic enzymes?

-The three major proteolytic enzymes are (1) trypsin, (2) chymotrypsin, and (3) carboxypeptidase. -These enzymes, which digest proteins, are secreted in inactive forms, whereas many of the other enzymes are secreted in active form. -The inactive forms are called trypsinogen, chymotrypsiongen, and procarboxypeptidase. -They are activated by proteolytic removal of certain peptides from the precursor proteins. -The proteolytic enzyme enterokinase, which is attached to the brush border of the small intestine, activates. trypsinogen. -Trypsin then activates more trypsinogen, as well as chymotrypsinogen and procarboxypeptidase. -This process of releasing inactive enzymes is necessary because if the enzymes were produced int heir active forms, they would start to digest the pancreas itself. -Inappropriate activation causes pancreatitis, which is a painful inflammation of the pancreas. -Pancreatitis can result from alcoholism, the use of certain drugs, pancreatic duct blockage, cystic fibrosis, viral infection, or pancreatic cancer. -Symptoms can range form abdominal pain to systemic shock and coma.

Describe the tongue.

-The tongue is a large, muscular organ that occupies most of the oral cavity proper when the mouth is closed. -Its major attachment in the oral cavity is through its posterior part. -The anterior part of the tongue is relatively free, except for attachment to the floor of the mouth by a thin fold of tissue called the lingual frenulum. -The muscles associated with the tongue are divided into two categories: Intrinsic muscle are within the tongue itself, and extrinsic muscles are outside the tongue but attached to it. -The intrinsic muscles are largely responsible for changing the shape of the tongue, such as flattening and elevating it during drinking and swallowing. -The extrinsic tongue muscles protrude and retract the tongue, move it from side to side, and change its shape.

What does the tongue do?

-The tongue moves food in the mouth and, in cooperation with the lips and gums, holds the food in place during mastication. -It also plays a major role in swallowing. -In addition, the tongue is a major sensory organ for taste and one of the primary organs for speech. -Patients with cancer of the tongue often have part or all of their tongue removed. -These patients can lear to speak fairly well but have difficulty chewing and swallowing.

Describe the peritoneum.

-The walls and organs of the abdominal cavity are lined with serous membranes. -These membranes are called the peritoneum. -Serous membranes are very smooth and secrete a serous fluid, which provides a lubricating film between the layers of membranes. -The membranes and fluid reduce friction as organs move within the abdominal cavity. -The serous membrane that covers the organ is the visceral peritoneum, and the one that covers the interior surface of the wall of the abdominal cavity is the parietal peritoneum. -Serous membranes also line other organs of the body.

What are the two hormones involved in the intestinal phase?

-There are 2 hormones involved in the intestinal phase: (1) secretin and (2) cholecystokinin. -Secretin is released in response to acidic solutions in the duodenum. -Secretin inhibits gastric secretions by inhibiting both parietal and chief cells. -Cholecystokinin is released in response to fatty acids, other lipids, and to a lesser degree protein digestion products in the duodenum and the proximal jejunum. -Cholecystokinin inhibits gastric secretions.

Describe what happens when the digestive system ages.

-There are a variety of diseases and disorders associated with the digestive system. -As a person ages, gradual changes occur throughout the digestive tract. -The connective tissue layers of the digestive tract-the submucosa and serosa-tend to thin. -The blood supply of the digestive tract decreases. -The number of smooth muscle cells in the muscularis also decreases, resulting in reduced motility in the digestive tract. -In addition, goblet cells within the mucosa secrete less mucus. -Glands along the digestive tract, such as the gastric glands, the liver, and the pancreas, also tend to secrete less with age. -However, these changes by themselves do not appreciably decrease the function of the digestive system.

What are several protective functions of saliva?

-There are several protective functions of saliva. 1. The large volume of saliva helps prevent bacterial infection int he mouth by continually washing the oral surface. 2. The bicarbonate ions in salvia act as a buffer to neutralize the acids produced by oral bacteria. -This reduces the harmful effects of bacterial acids on tooth enamel. 3. Saliva contains the proteins lysozyme, an enzyme that has a weak antibacterial action, and immunoglobulin A, which helps prevent bacterial infection. 4. The mucous in saliva helps protect the digestive tract from physical irritation and enzymatic digestion. -Any lack of salivary gland secretion increases the risk for ulceration and infection of the oral mucosa and for caries (cavities) in the teeth.

What are the three major types of enteric neurons?

-There are three major types of enteric neurons: (1) Enteric sensory neurons detect changes in the chemical composition of digestive tract contents or detect mechanical changes, such as stretch of the digestive wall; (2) enteric motor neurons stimulate or inhibit smooth muscle contraction and glandular secretion in the digestive system; and (3) enteric interneurons connect enteric sensory and motor neurons.

What are the two major types of mixing contractions?

-There are two major types of mixing contractions: a. Mixing waves in the stomach are gentle contractions that churn the food with gastric secretions. -Ingested food is stored and mixed in the stomach, from where it is slowly released into the small intestine. b. Segmental contractions mix food particles with digestive secretions in the small intestine.

Explain conscious control of the defecation reflex.

-There is also conscious control of the defecation reflex. -Action potentials form the sacral spinal cord ascend to the brain, where parts of the brainstem and hypothalamus inhibit or facilitate reflex activity in the spinal cord. -In addition, action potentials ascend to the cerebrum, where awareness of the need to defecate is realized. -The external anal sphincter is composed of skeletal muscle and is under conscious cerebral control. -If this sphincter is relaxed voluntarily, feces are expelled. -On the other hand, increased contraction of the external anal sphincter prevents defecation. -The defecation reflex persists for only a few minutes and quickly declines. -Generally, the reflex is reinitiated after a period that may be as long as several hours. -Mass movement in the colon are usually the reason for reinitiating of the defecation reflex.

What are the three major types of glands associated with the digestive tract?

-Three major types of glands are associated with the digestive tract: (1) unicellular mucous glands in the mucosa, (2) multicellular glands in the mucosa and submucosa, and (3) multicellular glands (accessory glands) outside the digestive tract.

What else happens when the digestive tract ages?

