The small intestine is the longest part of the digestive tract, and the adult's total length is about 5-7 meters. Small intestinal mucosa, especially jejunum, has many annular folds and villi, which greatly expands the surface area of mucosa and is beneficial to the digestion and absorption of nutrients. In the submucosa, there are intestinal glands formed by superficial epithelial subsidence, which open on the mucosal surface and secrete intestinal juice. Pancreatic juice and intestinal juice contain various digestive enzymes, which decompose protein, sugar and fat. Bile contributes to the digestion and absorption of fat. Protein, sugar and fat must be decomposed into simple materials. These substances can enter blood and lymph through columnar epithelial cells of intestinal villi, and can also enter capillaries and lymphatic capillaries through the gap between epithelial cells.