The villi and circular folds of the small intestine contribute to the digestion and absorption of nutrients in the small intestine. The normal adult small intestine is 4~5m long, with abundant circular folds in the mucosa of the small intestine, and a large number of villi on the circular folds, with the length of villi being 0.5~1.5mm. Inside the villi of the small intestine, there is a rich network of capillaries, capillary lymphatics, smooth muscle and nerve fibers, etc. Each villus has a layer of columnar epithelial cells on its outer surface, and the apical membrane of each columnar epithelial cell is covered with a layer of columnar epithelial cells. The outer surface of each villus is a layer of columnar epithelial cells, and there are about 1700 microvilli on the apical membrane of each columnar epithelial cell. The presence of circular folds, villi and microvilli ultimately increases the absorptive area of the small intestine by about 600 times over the area of a simple cylinder of the same length. In addition to the huge absorptive area of the small intestine, food stays in the small intestine for a long period of time, allowing food to be digested into small molecules that can be easily absorbed in the small intestine. Therefore, the villi and circular folds of the small intestine become favorable conditions for the small intestine to play a major role in digestion and absorption.