The small intestine consists of duodenum, jejunum and ileum, and its main functions are digestion and absorption, secretion and motility. 1. Digestion and absorption: the small intestine is the main place for digestion and absorption of food in the gastrointestinal tract, which can digest starch, protein, fat and other nutrients into glucose, amino acids and fatty acids that can be absorbed by the body. The small intestine is rich in capillaries and villi, and the food stays in the small intestine for a long time, so that the nutrients and vitamins and minerals produced by digestion can be fully absorbed. 2. Secretion: the small intestine also has the function of secretion of small intestinal fluid, which contains enterokinase that can activate trypsinogen to trypsin, assisting protein digestion. Small bowel fluid is alkaline, can neutralize gastric acid, the intestinal mucosa has a certain protective effect. In addition, small intestinal fluid can also dilute the digestive products, reduce osmotic pressure, so that it is easy to be absorbed. 3. Movement: The movement of the small intestine mainly includes tense contraction, segmentation, peristalsis, etc., which can stir and grind the food, mix it with the digestive juices and make it easy to be digested, and can push the food forward.