Villi are small finger-like projections located in the walls of the small intestine. Their function is to increase the surface area in order to maximise the absorption of digested food. Villi numbers vary between 10 to 40 per square metre of tissue and are typically 0.5 to 1 mm long.
Each villus consists of arteries, veins, a complex capillary system and a lymphatic vessel called lacteal. The lacteal absorbs fatty acids and glycerol and transports it away from the villi. The capillaries have several functions in the villi, they carry the absorbed nutrients such as proteins and carbohydrates away from the villi but also delivers oxygen to the microvilli so that they can perform cell respiration.
Microvilli are microscopic cells that project from the villus which further increases surface area for faster absorption. Microvilli are specialised for this function in other ways by having protein pumps that move monosaccharides and amino acids into the bloodstream and also containing mitochondria so that large amounts of DNA can be made for the active transport of nutrients.
Each villus has a muscle strand which allows the villi to contract and expand. This is so that the contents in the lumen of the small intestine are constantly moving and the concentration gradient is maintained for efficient absorption. It is also believed that these contractions empty the contents of the lacteal into larger lymphatic vessels.
The villi is covered by a mucosal epithelium which is mainly composed of multiple cell types, enterocytes, goblet cells, enteroendocrine cells, and at the base of the villi (also called a crypt) paneth cells and stem cells, Enterocytes absorb substances such as water and electrolytes. Other cells such as goblet cells secrete mucus into the intestinal cavity and enteroendocrine cells secrete hormones. Paneth cells secrete anti-microbial peptides.