The small intestine is the main place for digesting food and absorbing nutrients. 1. The small intestine is the main site for absorption. Food has been broken down into small molecules that can be absorbed through digestion in the small intestine. Food stays in the small intestine for a long time, usually 3 to 8 hours, which provides sufficient time for absorption. The small intestine is the longest part of the digestive tract. The small intestine is the main absorptive organ. The villi of the small intestine are the main site for absorbing nutrients. The total length of the small intestine is 5 to 7 meters. The mucosa of the small intestine forms many annular folds and a large number of villi protruding into the intestinal lumen. The surface of each villus is a layer of columnar epithelial cells. The cell membrane at the top of the columnar epithelial cells forms many small protrusions, called microvilli. . The annular folds on the small intestinal mucosa, the small intestinal villi, and the 1,000 to 3,000 microvilli on the free surface of each small intestinal villus cell increase the surface area of ??the small intestinal mucosa 600 times to about 200 square meters. The large absorption area of ??the small intestine is conducive to improving absorption efficiency. There are capillary networks, lymphatic capillaries, smooth muscle fibers, nerve networks and other tissues inside the villi (Figure 8-8). The relaxation and contraction of smooth muscle fibers can cause the villi to stretch and swing. The movement of the villi can accelerate the flow of blood and lymph and help absorption. 2. The absorption of three nutrients and water in the small intestine. When nutrients and water in the small intestine pass through the intestinal mucosal epithelial cells and finally enter the blood and lymph, they must pass through the luminal membrane and basement membrane (or lateral membrane) of the intestinal epithelial cells. ). The mechanism by which substances pass through these membranes, that is, the absorption mechanism, includes simple diffusion, facilitated diffusion, active transport, inbound and outbound transport, etc. Most of the glycerol and fatty acids in the small intestine are absorbed by the lymphatic capillaries in the villi of the small intestine and sent into the blood via lymphatic circulation. The remaining nutrients are absorbed by the capillaries in the villi of the small intestine and enter the blood directly.