-Through the years, the digestive tract, like the skin and lungs, is directly exposed to material from the outside environment. -Some of those substances can cause mechanical damage to the digestive tract, and other are toxic to the tissues. -because the connective tissue of the digestive tract becomes thin with age and because the protective mucous covering is reduced, an elderly person's digestive tract becomes less and less protected for these outside influences. -In addition, the mucosa of elderly people tends to heal more slowly following injury. -Declines also occur in the liver's ability to detoxify certain chemicals, the hepatic phagocytic cells' ability to remove particulate contaminants, and the liver's ability to store glycogen. -These problems worsen in people who smoke.

Summarize the intestinal phase of stomach secretion.

-To summarize, gastric acid secretion is controlled by negative-feedback loops involving nerves and hormones. -During the gastric phase, high acid levels in the stomach trigger a decrease in additional acid secretion. -Then, during the intestinal phase, acidic chyme entering the duodenum triggers a decreases in stomach acid secretion. -These negative-feedback loops ensure that the acidic chyme entering the duodenum is neutralized, which is required for the digestion of food by pancreatic and brush border enzymes in the intestine, and for the prevention of peptic ulcer formation.

What constitutes the enteric nervous system.

-Together, the submucosal and myenteric plexuses constitute the enteric nervous system (ENS), or the enteric plexus, which is extermely important in controlling secretion and movement.

Describe vomiting.

-Vomiting is usually a protective mechanism against the ingestion of toxic or harmful substances. -Vomiting can result from irritation (e.g., overdistension or overexcitation) anywhere along the digestive tract. -Action potentials travel through the vagus nerve and spinal visceral afferent nerves to the vomiting center in the medulla oblongata. -Once the vomiting center is stimulated and the reflex is initiated, the following events occur: (1) A deep breath is taken; (2) the hyoid bone and larynx are elevated, opening the upper esophageal sphincter, (3) the opening of the larynx is closed; (4) the soft palate is elevated, closing the connection between the oropharynx and the nasopharynx; (5) the diaphragm and abdominal muscles are forcefully contracted, strongly compressing the stomach and increasing the intragastric pressure; (6) the lower esophageal sphincter is relaxed; and (7) the gastric contents are forced out of the stomach, through the esophagus and oral cavity, to the outside.

What happens when H+ is pumped into the stomach lumen?

-While H+ is pumped into the stomach lumen, bicarbonate ions move down their concentration gradient from the parietal cell into the extracellular fluid. -During this process, HCO3- is exchanged for Cl- through an antiporter, which is located in the plasma membrane, and the Cl- subsequently moves into the cell. -This results in an elevated blood pH in the veins that carry blood away from the stomach, called the alkaline tide. -An alkaline tide normally occurs after eating a meal.

Describe the interior of the teeth.

-Within the center of the tooth, in the neck and root, is a pulp cavity, which is filled with blood vessels, nerves, and connective tissue called pulp. -The portion of the pulp cavity within the root is called the root canal. -The nerves and blood vessels of the tooth enter and exit the pulp through a hole at the point of each root called the apical foramen. -The pulp cavity is surrounded by living, cellular, calcified tissue called dentin. -The dentin of the tooth crown is covered by an extermely hard, nonliving, acellular substance called enamel, which protect the tooth against abrasion and acids produced by bacteria in the mouth. -The surface of the dentin in the root is covered with a bonelike substance called cementum, which helps anchor the tooth to the periodontal ligament in the jaw. -The teeth are set in alveoli along the alveolar processes of the mandible and maxilla. -Dense fibrous connective tissue and stratified squamous epithelium, referred to as the gingiva, cover the alveolar processes. -Peridontal ligaments secure the teeth in the alveoli.

Describe interstitial cells.

-Within the myenteric plexus, specialized interstitial cells form a network of pacemakers, which promote rhythmic contractions of smooth muscle along the digestive tract. -These cells also help transmit signals from neurons to muscles to regulate movement. -Dysfunction of these pacemakers decreases motility in the digestive tract.

Describe what chylomicrons are.

-Within the smooth endoplasmic reticulum of the intestinal epithelial cells, free fatty acids are combined with monoglyceride molecules to form triglycerides. -Proteins synthesized in the epithelial cells attach to droplets of triglycerides, phospholipids and cholesterol called chylomicrons. -Chylomicrons contain about 90% triglyceride, 5% cholesterol, 4% phospholipid, and 1% protein. -The chylomicrons leave the epithelial cells and enter the lacteals of the lymphatic system within the villi. -Chylomicrons enter the lymphatic capillaries rather than the blood capillaries because the lymphatic capillaries rather than the blood capillaries because the lymphatic capillaries lack a basement membrane and are more permeable to large particles, such as chylomicrons, which are about 0.3 mm in diameter. -They travel through the lymphatic system via the thoracic duct to the blood stream and then by the blood to adipose tissue. -Before entering the adipose cells, triglycerides break back down into fatty acids and glycerol, which enter the adipocytes and are once more converted to triglycerides. -Triglycerides are stored in adipose tissue until an energy source is needed elsewhere in the body. -In the liver, the chylomicron lipids are stored, converted into other molecules, or used as energy. -The chylomicron remnant, minus the triglyceride, is conveyed through the blood to the liver, where it breaks up.

List the steps of segmental contractions.

1. A bolus within the tract begins at one location. 2. Segments of the digestive tract alternate between contractions and relaxation 3. The bolus spread out in both directions.

List the steps of movements in the stomach.

1. A mixing wave initiated in the body of the stomach progresses toward the pyloric sphincter (pink arrows directed inward). 2. The more fluid part of the chyme is pushed toward the pyloric sphincter (blue arrows), whereas the more solid center of the chyme squeezes past the peristaltic constriction back toward the body of the stomach (orange arrow). 3. Peristaltic waves (purple arrows) move int eh same direction and in the same way as the mixing waves but are stronger. 4. Again, the more fluid part of the chyme is pushed toward the pyloric region (blue arrows), whereas the more solid center of the chyme squeezes past the peristaltic constriction back toward the body of the stomach (orange arrow). 5. Peristaltic contractions force a few milliliters of the mostly fluid chyme through the pyloric opening into the duodenum (small red arrows). Most of the chyme, including the more solid portion, is forced back toward the body of the stomach for further mixing (yellow arrow).

Describe the steps of Peristalsis and Segmental Contractions.

1. A wave of smooth muscle relaxation moves ahead of the bolus, allowing the digestive tract to expand. 2. A wave of contraction of the smooth muscle behind the bolus propels it through the digestive tract.

