Chapter 12 - Gastrointestinal Digestion and Absorption

Abstract

The gastrointestinal (GI) system or tract is responsible for digestion and absorption of nutrients and fluids. The tract extends from the mouth to the anus, and in an adult human, it is about 10 m long. Digestion is the hydrolysis of complex food substances into simpler units, such as monosaccharides, amino acids, fatty acids, and glycerol. Absorption is the transport of the products of digestion and of vitamins, minerals, and water across the intestinal epithelium to the lymphatic or blood circulatory systems. Digestion and absorption involve not only the gastrointestinal (GI) tract but also secretions from salivary glands, liver (and gallbladder), and pancreas. The system provides highly selective, efficient, and elaborate absorptive surfaces that contain enzymes and secretions of enzyme-containing fluids, electrolytes, and other substances required for digestion and absorption. The GI tract consists of mouth and esophagus, stomach, small intestine, and large intestine. In the mouth, the food is mixed with saliva, chewed to break up large particles, and propelled into the esophagus by swallowing. Saliva is secreted by parotid, submaxillary, and sublingual glands, and by numerous small buccal glands. The presence of food in the mouth and the act of chewing stimulate secretion of saliva by reflex nervous stimulation. The stomach stores food temporarily, retards its entry into the small intestine, and secretes pepsin to begin the digestion of protein. Hydrogen ions in the stomach activate pepsinogen to form pepsin and aid in maintaining the sterility of the upper GI tract.