Villi are tiny, finger-like structures that protrude from the epithelial lining of the intestinal wall and are crucial in the process of digestion and absorption. They are most abundant in the small intestine, the primary site for nutrient absorption. Each villus is approximately 0.5-1.5 mm in length and contains a network of capillaries and a lymphatic vessel known as a lacteal. The capillaries absorb small molecules such as amino acids and simple sugars directly from digested food, while the lacteal absorbs larger molecules, such as fats. The unique structure of the villi increases the surface area of the intestinal lining significantly, making nutrient absorption more efficient.
The surface of each villus is covered with even smaller structures called microvilli, which form a brush border. This border greatly enhances the absorptive capacity of the intestine by increasing its surface area up to 600-fold. The combination of villi and microvilli provides the intestine with a surprisingly large surface area—roughly the size of a tennis court in an average adult—optimized for maximal nutrient absorption. The epithelial cells of the villi also produce digestive enzymes which aid in processing nutrients at the point of contact.
The core of each villus contains a rich supply of blood vessels, which are essential for transporting absorbed nutrients throughout the body. Blood flow to the villi is regulated to meet the body’s nutritional demands, increasing during digestion. When nutrients are absorbed, they are first taken up by the enterocytes—the absorptive cells on the villi—and then transferred to the bloodstream or into the lymphatic system, depending on their type. The lacteal plays a particularly important role in the absorption of fats, which are reassembled into triglycerides and transported as chylomicrons in the lymphatic fluid.
Enterocytes also have an important protective role. They act as a barrier and an active part of the immune system by secreting antimicrobial peptides and carrying immune cells that help to prevent the colonization of pathogenic microbes. Additionally, these cells can influence the composition of the intestinal microbiota, fostering a community of microorganisms that further aids digestion and protects against infection. The efficient and selective absorption facilitated by the villi is crucial not only for nutrition but also for the body's overall health and defense mechanisms.