Summary

Introduction to the Digestive System 560

I. The digestion of food molecules involves the hydrolysis of these molecules into their subunits.

A. The digestion of food occurs in the lumen of the GI tract and is catalyzed by specific enzymes.

B. The digestion products are absorbed through the intestinal mucosa and enter the blood or lymph.

II. The layers (tunics) of the GI tract are, from the inside outward, the mucosa, submucosa, muscularis, and serosa.

A. 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 mucosae.

B. The submucosa is composed of connective tissue; the muscularis consists of layers of smooth muscles; the serosa is connective tissue covered by the visceral peritoneum.

C. The submucosa contains the submucosal plexus, and the muscularis contains the myenteric plexus of autonomic nerves.

From Mouth to Stomach 563

I. Peristaltic waves of contraction push food through the lower esophageal sphincter into the stomach.

II. The stomach consists of a cardia, fundus, body, and pylorus (antrum). The pylorus terminates with the pyloric sphincter.

A. The lining of the stomach is thrown into folds, or rugae, and the mucosal surface forms gastric pits that lead into gastric glands.

B. The parietal cells of the gastric glands secrete HCl; the chief cells secrete pepsinogen.

C. In the acidic environment of gastric juice, pepsinogen is converted into the active protein-digesting enzyme called pepsin.

D. 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 568

I. The small intestine is divided into the duodenum, jejunum, and ileum. The common bile duct and pancreatic duct empty into the duodenum.

II. 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.

A. New epithelial cells are formed in the crypts.

B. The membrane of intestinal epithelial cells is folded to form microvilli. This brush border of the mucosa increases the surface area.

III. Digestive enzymes, called brush border enzymes, are located in the membranes of the microvilli.

IV. The small intestine exhibits two major types of movements—peristalsis and segmentation.

Large Intestine 572

I. The large intestine is divided into the cecum, colon, rectum, and anal canal.

A. The appendix is attached to the inferior medial margin of the cecum.

B. The colon consists of ascending, transverse, descending, and sigmoid portions.

C. Bulges in the walls of the large intestine are called haustra.

II. The large intestine absorbs water and electrolytes.

A. Although most of the water that enters the GI tract is absorbed in the small intestine, 1.5 to 2.0 L pass to the large intestine each day. The large intestine absorbs about 90% of this amount.

B. Na+ is actively absorbed and water follows passively, in a manner analogous to the reabsorption of NaCl and water in the renal tubules.

III. Defecation occurs when the anal sphincters relax and contraction of other muscles raises the rectal pressure.

Liver, Gallbladder, and Pancreas 575

I. The liver, the largest internal organ, is composed of functional units called lobules.

A. Liver lobules consist of plates of hepatic cells separated by capillary sinusoids.

B. Blood flows from the periphery of each lobule, where the hepatic artery and portal vein empty, through the sinusoids and out the central vein.

C. Bile flows within the hepatocyte plates, in canaliculi, to the bile ducts.

D. Substances excreted in the bile can be returned to the liver in the hepatic portal blood. This is called an enterohepatic circulation.

E. Bile consists of a pigment called bilirubin, bile salts, cholesterol, and other molecules.

F. The liver detoxifies the blood by excreting substances in the bile, by phagocytosis, and by chemical inactivation.

G. The liver modifies the plasma concentrations of proteins, glucose, triglycerides, and ketone bodies.

II. The gallbladder stores and concentrates the bile. It releases bile through the cystic duct and common bile duct to the duodenum.

III. The pancreas is both an exocrine and an endocrine gland.

A. The endocrine portion, known as the islets of Langerhans, secretes the hormones insulin and glucagon.

B. The exocrine acini of the pancreas produce pancreatic juice, which contains various digestive enzymes and bicarbonate.

Neural and Endocrine Regulation of the Digestive System 583

I. The regulation of gastric function occurs in three phases.

A. In the cephalic phase, the activity of higher brain centers, acting via the vagus nerve, stimulates gastric juice secretion.

B. 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. C. In the intestinal phase, the activity of the stomach is inhibited by neural reflexes and hormonal secretion from the duodenum.

II. Intestinal function is regulated, at least in part, by short, local reflexes coordinated by the enteric nervous system.

A. The enteric nervous system contains interneurons, intrinsic sensory neurons, and autonomic motor neurons.

B. Peristalis is coordinated by the enteric nervous system, which produces smooth muscle contraction above the bolus and relaxation below the bolus of chyme.

C. Short reflexes include the gastroileal reflex, ileogastic reflex, and intestino-intestinal reflexes.

III. The secretion of the hormones secretin and cholecystokinin (CCK) regulates pancreatic juice and bile secretion. A. Secretin secretion is stimulated by the arrival of acidic chyme into the duodenum.

B. CCK secretion is stimulated by the presence of fat in the chyme arriving in the duodenum.

C. Contraction of the gallbladder occurs in response to a neural reflex and to the secretion of CCK by the duodenum.

IV. Gastrointestinal hormones may be needed for the maintenance of the GI tract and accessory digestive organs.

Digestion and Absorption of Carbohydrates, Lipids, and Proteins 587

I. The digestion of starch begins in the mouth through the action of salivary amylase.

A. Pancreatic amylase digests starch into disaccharides and short-chain oligosaccharides.

B. Complete digestion into monosaccharides is accomplished by brush border enzymes.

II. Protein digestion begins in the stomach through the action of pepsin. A. Pancreatic juice contains the protein-digesting enzymes trypsin and chymotrypsin, among others.

B. The brush border contains digestive enzymes that help to complete the digestion of proteins into amino acids.

C. Amino acids, like monosaccharides, are absorbed and secreted into capillary blood entering the portal vein.

III. Lipids are digested in the small intestine after being emulsified by bile salts.

A. 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.

B. Once inside the mucosal epithelial cells, these subunits are used to resynthesize triglycerides.

C. Triglycerides in the epithelial cells, together with proteins, form chylomicrons, which are secreted into the central lacteals of the villi.

D. Chylomicrons are transported by lymph to the thoracic duct and from there enter the blood.

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