2 The total length of the small intestine is 5-6 meters. There are many annular folds and a large number of villi protruding into the intestinal cavity. The surface of each villi is a layer of columnar epithelial cells, and the cell membrane at the top of the columnar epithelial cells forms many tiny protrusions, called microvilli. The annular folds on the small intestinal mucosa, the small intestinal villi and 1 ~ 3 microvilli on the free surface of each small intestinal villi cell increase the surface area of the small intestinal mucosa by 6 times. It is about 2 square meters. The epithelial cells of small intestine face the intestinal cavity. It is estimated that the internal surface area of an adult's small intestine is 2 square meters. The larger the internal surface area, the more it will be absorbed.
3 There are capillaries in the small intestine villi, and the walls of small intestine villi and capillaries are very thin, all of which are composed of only one layer of epithelial cells. These structural characteristics make nutrients easily absorbed into the blood. The huge absorption area of small intestine is conducive to improving absorption efficiency. There are capillary network, capillary lymphatic vessel, smooth muscle fiber and neural network in villi. The relaxation and contraction of smooth muscle fiber can make villi stretch and swing, and the movement of villi can accelerate the flow of blood and lymph, which is helpful for absorption. < P > Summary:
1. Food stays in the small intestine for the longest time
2. The small intestine absorbs the most nutrients during digestion
3. The structure and function of the small intestine determine it.