Chapter Summary

Introduction to the Digestive System

1. The digestion of food molecules involves the hydrolysis of these molecules into their subunits.
1. The digestion of food occurs in the lumen of the GI tract and is catalyzed by specific enzymes.
2. The digestion products are absorbed through the intestinal mucosa and enter the blood or lymph.
2. The layers (tunics) of the GI tract are, from the inside outward, mucosa, submucosa, muscularis, and serosa.
1. The mucosa consists of a simple columnar epithelium, a layer of connective tissue called the lamina propria, and a thin layer of smooth muscle called the muscularis mucosa.
2. The submucosa is composed of connective tissue; the muscularis consists of layers of smooth muscles; and the serosa is connective tissue covered by the visceral peritoneum.
3. The submucosa contains the submucosal plexus, and the muscularis contains the myenteric plexus of autonomic nerves.

Esophagus and Stomach

1. Peristaltic waves of contraction push food through the lower esophageal sphincter into the stomach.
2. The stomach consists of a cardia, fundus, body, and pyloris (antrum). The pylorus with the pyloric sphincter.
1. The lining of the stomach is thrown into folds, or rugae, and the mucosal surface forms gastric pits that lead into gastric glands.
2. The parietal cells of the gastric glands secrete HCl; the chief cells secrete pepsinogen.
3. In the acidic environment of gastric juice, pepsinogen is converted into the active protein- digesting enzyme called pepsin.
4. Some digestion of protein occurs in the stomach, but the most important function of the stomach is the secretion of intrinsic factor, which is needed for the absorption of vitamin B₁₂ in the intestine.

Small Intestine

1. The small intestine is divided into the duodenum, jejunum, and ileum. The common bile duct and pancreatic duct empty into the duodenum.
2. Fingerlike extensions of mucosa called villi project into the lumen, and at the bases of the villi the mucosa forms narrow pouches called the crypts of Lieberkuhn.
1. New epithelial cells are formed in the crypts.
2. The membrane of intestinal epithelial cells is folded to form microvilli; this is called the brush border of the mucosa and serves to increase surface area.
3. Digestive enzymes, called brush border enzymes, are located in the membranes of the microvilli.
4. The small intestine exhibits two major types of movements, peristalsis and segmentation.

Large Intestine

1. The large intestine is divided into the cecum, colon, rectum, and anal canal.
1. The appendix is attached to the inferior medial margin of the cecum.
2. The colon consists of ascending, transverse, descending, and sigmoid portions.
3. Bulges in the walls of the large intestine are called haustra.
2. The large intestine absorbs water and electrolytes.
1. Although most of the water that enters the GI tract is absorbed in the small intestine, 1 to 1.5 L/day pass to the large intestine each day. The large intestine absorbs about 90% of this amount.
2. Na⁺ is actively absorbed and water follows passively, in a manner analogous to the reabsorption of NaCl and water in the renal tubules.
3. Defecation occurs when the anal sphincters relax and contraction of other muscles raises the rectal pressure

Liver, Gallbladder, and Pancreas

1. The liver, the largest internal organ, is composed of functional units called lobules.
1. Liver lobules consists of plates of hepatic cells separated by capillary sinusoids.
2. Blood flows from the periphery of each lobule, where the hepatic artery and portal vein empty, through the sinusoids and out the central vein.
3. Bile flows within the hepatocyte plates, in canaliculi, to the bile ducts.
4. Substances excreted in the bile can be returned to the liver in the hepatic portal blood; this is called an enterohepatic circulation.
5. Bile consists of a pigment called bilirubin, bile salts, cholesterol, and other molecules.
6. The liver detoxifies the blood by excreting substances in the bile, by phagocytosis, and by chemical inactivation.
7. The liver modifies the plasma concentrations of proteins, glucose, triglycerides, and ketone bodies.
2. The gallbladder stores and concentrates the bile. It releases bile through the cystic duct and common bile duct to the duodenum.
3. The pancreas is both an exocrine and endocrine gland.
1. The endocrine portion, known as the islets of Langerhans, secretes the hormones insulin and glucagon.
2. The exocrine acini of the pancreas produce pancreatic juice, which contains various digestive enzymes and bicarbonate.

