Definition:
Villi are small finger-like projections that line the small intestine. These projections increase the surface area of the intestine, allowing for greater absorption of nutrients.

Location:
The villi are located in the small intestine, specifically in the mucosa layer.

Function:
The main function of villi is to increase the surface area of the small intestine, which in turn increases the efficiency of nutrient absorption. The villi accomplish this through several mechanisms:

1. Absorption of Nutrients: The villi contain specialized cells called enterocytes, which are responsible for absorbing nutrients from the food that passes through the small intestine. The increased surface area of the villi provides more enterocytes, allowing for greater absorption of nutrients.

2. Secretion of Enzymes: The villi also contain specialized cells called goblet cells and Paneth cells, which secrete enzymes and mucus to aid in digestion and protect the intestinal lining.

3. Movement of Food: The villi contain smooth muscle cells that aid in the movement of food through the small intestine.

4. Immune Function: The villi contain immune cells that help protect the body from harmful bacteria and other pathogens.

In summary, villi are small finger-like projections that line the small intestine and increase the surface area for nutrient absorption. They accomplish this through absorption of nutrients, secretion of enzymes, movement of food, and immune function.