HUMAN DIGESTIVE SYSTEM

In the digestive system, ingested food is converted into a form that can be absorbed into the circulatory system for distribution to and utilization by the various tissues of the

body. This is accomplished both physically, by mastication

in the mouth and churning of the stomach, and chemically, by

secretions and enzymes of the gastrointestinal tract. Beginning at the mouth, all food passes through the alimentary canal ( pharynx, esophagus, stomach, and intestines) before it reaches the anus, where undigested matter is eliminated as waste. The outer walls of the digestive tract are composed of layers of muscle and tissue that undergo waves of contraction (peristalsis), thereby pushing the food along its digestive path.

The inner lining contains glands that secrete the acids and enzymes necessary to break down food into a form

utilizable by the body. Digestion begins in the

mouth, where chewing reduces the food to fine

texture, and saliva moistens it and begins

the conversion of starch into simple sugars by

means of an enzyme, salivary amylase. The

food is then swallowed, passing through the

pharynx and down the muscular esophagus, or

gullet, to the expanded muscular pouchlike

section of the gastrointestinal tract, the stomach. Specialized

cells in the stomach secrete digestive enzymes and gastric

juices, which act on the partially digested food.

The stomach also physically churns and mixes the food. The

stomach secretions include the enzyme pepsin, which acts on

proteins; hydrochloric acid, essential for the action of pepsin; and an enzyme, gastric lipase, which begins the breakdown of fats. The gastric juices of young children contain, in addition to those just mentioned, the enzyme rennin, which acts on

milk. Some foods, including simple sugars and alcohol, are

absorbed directly through the stomach wall and do not remain

in the stomach. Most food, however, is not absorbed in the stomach and passes into the duodenum (first section

of the small intestine) in the form of a thick liquid

called chyme. Digestive enzymes from the pancreas and bile

from the liver act on the chyme in the duodenum.

These enzymes include pancreatic lipase, which

breaks down fats into glycerol and fatty acids; pancreatic amylase, which continues the breakdown of starches and most other carbohydrates into disaccharides; and trypsin and erepsin, which break down whole and partially digested proteins (proteoses and peptones) into amino acids, the end products of protein digestion. Bile is essential for emulsifying large fat globules into smaller ones that are more easily digested by pancreatic lipase. In addition, intestinal juices are

secreted by small glands in the intestinal wall called the crypts of Lieberkühn. Like the pancreatic juices, intestinal juices contain enzymes that continue the digestion of proteins and fats and also contain three enzymes that  break down disaccharides into glucose, galactose, and fructose (simple sugars). The digested food is absorbed into the circulatory and lymphatic systems through small fingerlike projections of the intestinal wall, called villi. Undigested material passes into the large intestine, where most of the water is absorbed and the solid material, or feces, is excreted through the anus.