List the steps of Amino Acid Transport Across the Intestinal Epithelium.

1. Acidic and most neutral amino acids are absorbed by symport into intestinal epithelia cells. 2. Symport is driven by a sodium gradient established by a Na+-K+ pump. 3. Amino acids move out of intestinal epithelial cells. 4. Amino acids enter the capillaries of the intestinal villi and are carried through the hepatic portal vein to the liver.

List the steps of Transport of Lipids Across the Intestinal Epithelium

1. Bile salts surround fatty acids and monoglycerides to form micelles. 2. Micelles attach to the plasma membranes of intestinal epithelial cells, and the fatty acids and monoglycerides pass by simple diffusion into the intestinal epithelial cells. 3. Within the intestinal epithelia cell, the fatty acids and monoglycerides are converted to triglycerides; proteins coat the triglycerides to form chylomicrons, which move out of the intestinal epithelial cells by exocytosis. 4. The chylomicron enter the lacteals of the intestinal villi and are carried through the lymphatic system to the general circulation.

List the steps of Hydrochloric Acid Production by Parietal Cells in the Gastric Glands of the Stomach.

1. Carbon dioxide (CO2) diffuses into the parietal cell. 2. Carbon dioxide combines with water (H2O) in an enzymatic reaction that is catalyzed by carbonic anhydrase (CA) to form carbonic acid (H2CO3). 3. Carbonic acid dissociates into a bicarbonate ion (HCO3-) and a hydrogen ion (H+). 4. Bicarbonate ions are transported back into the bloodstream. An anti porter in the plasma membrane exchanges HCO3- for a chloride ion (Cl-). 5. A H+-K+ pump moves H+ into the duct of the gastric gland and K+ into the parietal cell. 6. Chloride ions diffuse into the gastric gland duct.

List the steps of Transport of LDL into Cells.

1. Cells have pits on the surface, which contain LDL receptors. 2. LDL binds to the LDL, receptors in the pits. 3. The LDL, bound to LDL, receptors, is taken into the cell by endocytosis.

Describe the cephalic phase of stomach secretions.

1. Cephalic phase. -"Get started!" -The cephalic phase is the brain phase of stomach secretion. -It is controlled by the CNS. -It begins even before the bolus of food enters the stomach. -Several types of stimuli act not the centers within the medulla oblongata to influence gastric secretions. -These stimuli include the taste and smell of food, the stimulation of tactile receptors during the process of chewing and swallowing, and pleasant thoughts of food. -Action potentials are sent from the medulla oblongata along parasympathetic neurons within the vagus (X) nerves to the stomach. -Within the stomach wall, the preganglionic neurons stimulate the postganglionic neurons in the ENS. -The postganglionic neurons, which are primarily cholinergic, stimulate secretory activity in the cells of the stomach mucosa. -Parasympatehtic stimulation of the stomach mucosa results in the release of the neurotransmitter acetylcholine, which increases the secretory activity of both the parietal and the chief cells and stimulates the secretion of gastrin and histamine form endocrine cells. -The gastrin released into the circulation travels to the parietal cells, where it stimulates additional hydrochloric acid and pepsinogen secretion. -In addition, gastrin stimulates enterochromaffin-like cells to release histamine, which stimulates parietal cells to secrete hydrochloric acid. -Histamine acts as both a local paracrine chemical messenger and a hormone in the blood to stimulate gastric gland secretory activity. -Acetylcholine, histamine, and gastrin working together causes. greater secretion of hydrochloric acid than any of them does separately. -Of the three, histamine has the greatest stimulatory effect. -Drugs that block the actions of histamine are used to lower acid levels.

List the steps of the Intestinal phase.

1. Chyme in the duodenum with a pH less than 2 or counting lipids inhibits gastric secretions by three mechanisms (2-4). 2. Chemoreceptors in the duodenum are stimulated by H+ (low pH) or lipids. Action potentials generated by the chemoreceptors are carried by the vagus nerves to the medulla oblongata (light green arrow), where they inhibit parasympathetic action potentials (pink arrow), thereby decreasing gastric secretions. 3. Local reflexes activated by H+ or lipids also inhibit gastric secretion (orange arrows). 4. Secretin and cholecystokinin produced by the duodenum (dark red arrows) decrease gastric secretions in the stomach.

List the steps of Control of Defecation.

1. Distension of the stomach and chyme in the duodenum can stimulate the gastrocolic and duodenocolic reflexes. (green arrows). 2. Mass movements occur in the colon, which propel the contents toward the rectum (orange arrow). 3. Distension of the rectum by feces stimulates local defecation reflexes that cause contractions of the colon and rectum (brown arrows), which move feces toward the anus. 4. Local reflexes cause relaxation of the internal anal sphincter (brown arrows). 5. Distension of the rectum by feces stimulates parasympathetic reflexes. Action potentials are propagated to the defecation reflex center located in the spinal cord (yellow arrow). 6. Action potentials are propagated through ascending nerve tracts to the brain (blue arrow). 7. Action potentials are propagated through ascending nerve tracts to the brain (blue arrow). 8. Descending nerve tracts form the brain regulate the defecation reflex center (pink arrow). 9. Action potentials form the brain control the external anal sphincter (purple arrow).

List the steps of the Gastric phase.

1. Distension of the stomach stimulate mechanoreceptors (stretch receptors) and activates a parasympathetic reflex. Action potentials generated by the mechanoreceptors are carried by the vagus nerves to the medulla oblongata (light green arrow). 2. The medulla oblongata increases action potentials in the agues nerves that stimulate secretions by parietal and chief cells and stimulate gastrin and histamine secretion by endocrine cells (pink arrow). 3. Distension of the stomach also activates local reflexes that increase stomach secretions (orange arrow). 4. Gastrin is carried though the blood back to the stomach (dark green arrow), where, along with histamine, it stimulates secretion.

List the phases of swallowing.

