The gastrointestinal tract mainly includes the stomach, small intestine and large intestine, etc. It has the functions of digesting food, absorbing nutrients, secretion, movement and immunity. 1. Digestion of food: Pepsin in the stomach can carry out preliminary digestion of proteins, while the small intestine is the main part of the gastrointestinal tract for digestion and absorption, in which pancreatic protease, pancreatic amylase, pancreatic lipase and so on, can digest starch, proteins, fats and other substances. 2. Absorption of nutrients: the absorption of food in the stomach is very little, mainly a small amount of water and ethanol, the absorption is mainly the small intestine. Glucose, amino acids, fatty acids and other nutrients are mainly absorbed in the duodenum and jejunum, the ileum mainly absorbs bile salts, vitamin B12, etc., while the large intestine mainly absorbs water, electrolytes and other substances. 3. Secretory function: gastric mucosa can secrete gastric juice, in which hydrochloric acid can activate pepsinogen, kill bacteria, etc., and the secreted mucus and bicarbonate can protect the gastric mucosa; small intestinal secretion of the small intestinal fluid can neutralize gastric acid and protect the intestinal mucosa, and the enterokinase can activate the pancreatic enzymes, assisting in the digestion of proteins; colon secretion of the large intestine can protect the mucous membrane of the large intestine, lubricate the bowel. 4. Movement: the movement of the stomach can hold food, promote food emptying to the small intestine; the movement of the small intestine can stir and grind the food, mix it with the digestive juices, easy to be digested, and can push the food forward; the movement of the large intestine can promote the discharge of stool. 5. Immunity: The intestinal flora in the large intestine can maintain the normal immune state of the intestine, and also synthesize B vitamins and vitamin K, etc., to provide nutrition for the body.