Villi are microscopic, slender, hair-like projections that line the small intestine to facilitate food absorption, explains Encyclopædia Britannica. Villi number between 6,000 and 25,000 per square inch of the small intestine, with greater densities near the beginning of the organ closer to the stomach. Villi are also found in the placenta surrounding a fetus in the womb.
Intestinal villi (singular villus) consist of a central core made of one artery, one vein, a small muscle, a lymphatic capillary and connective tissues that support the overall structure of a single villus. The blood vessels transport proteins and carbohydrates away from food and into the body. The muscle strand allows villi to expand and contract.
Villi give the small intestine a velvety feel. The outer surface of the villi contains mucous secreted into the small intestine. The tip of each villi is covered with up to 600 microvilli that increase the surface area of each projection. Humans have around 130 billion microvilli over the entire inner surface of the small intestine. The increased surface area means absorption of nutrients increases. Villi process around 2 gallons of food and water every day.
In the fetus, chronic villus sampling, or CVS, is a procedure by which doctors extract a sample of villi from the placenta to test the health of the fetus. This process is done between 10 and 13 weeks into a pregnancy. DNA from the sample can test for genetic abnormalities and other prenatal diseases.