The small intestine is the main place for digestion and absorption. Most macromolecular organic substances such as protein, fat and sugar are digested in the small intestine. Although the mouth and stomach have certain digestion effects on starch and protein, they are very small, so it can be said that the small intestine bears the main digestion effect. Starch is hydrolyzed into maltose by intestinal amylase and pancreatic amylase, and then hydrolyzed into glucose by intestinal amylase and pancreatic amylase. Fat is first emulsified into particles under the emulsification of bile, and then hydrolyzed into glycerol and fatty acids under the action of intestinal and pancreatic lipase. Protein is hydrolyzed into polypeptide by trypsin, and then into amino acid by intestinal peptidase. For absorption, the main place is the small intestine. The length of the small intestine is the longest in the digestive system, and there are annular folds, villi and microvilli inside, which expands the internal area of the small intestine and makes the internal surface area of the small intestine reach more than 200 square meters. The epithelium of the small intestine is a single-layer columnar epithelium, and there are abundant capillaries and lymphatic capillaries in the villi of the small intestine. After the food in the small intestine is digested, most of the substances are absorbed into the blood through capillaries.