1. During the voluntary phase, a bolus of food (yellow) is pushed by the tongue against the hard and soft palates and posteriorly toward the oropharynx (blue arrow indicates tongue movement; black arrow indicates movement of the bolus). Tan bone; purple; cartilage; red: muscle. 2. During the pharyngeal phase, the soft palate is elevated, closing off the nasopharynx. The pharynx and larynx are elevated (blue arrows indicate muscle movement; green arrow indicates elevation of the larynx). 3. Successive constriction of the pharyngeal constrictors from superior to inferior (blue arrows) forces the bolus through the pharynx and into the esophagus. As this occurs, the vestibular and vocal folds expand medially to close the passage of the larynx. The epiglottis (green arrow) is bent down over the opening of the larynx largely by the force of the bolus pressing against it. 4. As the inferior pharyngeal constrictor contracts, the upper esophageal sphincter relaxes (outwardly directed blue arrows) allowing the bolus to enter the esophagus. 5. During the esophageal phase, the bolus is moved by peristaltic contractions of the esophagus toward the stomach (inwardly directed blue arrows).

List the steps of Transport of Glucose Across the Intestinal Epithelium

1. Glucose is absorbed by symport with Na+ into intestinal epithelial cells. 2. Symport is driven by a sodium gradient established by a Na+ - K+ pump. 3. Glucose moves out of the intestinal epithelial cells by facilitated diffusion. 4. Glucose enters the capillaries of the intestinal villi and is carried through the hepatic portal vein to the liver.

Describe ingestion and mastication.

1. Ingestion and Mastication. Ingestion is the intake of solid or liquid food into the stomach. -The normal route of ingestion is through the oral cavity. -Mastication is the process by which the teeth chew food in the mouth to begin the process of digestion. -Digestive enzymes cannot easily penetrate solid food particles and can work effectively only on particle surfaces. -It is vital, therefore, that solid foods be mechanically broken down by mastication into smaller particles to increase the total surface area of food for digestion.

What do the digestive tract and associated accessory organs include?

1. Oral cavity, including the tongue and teeth, with the salivary glands as accessory organs. 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine, consisting of the duodenum, jejunum, and ileum, with the liver, gallbladder, and pancreas as accessory organs. 6. Large intestine, including the cecum, colon, rectum, anal canal, and anus.

List the regions of the digestive tract, from beginning to end.

1. Oral cavity, including the tongue and teeth, with the salivary glands as accessory organs. 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine, consisting of the duodenum, jejunum, and ileum, with the liver, gallbladder, and pancreas as accessory organs. 6. Large intestine, including the cecum, colon, rectum, anal canal, and anus.

List the steps of Control of Pancreatic Secretion.

1. Parasympathetic stimulate from the vagus nerve (red arrow) causes the release of enzyme-rich pancreatic juice. 2. Secretin (purple arrows) released from the duodenum stimulates, release of aqueous pancreatic juice rich in bicarbonate ions. 3. Cholecystokinin (pink arrows), released form the duodenum causes the release of enzyme-rich pancreatic juice.

Describe surface mucous cells.

1. Surface mucous cells are found on the surface around the gastric pit. -These cells protect the stomach wall form being damaged by acid and digestive enzymes. -The cells produce an alkaline mucus on their surface that neutralizes the acid and is a barrier to the digestive enzymes. -The surface mucous cells are connected by tight junctions, which provide an additional barrier that prevents acids and enzymes from reaching deeper tissues. -In addition, when surface mucous cells are damaged, they are rapidly replaced.

List the steps of Blood and Bile Flow Through the Liver.

1. The hepatic artery (red) carries oxygenated blood from the aorta through the aorta of the liver. Hepatic artery branches become part of the portal triads. Blood from the hepatic artery branches enters the hepatic sinusoids and supplies hepatocytes in the hepatic cords with oxygen. 2. The hepatic portal vein (blue) carries nutrient-rich deoxygenated blood from the intestines through the aorta of the liver. Hepatic portal vein branches become part of the portal triads. Blood from the hepatic portal vein branches enter the hepatic sinusoids and supplies hepatocytes in the hepatic cords with nutrients. 3. Blood in the hepatic sinusoids that comes from the hepatic artery and hepatic portal vein (purple)picks up plasma proteins, processed molecules, and waste products produced by the hepatocytes of the hepatic cords. 4. The hepatic sinusoids empty into central veins (blue). The central veins connect to hepatic veins, which connect to the inferior vena cava. 5. Bile produced by hepatocytes in the hepatic cords enters bile canaliculi (green), which connect to hepatic duct branches that are part of the portal triads. 6. The hepatic duct branches converge to form the left and right hepatic ducts, which carry bile out the porta of the liver.

List the steps of Flow of Bile and Pancreatic Secretions Through the Duct System of the Liver, Gallbladder, and Pancreas

1. The hepatic ducts, which carry bile from the liver lobes, combine to form the common hepatic duct. 2. The common hepatic duct combines with the cystic duct from the gallbladder to form the common bile duct. 3. The common bile duct and the pancreatic duct combine to form the hepatopancreatic ampulla. 4. The hpeatopancreatic ampulla empties bile and pancreatic secretions into the duodenum at the major duodenal papilla. 5. The accessory pancreatic duct empties pancreatic secretions into the duodenum at the minor duodenal papilla.

Describe the three modifications of the duodenum.

1. The mucosa and submucosa form a series of folds called the circular folds, or place circulates, which run perpendicular to the long axis of the digestive tract. 2. Tiny, fingerlike projections of the mucosa form numerous villi, which are 0.5-1.5 mm in length. Each villus is covered by simple columnar epithelium and contains a blood capillary network and a lymphatic capillary called a lacteal. 3. Most of the cells that make up the surface of the villi have numerous cytoplasmic extension (about 1 micrometer long) called microvilli, which further increase the surface area. The combined microvilli on the entire epithelial surface form the bush border.

List the steps of the Cephalic Phase.

1. The taste, smell, or though of food or tactile sensation of food in the mouth stimulate the medulla oblongata (light green arrows). 2. Vagus nerves carry parasympathetic action potentials to the stomach (pink arrow), where enteric plexus neurons are activated. 3. Postganglionic neurons stimulate secretion by parietal and chief cells and stimulate gastrin and histamine secretion by endocrine cells. 4. Gastrin is carried through the blood back to the stomach (dark green arrow), where, along with histamine, it stimulates secretion.

List the steps of Control of Bile Secretion and Release.

1. Vagal nerve stimulation (red arrow) stimulates bile secretion by the liver (green arrows inside the liver) and causes the gallbladder to contract, thereby releasing bile into the duodenum. 2. Secretin, produced by the duodenum (purple arrows) and carried through the blood to the liver, stimulates bile secretion by the liver (green arrows inside the liver). 3. Cholecystokinin, produced by the duodenum (pink arrows) and carried through the blood to the gallbladder, stimulates the gallbladder to contract and sphincters to relax, thereby releasing bile into the duodenum (green arrow outside the liver). 4. Bile salts also stimulate bile secretion. Over 90% of bile salts are reabsorbed in the ileum and returned to the liver (green arrows), where they stimulate additional secretion of bile salts.

