Gi tract structure and function Anatomy and Absorptive Function

With normal volitional feeding, food is ingested via the mouth. Here, the process of breaking down complex foodstuffs into simpler forms that can be absorbed by the small bowel begins. Solid food is chewed in the mouth, and enzymes begin digestion. The trigger for release of many enzymes is presence of food in specific regions of the GI tract. Food is swallowed and passes through the esophagus and the esophageal sphincter to the stomach, where additional digestive enzymes and acids further break it down. The stomach also serves a mixing and grinding function.

The food, now in a liquid form known as chyme, passes through the pyloric sphincter into the duodenum, where stomach acid is neutralized. There is wide variation in lengths of the components of the small intestine (i.e., duodenum, jejunum, and ileum) between individuals (Table 101-1). Most absorption of digested carbohydrate and protein occurs within the jejunum. Most fat absorption occurs within the jejunum and ileum. In the small bowel, breakdown of macronutrients (i.e., carbohydrate, protein, and fat) occurs both within the lumen and at the intestinal mucosal membrane surface. The absorptive units on the intestinal mucosal membrane are infoldings known as villi. These villi are made up of epithelial cells called enterocytes. Projections (stri-ations) from these enterocytes called microvilli increase the surface area of the small bowel and make up what is known as the brush-border membrane.

Table 101-1

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Length (ft)









Digestive enzymes secreted by the pancreas play a role in food breakdown. The pancreas secretes large amounts of sodium bicarbonate that neutralize stomach acid. These substances flow from the pancreas through the pancreatic duct. The pancreatic duct typically joins the hepatic duct to become the common bile duct that empties through the sphincter of Oddi into the duodenum. Bile secreted by the liver does not contain digestive enzymes, but bile salts help to emulsify fat and facilitate fat absorption. Bile flows through bile ducts into the hepatic duct and common bile duct. Bile is stored in the gallbladder until needed in the gut to aid fat digestion, at which time it empties through the cystic duct to the common bile duct to the duodenum. Pathways through which carbohydrate, protein, and fat are digested and absorbed through the small bowel are illustrated in Fig. 101-1.

After absorption in the small bowel, remaining undigested food passes from the ileum through the ileocecal valve to the colon. A major role of the colon is fluid absorption. Some of the water and sodium absorption achieved by the colon is facilitated by short-chain fatty acids (SFCAs) formed from digestion of certain dietary fibers by colonic bacterial enzymes.

FIGURE 101-1. Schematic of carbohydrate, fat, and protein digestion. (From Kumpf VJ, Chessman KH. Enteral nutrition. In: DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotherapy: A Pathophysiologic Approach, 7th ed. New York: McGraw-Hill, 2008:2400.)

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