The interior walls of a human being's small intestine are covered with a multitude of threadlike, tubular projections called the intestinal villi. These fingerlike projections, although tiny, are very complex and serve as sites for the absorption of necessary nutrients and fluids into the body. To aid in this process, the villi increase the small intestine's surface area, facilitating absorption of nutrients. In this manner, they play a crucial role in proper digestion.
Intestinal villi coat the interior mucus membrane of the small intestine like a carpet. Each villus extends approximately 0.04 inches (about 1 mm) into the lumen, which is the empty chamber inside the small intestine. Inside each villus, a capillary bed and lymphatic vessel can be found. The outsides of the villi are covered by layers of cells. Nutrients pass through certain cells in this layer, are taken up by the capillary network and lymphatic vessels, and are thus transported by the blood and lymphatic system to the rest of the body.
The types of cells that cover the surfaces of intestinal villi include mature absorptive enterocyte cells, mucus secreting goblet cells, and antimicrobial paneth cells. The surface of the enterocyte cells are covered with microvilli, which allow the cells to absorb the nutrients. The cells that cover the villi only live for a few days. When the cells die, they are shed into the lumen, digested, and absorbed into the body.
In between the villi are areas called crypts, which are moatlike structures that produce the cells found on the surface of the villi. At the bases of the crypts are stem cells, and to replace dying cells, the stem cells keep dividing, creating daughter cells continuously. While some of these daughter cells remain to become stem cells, most migrate up the villi and divide into other types of cells. Some become mature absorptive enterocyte cells, while others become mucus-producing goblet cells. Other migrating cells become paneth cells, whose job it is to sterilize the interior of the small intestine by secreting antimicrobial peptides.
Thanks to the intestinal villi, the surface area of the small intestine is much larger than any person would guess. It is about 656 square feet (200 square meters) — that's 100 times the surface area of a person's skin. Without the intestinal villi, the human body would not be able to absorb the nutrients necessary to survive.