List the steps of Bicarbonate Ion Production in the Pancreas

1. Water (H2O) and carbon dioxide (CO2) combine under the influence of cabronic anhydrase (CA) to form carbonic acid (H2CO3). 2. Carbonic acid dissociates to form hydrogen ions (H+) and bicarbonate ions (HCO3-). 3. The H+ is exchanged for sodium ions (Na+) by an anti porter. Sodium ions are removed by the Na+ -K+ pump. 4. The HCO3- is transported into the intercalated ducts in exchanged for Cl-, which returns to the lumen by a channel. Sodium ions and H2O follows he HCO3- into the ducts.

Describe the gastric phase of stomach secretions.

2. Gastric phase. -"Go for it!" -The gastric phase of stomach secretion produces the greatest volume of gastric secretions. -The presence of food int eh stomach initiates the gastric phase. -The primary stimuli are dissension of the stomach and the presence of amino acids and peptides in the stomach. -Distension of the stomach wall, especially in the body or fungus, stimulates mechanoreceptors. -Action potentials are generated by these receptors initiate reflexes that involve both the CNS and the ENS. -These reflexes result in acetylcholine release and the cascade of events that increase secretion, as in the cephalic phase. -The presence of partially digested proteins or moderate amounts of alcohol or caffeine int eh stomach also stimulates gastrin secretion. -When the pH of the stomach contents falls below 2, increased gastric secretion produced by distension of the stomach is blocked. -This negative-feedback mechanism limits the secretion of gastric juice.

Describe mucous neck cells.

2. Mucous neck cells are located near the openings of the glands and produce mucus.

Describe propulsion and mixing.

2. Propulsion and Mixing. -Propulsion is the movement of food from one end of the digestive tract to the other. -Mixing is the movement of food back and forth in the digestive tract, without forward movement. -The total time it takes food to travel the length of the digestive tract is usually about 24-36 hours. -Each segment of the digestive tract is specialized to assist in moving its contents from the oral end to the anal end. -The propulsive movement begin with swallowing, followed by peristalsis, and finally mass movements.

Describe the intestinal phase of stomach secretions.

3. Intestinal phase. -"Slow down!" -The intestinal phase of gastric secretion inhibits gastric secretions. -It is initiated by acidic chyme entering the duodenum of the small intestine, which activates both neural and hormonal mechanisms. -Secretions are inhibited when the pH of the chyme entering the duodenum drops to 2 or below. -In addition, when the chyme contains lipid digestion products, gastric secretions are inhibited.

Describe parietal cells.

3. Parietal cells produce hydrochloric acid and intrinsic factor.

Describe secretion.

3. Secretion. -Secretions are added to lubricate, liquefy, buffer, and digest the food as it moves through the digestive tract. -Mucus, secreted along the entire digestive tract, lubricates the food and the lining of the tract. -The mucus coats and protects the epithelial cells of the digestive tract from mechanical abrasion, stomach acid, and digestive enzymes. -The secretions also contain large amounts of water, which liquefies the food, making it easier to digest and absorb. -Water also moves into the intestine by osmosis, -Liver secretions break large lipid droplets into much smaller droplets, which makes the digestive and absorption of lipids possible. -Enzymes secreted by the oral cavity, stomach, small intestine, and pancreas breakdown large food molecules into smaller molecule that can be absorbed by the intestinal wall.

Describe chief cells.

4. Chief cells produce the enzyme pepsinogen. -They also produce the enzyme gastric lipase, which can digest lipids in the stomach.

Describe digestion.

4. Digestion. -Digestion is the breakdown of large organic molecules into their component parts. -Digestion consists of mechanical digestion, which involves the mastication and mixing of food, and chemical digestion, which is accomplished by digestive enzymes secreted along the digestive tract. -Large organic molecules must be digested into their component parts before they can be absorbed by the digestive tract. -Carbohydrates are broken into monosaccharides. -Proteins are broken into amino acids, and triglycerides into fatty acids and glycerol. -Minerals and water are not broken down before being absorbed. -Vitamins are also absorbed without digestion; in fact, they lose their function if their structure is altered by digestion.

Describe absorption.

5. Absorption. -Absorption is the movement of molecules out of the digestive tract and into the blood or into the lymphatic system. -The mechanism by which absorption occurs depends on the type of molecule involved. -Molecules pass out of the digestive tract by diffusion, facilitated diffusion, active transport, symport, or endocytosis.

Describe endocrine cells.

5. Endocrine cells produce regulatory hormones and paracrine factors. -There are several types of endocrine cells. -Enterochromaffin-like cells produce histamine, which stimulates acid secretion by parietal cells. -Gastrin-containing cells secrete gastrin, and somatostatin-containing cells secrete somatostatin, which inhibits gastrin and insulin secretion.

Describe Elimination.

6. Elimination -Elimination is the process by which the waste product of digestion are removed from the body. -During this process, which occurs primarily in the large intestine, water and salts are absorbed, changing the material in the digestive tract from liquefied to semisolid. -These semisolid waste products, called feces, are stored in the distal large intestine, and then eliminated by the processionals of defecation.

PROCESS FIGURE 24.12 Hydrochloric Acid Production by Parietal Cells in the Gastric Glands of the Stomach

A series of steps involving carbonic anhydrase, a proton pump, and a HCO3- -Cl- anti porter produce HCI in the gastric gland.

FIGURE 24.8 Molar Tooth in Place in the Alveolar Bone

A tooth consists of a crown (anatomical clinical), a neck, and a root. The root is covered with cementum, and the tooth is held in the socket by periodontal ligaments. Nerves and vessels enter and exit the tooth through the apical foramen.

Describe absorption of the stomach.

Absorption -Absorption of a few substances (e.g., water, alcohol, aspirin) takes place in the stomach.

Describe absorption of the small intestine.

Absorption -The circular folds, villi, and microvilli increase surface area. -Most nutrients are actively or passively absorbed. -Most of the ingested water or the water in digestive tract secretions is absorbed.

Describe absorption of the oral cavity.

