digestion

the process of breaking down food into substances that can be absorbed and used by the body. Digestion begins with the chewing and grinding of food, at which point it is mixed with saliva to commence the process of breakdown. Most digestion occurs in the STOMACH and small INTESTINE . In the stomach the food is subjected to GASTRIC JUICE , which contains PEPSINS , to break down PROTEINS , and hydrochloric acid. The food is mixed and becomes totally soluble before passing into the small intestine as CHYME , where it is acted on by pancreatic juice, BILE , BACTERIA and succus entericus (intestinal juices). Water is absorbed in the intestine in a very short time, while the bulk of the food may take several hours to be processed. The chyme forms chyle because of the action of bile and pancreatic juice. Fats are removed from this in emulsion form into the lymph vessels (see LACTEAL VESSELS ) and then into the blood. Sugars, salts and AMINO ACIDS move directly into the small blood vessels in the intestine, and the whole process is promoted by microfolding of the intestine wall producing finger-like projections (villi). The food passes down the intestine as a result of muscular contractions of the intestine wall ( PERISTALSIS ), and ultimately the residue and waste are excreted.