Villi are small, finger-like projections that line the surfaces of specific internal organs. These structures, generally between 0.5 and 1.6 millimeters in length, maximize the area available for exchange. Their main purpose is to dramatically increase the surface area within a confined space, facilitating the efficient movement of substances, such as nutrients or gases, across a membrane. While villi exist in different biological contexts, they are most recognized within the digestive system.
The Primary Location: The Small Intestine
The most prominent location for villi is along the inner lining of the small intestine, where they are densely packed across the entire length of the organ. The intestinal wall is covered with millions of these projections, with estimates ranging from 10 to 40 villi per square millimeter of tissue. This high concentration transforms the relatively narrow tube into an enormous absorptive surface.
Villi are present across all three segments of the small intestine: the duodenum, the jejunum, and the ileum. They are generally most prevalent and longest in the initial segment, the duodenum, where the bulk of digestion and absorption begins. They diminish in number and become shorter as they progress toward the ileum, the final segment before the large intestine.
Structure and Function of Intestinal Villi
The primary purpose of the intestinal villi is the absorption of digested food molecules into the bloodstream and lymphatic system. Each villus is covered by a single layer of epithelial cells, which makes the diffusion path for nutrients short. This thin wall allows for quick transport of simple sugars, amino acids, fatty acids, and other molecules resulting from the breakdown of food.
The absorptive capacity is amplified by tinier projections called microvilli, found on the surface of the epithelial cells covering each villus. These microvilli form the brush border, increasing the total surface area by up to 600-fold. This nested system of folding makes the small intestine highly efficient at nutrient uptake.
Each intestinal villus contains a specialized internal network to transport absorbed molecules. Simple sugars and amino acids are moved into a dense network of blood capillaries located beneath the surface cells. Fat-soluble molecules, such as dietary lipids and certain vitamins, are absorbed into a specialized lymphatic vessel at the center of the villus, known as a lacteal. Lacteals transport these fats as chylomicrons, bypassing the liver initially before delivery to the general circulation.
Villi in Other Biological Systems
While intestinal villi are important for digestion, similar structures facilitate other forms of biological exchange. One example is the chorionic villi, which form part of the placenta during pregnancy. These finger-like projections grow out from the chorion, the outermost membrane surrounding the embryo, and anchor into the wall of the uterus.
The function of the chorionic villi is analogous to intestinal villi, but they facilitate the passage of substances between the mother and the developing fetus. They maximize the surface area for the exchange of oxygen, nutrients, and antibodies from the mother’s blood into the fetal circulation. These villi also ensure the efficient transfer of waste products, such as carbon dioxide and urea, from the fetus back into the maternal circulation.
Health Consequences of Villi Damage
Damage to the intestinal villi severely compromises nutrient absorption, a condition medically described as villous atrophy. The flattening or erosion of these projections reduces the absorptive surface area, leading to malabsorption. This process results in nutritional deficiencies and a wide range of symptoms.
Celiac disease is the most common example of villous atrophy, where gluten ingestion triggers an autoimmune response in susceptible individuals. This immune attack causes chronic inflammation that damages and flattens the villi. As the villi erode, the body cannot absorb adequate iron, calcium, or B vitamins, potentially leading to issues like iron-deficiency anemia or osteoporosis.
The consequences of persistent villous atrophy extend beyond nutritional deficiencies. Ongoing damage has been linked to an increased risk of severe complications, including certain types of intestinal lymphoma. While Celiac disease is the primary cause, infections or other conditions can also temporarily or permanently damage the villi.