Absorption -There is no absorption of nutrients in the mouth, although some drugs can be absorbed across the oral mucosa.

Describe absorption of the large intestine.

Absorption. -The proximal half of the colon absorbs salts (e.g., NaCl), water, and vitamins (e.g., K) produced by bacteria.

PROCESS FIGURE 24.23 Bicarbonate Ion Production in the Pancreas

Bicarbonate (HCO3-) is produced by carbonic anhydrase in duct epithelial cells. It is then secreted into the intercalated duct lumen in exchange for Cl-.

PROCESS FIGURE 24.20 Blood and Bile Flow Through the Liver

Blood enters the porta of the liver through the hepatic portal vein and hepatic artery, which branch to become part of the portal triads of liver lobules. The blood then empties into the central vein and is sent to the heart. Bile is produced in the liver and leaves the lobules via the hepatic duct branches that are part of the portal triad. E

Infections of the Digestive Tract: Cholera

Caused by a bacterium, Vibrio cholera, in contaminated water; bacteria produce a toxin that stimulates the secretion of chloride, bicarbonate, and water into the large intestine, resulting in sever diarrhea; the loss of as much as 12-20 L of fluid per day causes shock and even death; still a major health problem in parts of Asia.

Infections of the Digestive Tract: Giardiasis

Caused by a protozoan, Giardia lambda, invading the large intestine; symptoms include nausea, abdominal cramps, weakness, weight loss, and malaise; the protozoans are transmitted in the feces of humans and other animals, often by drinking from contaminated wilderness streams.

Infections of the Digestive Tract: Typhoid fever

Caused by a virulent strain of the bacterium Salmonella type, which can cross the intestinal wall and invade other tissues; symptoms include sever fever, headaches, and diarrhea; usually transmitted through poor sanitation practices; leading cause of death in may developing countries.

Infections of the Digestive Tract: Food poisoning

Caused by ingesting bacteria or toxins, such as Staphylococcus aureus, Salmonella, or Escherichia coli; symptoms include nausea, abdominal pain, vomiting, and diarrhea; in severe cases, death can occur.

List some characteristics of propulsive movement.

Characteristics of these propulsive movements are: a. Swallowing, or deglutition b. Peristalsis c. Mass movements d. Mixing contractions 1. Mixing waves 2. Segmental contractions.

Liver: Cirrhosis

Characterized by damage and death of hepatic cells and replacement by connective tissue; results in loss of normal liver function and interference with blood flow through the liver; a common consequence of alcoholism

PROCESS FIGURE 24.24 Control of Pancreatic Secretion

Cholecystokinin and parasympathetic impulses stimulate pancreatic enzyme secretion. Secretin stimulates secretion of bicarbonate from the pancreas.

Infections of the Digestive Tract: Intestinal parasites

Common under conditions of poor sanitation; parasites include tapeworms, pinworms, hookworms, and roundworms

Stomach: Vomiting

Contraction of the diaphragm and abdominal muscles and relaxation of the esophageal sphincters to forcefully expel gastric contents; vomiting reflex is initiated by irritation of the stomach or small intestine.

PROCESS FIGURE 24.27 Control of Defecation

Defecation involves both reflex and voluntary neural signals that are triggered by dissension of the rectum.

Describe digestion of the oral cavity.

Digestion -Mastication begins mechanical digestion of food. -Amylase in saliva begins carbohydrate (starch) digestion.

Describe digestion of the stomach.

Digestion. -Mechanical digestion occurs as food is churned in the stomach by mixing waves. -Protein digestion begins as a result of the actions of hydrochloric acid and pepsin. -Gastric lipase and lingual lipase digest a small amount of lipids.

Describe digestion of the small intestine.

Digestion. -Segmental contractions aid mechanical digestion. -Enzymes from the pancreas and the lining of the small intestine complete the breakdown of food molecules. -Bile salts from the liver emulsify lipids to allow lipid digestion.

Where do parotid ducts ext?

Each parotid duct exits the gland on its anterior margin, crosses the lateral surface of the master muscle, pierces the buccinator muscle, and enters the oral cavity adjacent to the second upper molar.

Describe elimination of the large intestine.

Elimination. -The distal half of the colon holds feces until they are eliminated.

FIGURE 24.34 Fluid Volumes in the Digestive Tract

Fluid movement across the digestive tract varies depending on the particular segment.

PROCESS FIGURE 24.14 Movements in the Stomach

Food is mixed with hydrochloric acid and other secretions in the stomach to create chyme.

Small Intestine: Enterokinase

Function: Activates trypsin from tryspinogen

Gastric: Hydrochloric acid

Function: Antibacterial; decreases stomach pH to activate pepsinogen to pepsin

Liver: Bile

Function: Bile salts in bile emulsify lipids, making them available to lipase, and help make end products soluble and available for absorption by the intestinal mucosa; many of the other bile contents are waste products, such as bile pigments, that are transported to the intestines for disposal.

Gastric: Intrinsic factor

Function: Binds to vitamin B13 and aids in its absorption in the small intestine

Oral Cavity: Lingual Lipase

Function: Digests a mini amount of lipids

Gastric: Gastric lipase

Function: Digests a minor amount of lipids

Oral Cavity: Salivary amylase

Function: Digests carbohydrates

Pancreas: Pancreatic amylase

Function: Digests carbohydrates (hydrolyzes starches and glycogen to form maltose and isomaltose)

Pancreas: Cholesterol esterase

Function: Digests cholesterol esters (breaks down into cholesterol and free fatty acid)

Pancreas: Deoxyribonuclease

Function: Digests deoxyribonucleic acid (hydrolyzes phosphodiester bonds)

Pancreas: Pancreatic lipase

Function: Digests lipids (breaks down triglycerides into monoglycerides and free fatty acids)

Gastric: Pepsin

Function: Digests protein into smaller peptide chains; activates pepsinogen

Pancreas: Trypsin

Function: Digests proteins (cleavers at arginine or lysine amino acids); activates trypsinogen and other digestive enzymes

Pancreas: Chymotrypsin

Function: Digests proteins (cleaves at hydrophobic amino acids)

Pancreas: Carboxypeptidase

Function: Digests proteins (removes amino acids from the carboxyl end of proteins)

Pancreas: Ribonuclease

Function: Digests ribonucleic acid (hydrolyzes phosphodiester bonds)

Esophagus: Mucus

Function: Lubricates esophagus; protects lining of esophagus from abrasion and allows food to move more smoothly through esophagus

Oral Cavity: Serous saliva (mostly water, bicarbonate ions)

Function: Moistens food and mucous membrane; neutralizes bacterial acids; flushes bacteria from oral cavity; has weak antibacterial activity

Pancreas: Bicarbonate ions

Function: Neutralize acid form stomach; provide appropriate pH for pancreatic enzymes

Small Intestine: Mucus

Function: Protects duodenum from stomach acid and intestinal wall from digestive enzymes

Gastric: Mucus

Function: Protects stomach lining from acid and digestion

Large Intestine: Mucus

Function: Provides adhesion for fecal matter; protects intestinal wall form bacterial acids and actions.

