Concepts of Human Anatomy & Physiology   5/e   Van De Graaff/Fox
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I. Introduction to the Digestive System

Concept: Within the lumen of the gastrointestinal (GI) tract, large food molecules are hydrolyzed into their monomers (subunits). These monomers pass through the inner layer, or mucosa, of the small intestine to enter the blood or lymph in a process called absorption. Digestion and absorption are aided by specializations of the mucosa and by characteristic movements caused by contractions of the muscle layers of the GI tract.

II. Mouth, Pharynx, and Associated Structures

Concept: Ingested food is mechanically broken down by the action of teeth and chemically broken down by the activity of saliva. The resulting bolus is swallowed in the process of deglutition.

III. Esophagus and Stomach

Concept: Swallowed food is passed through the esophagus to the stomach by wavelike contractions known as peristalsis. The mucosa of the stomach secretes hydrochloric acid and pepsinogen. Upon entering the lumen of the stomach, pepsinogen is converted into the active protein-digestive enzyme known as pepsin. The stomach partially digests proteins and functions to store its contents, called chyme, for later processing by the small intestine.

IV. Small Intestine

Concept: The mucosa of the small intestine is folded into villi that project into the lumen. In addition, the cells that line these villi have foldings of their cell membrane called microvilli. This arrangement greatly increases the surface area for absorption. It also improves digestion, since the digestive enzymes of the small intestine are embedded within the cell membrane of the microvilli.

V. Large Intestine

Concept: The large intestine absorbs water, electrolytes, and certain vitamins from the chyme it receives from the small intestine. In a process regulated by the action of sphincter muscles, the large intestine then passes undigested waste products out of the GI tract through the rectum and anal canal.

VI. Liver, Gallbladder, and Pancreas

Concept: In addition to regulating the chemical composition of the blood in numerous ways, the liver produces and secretes bile, which is stored and concentrated in the gallbladder prior to its discharge into the duodenum. The pancreas produces pancreatic juice, an exocrine secretion containing bicarbonate and important digestive enzymes, which is passed into the duodenum via the pancreatic duct.

VII. Neural and Endocrine Regulation of the Digestive System

Concept: The activities of different regions of the GI tract are coordinated by the actions of the vagus nerves and various hormones. The stomach begins to increase its secretion in anticipation of a meal, and further increases its activities in response to the arrival of food. The entry of chyme into the duodenum stimulates the secretion of hormones that promote contractions of the gallbladder, secretion of pancreatic juice, and inhibition of gastric activity.

VIII. Digestion and Absorption of Carbohydrates, Lipids, and Proteins

Concept: Polysaccharides and polypeptides are hydrolyzed into their subunits. These subunits enter the epithelial cells of the intestinal villi and are secreted into blood capillaries. Fat is emulsified by the action of bile salts, hydrolyzed into fatty acids and monoglycerides, and absorbed into the intestinal epithelial cells. Once inside the cells, triglycerides are resynthesized and combined with proteins to form particles that are secreted into the lymphatic fluid.

IX. Development of the Digestive System

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