The digestive system is made up of organs for changing food chemically for absorption by body tissues. This process, called digestion, varies among vertebrates; it is unique in ruminants, which use intestinal symbiotic organisms to prepare such foodstuffs as cellulose for use. Digestion involves breaking organic compounds into simple soluble substances absorbable by tissues. This process involves catalytic reactions between ingested food and enzymes secreted into the intestinal tract Digestion of fatty substances appears to involve the assembly of bile salts, phospholipids, fatty acids, and monoglycerides permeable to intestinal cells. Other nutrients such as iron and vitamin B12 are absorbed by specific "carrier proteins" that make them transferable by the intestinal cells. The process described here is typical of all vertebrates except ruminants .
Digestion includes both mechanical and chemical processes. The mechanical processes include chewing to reduce food to small particles, the churning action of the stomach, and intestinal peristaltic action.

These forces move the food through the digestive tract and mix it with various secretions. Three chemical reactions take place: conversion of carbohydrates into such simple sugars as glucose , breaking down of protein into such amino acids as alanine, and conversion of fats into fatty acids and glycerol . These processes are accomplished by specific enzymes.

When food is eaten, the six salivary glands produce secretions that are mixed with the food. The saliva breaks down starches into dextrin and maltose, dissolves solid food to make it susceptible to the action of later intestinal secretions, stimulates secretion of digestive enzymes, and lubricates the mouth and oesophagus for the passage of solids.Stomach and Intestinal Action. Gastric juice in the stomach contains agents such as hydrochloric acid and some enzymes, including pepsin, rennin, and lipase. (The surface of the stomach itself is thought to be protected from acid and pepsin by its mucous coating.) Pepsin breaks proteins into peptones and proteoses. Rennin separates milk into liquid and solid portions; lipase acts on fat. Another function of stomach digestion is gradually to release materials into the upper small intestine, where digestion is completed. Some constituents of gastric juice become active only when exposed to the alkalinity of the small intestine; secretion is stimulated by chewing and swallowing and even by emotion precipitated by seeing or thinking of food . The presence of food in the stomach also stimulates production of gastric secretions; in turn, these stimulate the production of digestive substances in the small intestine.
The most extensive part of digestion occurs in the small intestine; here most food products are further hydrolysed and absorbed . Pre-digested material supplied by the stomach is subjected to the action of three powerful digestive fluids: pancreatic fluid, intestinal juice, and bile . These fluids neutralise the gastric acid, ending the gastric phase of digestion.
Pancreatic fluid is introduced into the small intestine through several ducts . It contains trypsin and chymotrypsin, enzymes that split complex proteins into simpler components that can be absorbed and used in reconstructing body proteins. Steapsin breaks down fat; amylopsin hydrolyses starches into maltose. Other enzymes then break these into glucose and fructose . Secretion of pancreatic juice is stimulated by the ingestion of proteins and fats.
Intestinal juice is secreted by the small intestine. It contains a number of enzymes; its function is to complete the process begun by the pancreatic juice. The flow of intestinal juice is stimulated by the mechanical pressure of food partly digested in the intestine.
The role of bile in digestion is to aid in absorption of fats by combining with them to form structures called micelles, which are soluble in the blood and more accessible to lipases. Secreted by the liver and stored in the gallbladder , bile flows in response to fat in the stomach and upper intestine. Observation of obstructive jaundice makes it clear that digestion of fat is ineffective in the absence of bile.
Transport of the products of digestion through the wall of the small intestine may be either passive or active. Sodium, glucose, and many amino acids are actively transported. The products of digestion are thus assimilated into the body through the intestinal wall, which is able to absorb nutritive substances selectively, rejecting other similar substances. The stomach and the colon or large intestine also have the ability to absorb water, certain salts, alcohol, and some drugs and crystalloids. Certain whole proteins are also believed to pass through the intestinal barrier. Intestinal absorption has another unique feature. Many nutrients are more efficiently absorbed when the body need is greater. The absorptive, extensively convoluted surface of the intestine amounts to 140 sq. m (1500 sq. ft) in an adult. Absorption is also aided by the length of the small intestine, 6.7 to 7.6 m (22 to 25 ft).
The water-soluble substances, including minerals, amino acids, and carbohydrates, are transferred into the venous drainage of the intestine and through the portal blood channels directly to the liver. Many of the fats, however, are resynthesized in the wall of the intestine and are picked up by the lymphatic system , which carries them into the systemic blood flow as it returns through the vena caval system , bypassing an original passage through the liver .Excretion. Undigested material is formed into a solid mass in the colon by reabsorption of water into the body. If colonic muscles propel the excretory mass through the colon too quickly, it remains semiliquid. The result is diarrhea. Insufficient activity of the colonic musculature, on the other hand, produces constipation. The stool is held in the rectum until excreted through the anus.