Small Intestine: Peptidases

Function: Split amino acids from polypeptides

Small Intestine: Isomaltase

Function: Splits isomaltose into two glucose molcules

Small Intestine: Lactase

Function: Splits lactose into glucose and galactose

Small Intestine: Maltase

Function: Splits maltose into two glucose molecules

Small Intestine: Sucrase

Function: Splits sucrose into glucose and fructose

PROCESS FIGURE 24.29 Transport of Glucose Across the Intestinal Epithelium

Glucose absorption occurs by symport powered by a Na+ gradient.

Rebecca kept attributing her recurring abdominal pain to "something I ate"--and she was partly right about that. Several times during the past year, eating high-fat meals had led to episodes of serious abdominal pain. During the most recent attack, the discomfort became so intense that Rebecca went to the emergency room, where she was given medication to relieve the pain. Still, over the next few hours, her skin took on a yellowish tint, and the next morning she had diarrhea and clay-colored feces. Following lab tests and ultrasonography, a physician diagnosed gallstones and recommended the removal of Rebecca's gallbladder. Explain how gallstones led to Rebeeca's pain and other symptoms.

In this chapter we learned that the gallbladder stores bile, a secretion of the liver that neutralizes stomach acids and emulsifies lipids. We also learned that gallstones may form when there is an abundance of cholesterol in the bile, such as from a high-cholesterol diet. Gallstones can block the cystic duct, blocking the flow of bile from the gallbladder to the duodenum. Eating food high in fat causes gallbladder contractions. Specifically, gallbladder contraction is stimulated hormonally by cholecystokinin from the duodenum and parasympathetic stimulation. If Rebecca's cystic duct were blocked by gallstones, the increased pressure int eh contracting gallbladder would result in pain and inflammation. Bu why did Rebecca's skin turn yellow? Recall that bile contains bilirubin, a yellow pigment produced from the breakdown of hemoglobin that when further processed in the intestine turns brown and contributes to the normal color of feces. The gallstones blocked Rebecca's common bile duct, preventing bile from passing form the liver to the duodenum and reducing the amount of bilirubin removed form the blood. Rebecca's skin turned yellow do to the accumulation entering the duodenum resulted in poor emulsification of lipids. The intestine, and the undigested lipids passing through the small and large intestine were responsible for the diarrhea. The lack of bilirubin resulted in the clay-colored feces.

Liver: Hepatitis A

Infectious hepatitis; usually transmitted by poor sanitation practices or from mollusks living in contaminated waters

Liver: Hepatitis

Inflammation of the liver that causes liver cell death and replacement by scar tissue; if not corrected, results in loss of liver function and eventually death; symptoms include nausea, abdominal pain, fever, chills, malaise, and jaundice; caused by any of seven distinct viruses.

Describe ingestion and mastication of the oral cavity.

Ingestion and Mastication. -Solid foods and fluids are taken into the digestive tract through the oral cavity. -Movement of the mandible by the muscles of mastication causes the teeth to break food into smaller pieces. -The tongue and cheeks help place the food between the teeth. -The food is mixed with saliva, which plays important protective roles in the oral cavity and allows the food to be tasted.

Infections of the Digestive Tract: Diarrhea

Intestinal mucosa secretes large amounts of water and ions due to irritation, inflammation, or infection; diarrhea moves feces out of the large intestine more rapidly and speeds recovery.

Stomach: Peptic ulcer

Lesions in the lining of the stomach or duodenum, usually due to infection by the bacterium Helicobacter pylori; stress, diet, smoking, or alcohol may be a predisposing factor; antibiotic therapy is the accepted treatment

PROCESS FIGURE 24.30 Transport of Lipids Across the Intestinal Epithelium

Lipid absorption occurs when micelles enter intestinal epithelial cells and exit by exocytosis.

Intestine: Inflammatory bowel disease (IBD)

Localized inflammatory degeneration that may occur anywhere along the digestive tract but most commonly involves the distal ileum and proximal colon; the intestinal wall often becomes thickened, constricting the lumen, with ulcers and fissures int eh damaged areas; symptoms include diarrhea, abdominal pain, fever, fatigue, and weight loss; cause is unknown; treatments involve anti-inflammatory drugs, avoidance of foods that produce symptoms, and surgery in some cases; also called Crohn disease or ulcerative colitis.

Oral Cavity: Mucus

Lubricates food; protects digestive tract from digestion

Intestine: Gluten enteropathy (celiac disease)

Malabsorption in the small intestine due to the effects of gluten, a protein in certain grains, especially wheat; the reaction can destroy newly formed epithelial cells, causing the intestinal villi to become blunted and decreasing the intestinal surface, which reduces absorption of nutrients.

Site of Production: Secretin Duodenum

Method of Stimulation: Acidity of chyme Secretory Effects: Decreases gastric secretion; stimulates pancreatic and bile secretions high in bicarbonate ions Motility Effects: Decreases gastric motility

Site of Production: Gastrin Stomach

Method of Stimulation: Distension; partially digested proteins, autonomic stimulation, ingestion of alcohol or caffeine Secretory Effects: Increases gastric secretion Motility Effects: Causes a minor increase in gastric motility

Site of Production: Cholecystokinin Duodenum

Method of Stimulation: Fatty acids and peptides Secretory Effects: Slightly decreases gastric secretion; stimulates pancreatic secretions high in digestive enzymes; causes contraction of the gallbladder and relaxation of the hepatopancreatic ampullae sphincter Motility Effects: Strongly decreases gastric motility

PROCESS FIGURE 24.33 Amino Acid Transport Across the Intestinal Epithelium

Most amino acid absorption occurs by symport powerd by a Na+ gradient.