Neural and Endocrine Regulation of the Digestive System

1. The regulation of gastric function occurs in three phases.
1. In the cephalic phase, the activity of higher brain centers, acting via the vagus nerve, stimulates gastric juice secretion.
2. In the gastric phase, the secretion of HCl and pepsin is controlled by the gastric contents and by the hormone gastrin, secreted by the gastric mucosa.
3. In the intestinal phase, the activity of the stomach is inhibited by neural reflexes and hormonal secretion from the duodenum.
2. Intestinal function is regulated, in part, by local short reflexes.
1. The submucosal and myenteric plexuses contain autonomic motor neurons, sensory neurons, and interneurons.
2. Short reflexes include the gastroileal reflex, ileogastric reflex, and intestino-intestinal reflexes.
3. The secretion of the hormones secretin and cholecystokinin (CCK) regulates pancreatic juice and bile secretion.
1. Secretin secretion is stimulated by the arrival of acidic chyme into the duodenum.
2. CCK secretion is stimulated by the presence of fat in the chyme arriving in the duodenum.
3. Contraction of the gallbladder occurs in response to a neural reflex and to the secretion of CCK by the duodenum.
4. Gastrointestinal hormones may be needed for the maintenance of the GI tract and accessory digestive organs.

Digestion and Absorption of Carbohydrates, Lipids, and Proteins

1. The digestion of starch begins in the mouth through the action of salivary amylase.
1. Pancreatic amylase digests starch into disaccharides and short-chain oligosaccharides.
2. Complete digestion into monosaccharides is accomplished by brush border enzymes.
2. Protein digestion begins in the stomach through the action of pepsin.
1. Pancreatic juice contains the protein-digesting enzymes trypsin, chymotrypsin, among others.
2. The brush border contains digestive enzymes that help to complete the digestion of proteins into amino acids.
3. Amino acids, like monosaccharides, are absorbed and secreted into capillary blood entering the portal vein.
3. Lipids are digested in the small intestine after being emulsified by bile salts.
1. Free fatty acids and monoglycerides enter particles called micelles, formed in large part by bile salts, and they are absorbed in this form or as free molecules.
2. Once inside the mucosal epithelial cells, these subunits are used to resynthesize triglycerides.
3. Triglycerides in the epithelial cells, together with proteins, form chylomicrons, which are secreted into the central lacteals of the villi.
4. Chylomicrons are transported by lymph to the thoracic duct and there enter the blood.

After studying this chapter, students should be able to . . .

1. describe the functions of the digestive system and lists its structures and regions.
2. explain how one-way transport is accomplished in the digestive tract.
3. describe the layers of the gastrointestinal tract and the function(s) of each.
4. describe the structure of the gastric mucosa, list the secretions of the mucosa and their functions, and identify the cells that produce each of these secretions.
5. explain the roles of HCl and pepsin in digestion and explain why the stomach does not normally digest itself.
6. describe the structure and function of the villi, microvilli, and crypts of Lieberkuhn in the small intestine.
7. describe the location and functions of the brush border enzymes of the intestine.
8. explain the electrical activity that occurs in the intestine and describe the nature of peristalsis and segmentation.
9. explain how the large intestine absorbs fluid and electrolytes.
10. describe the flow of blood in the liver and explain how the liver modifies the chemical composition of the blood.
11. describe the composition and functions of bile and explain how bile is kept separate from blood in the liver.
12. trace the pathway of the formation, conjugation, and excretion of bilirubin and explain how jaundice may be produced.
13. explain the significance of the enterohepatic circulation of various compounds and describe the enterohepatic circulation of bile pigment.
14. explain the endocrine and exocrine structures of the pancreas and describe the composition and functions of pancreatic juice.
15. explain how gastric secretion is regulated during the cephalic, gastric, and intestinal phases.
16. describe the structure and function of the enteric nervous system.
17. explain how pancreatic juice and bile secretion is regulated by nerves and hormones.
18. discuss the nature and action of the different gastrointestinal hormones.
19. describe the enzymes involved in the digestion of carbohydrates, lipids, and proteins and explain how monosaccharides and amino acids are absorbed.
20. describe the roles of bile and pancreatic lipase in fat digestion and trace the pathways and structures involved in the absorption of lipids.