Turn a fresh pig small intestine upside down, so that the inner surface faces outwards, immerse it in a small beaker filled with clear water, and observe carefully. As shown in the figure, you can see the inside of the small intestine. The main organ for human body to digest food and absorb nutrients is the small intestine, as shown in the structural diagram of the small intestine. It can be seen that there are many annular folds on the inner surface of the small intestine, and fluffy protrusions on the folds are called small intestine villi, which greatly increases the inner surface area of the small intestine; There are capillaries and lymphatic capillaries in the villi of small intestine, and the villi wall, capillary wall and lymphatic capillary wall are all composed of a layer of epithelial cells, which is beneficial to the absorption of nutrients.

Therefore, the answer is: folds; Small intestinal villi