Liver: Gallstones

Most often due to excess cholesterol in the bile; gallstones can enter the cystic duct, where they block the release of bile and/or pancreatic enzymes, which interferes with digestion.

FIGURE 24.36 Symptoms and Treatments of Diarrhea

Multiple system are affected by diarrhea.

Liver: Hepatitis C

Often a chronic disease leading to cirrhosis and possibly cancer of the liver

PROCESS FIGURE 24.21 Control of Bile Secretion and Release

Parasympathetic nerve impulses, cholecystokinin, and secretin all stimulate bile secretion and gallbladder contractions.

Describe propulsion and mixing of the esophagus.

Propulsion and Mixing. -Peristaltic contractions move the bolus from the pharynx to the stomach. -The lower esophageal sphincter limits reflux of the stomach contents into the esophagus.

Describe propulsion and mixing of the small intestine.

Propulsion and Mixing. -Segmental contractions mix the chyme, and peristaltic contractions move the chyme into the large intestine.

Describe propulsion and mixing of the stomach.

Propulsion and Mixing. -Mixing waves churn ingested materials and stomach secretions into chyme. -Rugae allow the stomach to expand and stored food. -This allows further mixing in the stomach prior to propulsion of small amounts of chyme into the small intestine. -Peristaltic waves move the chyme into the small intestine.

Describe propulsion and mixing of the large intestine.

Propulsion and Mixing. -Slight segmental mixing occurs. -Mass movements propel feces toward the anus, and defecation eliminates the feces.

Describe propulsion and mixing of the pharynx.

Propulsion and Mixing. -The involuntary phase of swallowing moves the bolus from the oral cavity to the esophagus. -Materials are prevented from entering the nasal cavity by the soft palate and kept out of the lower respiratory tract by the epiglottis and vestibular folds.

Describe propulsion and mixing of the oral cavity.

Propulsion and Mixing. -The tongue forms food into a bolus and pushes the bolus into the pharynx to begin the swallowing reflex.

Describe secretion of the small intestine.

Secretion -Bicarbonate ions from the pancreas and bile from the liver neutralize stomach acid to form a pH environment suitable for pancreatic and intestinal enzymes. -Mucus provides lubrication, prevents digestion of the intestinal wall, and protects the small intestine from stomach acid. -Bile from the liver contains bilirubin and excess cholesterol that will be eliminated in the feces.

Describe secretion of the esophagus.

Secretion. -Mucus provides lubrication and protects the inferior esophagus from stomach acid.

Describe secretion of the pharynx.

Secretion. -Mucus provides lubrication.

Describe secretion of the large intestine.

Secretion. -Mucus provides lubrication; mucus and bicarbonate ions protect against acids produced by bacteria.

Describe secretion of the stomach.

Secretion. -Release of hydrochloric acid creates the acidic stomach environment. -The acid kills most microorganisms and activates the precursor of the proteolytic enzyme pepsin. -Gastric lipase and intrinsic factor are secreted. -Mucus provides lubrication and prevents digestion of the stomach wall.

Describe secretion of the oral cavity.

Secretion. -Salvia contains mucin, bicarbonate, and water, which provide lubrication and protection of the oral cavity, and lysozyme (an enzyme that lyses cells) kills microorganisms. -Amylase and lingual lipase are also released in salvia.

Liver: Hepatitis B

Serum hepatitis; usually transmitted through blood or other body fluids through either sexual contact or contaminated hypodermic needles

Infections of the Digestive Tract: Dysentery

Severe form of diarrhea with blood or mucus in the feces; can be caused by bacteria, protozoa, or amoebae.

Intestine: Constipatoin

Slow movement of feces through the large intestine, causing the feces to become dry and hard because of increased fluid absorption while being retained; often results from inhibiting normal defecation reflexes; spasms of the sigmoid colon resulting from irritation can also result in slow feces movement and constipation; high-fiber diet can be preventive.

PROCESS FIGURE 24.3 Segmental Contractions

Smooth muscle contractions in the wall of the small intestine disperse digesting food throughout its lumen.

PROCESS FIGURE 24.32 Transport of LDL into Cells

The LDL receptor transports LDL into cells by endocytosis.

FIGURE 24.1 Digestive System

The digestive system consists of the digestive tract form the oral cavity to the anus, and the associated accessory organs: salivary glands, liver, gallbladder, and pancreas.

FIGURE 24.28 Digestion of the Three Major Food Types

The enzymes involved in digesting carbohydrates, lipids, and proteins are depicted in relation to the region of the digestive tract where each functions.

FIGURE 24.4 Digestive Tract Histology

The four tunics are the mucosa, the submucosa, the muscularis, and a serosa or an adventitia. In this image, the serosa is also called the visceral peritoneum, which forms part of the mesentery. Glands may exist along the digestive tract as part of the epithelium, as glands within the submucosa, or as large glands outside the digestive tract.

FIGURE 24.19 Histology of Hepatic Lobules in the Liver

The liver consists of hexagon-shaped lobules with a portal triad at each corner. A central vein is located in the center of each lobule.

PROCESS FIGURE 24.18 Flow of Bile an Pancreatic Secretions Through the Duct System of the Liver, Gallbladder, and Pancreas

The liver produces bile, some of which is stored in the gallbladder. The bile enters the duodenum through the common bile duct.

FIGURE 24.6 Oral Cavity

The oral cavity is the normal location for ingestion of liquid and solid food.

FIGURE 24.15 Small Intestine

The small intestine consists of three segments: the duodenum, jejunum, and ileum.

PROCESS FIGURE 24.10 Phases of Swallowing

The three phases of swallowing are voluntary, pharyngeal, and esophageal.

PROCESS FIGURE 24.13 Regulation of Stomach Secretions

There are three phases of stomach secretions: cephalic, gastric, and intestinal.

PROCESS FIGURE 24.2 Peristalsis and Segmental Contractions

Waves of smooth muscle contraction push food and waste through the digestive tract.

Describing swallowing.

a. Swallowing, or deglutition, moves liquids or a soft mass of food and liquid, called a bolus, from the oral cavity into the esophagus.

Describe peristalsis.

b. Peristalsis propels material through most of the digestive tract.

Describe mass movements.

c. Mass movements are contractions that move material in the distal parts of the large intestine to the anus.