Section of duodenum of cat. X 60.
Intestinal villi (singular: villus) are small, finger-like projections that protrude from the epithelial lining of the intestinal wall. Each villus is approximately 0.5-1.6 (millimetres) in length and has many microvilli (singular: microvillus)(which together form the striated border)(on the enterocytes of the epithelium), each of which are much smaller than a single villus. The intestinal villi should not be confused with the larger folds of mucous membrane in the bowel known as the plicae circulares. A villus is much smaller than a single fold of plicae circulares.
Villi increase the internal surface area of the intestinal walls. Increased surface area allows for increased intestinal wall area that is available for absorption. Increased absorptive area is useful because digested nutrients (including monosaccharide and amino acids) pass into the semipermeable villi through diffusion, which is effective only at short distances. In other words, increased surface area (in contact with the fluid in the lumen) decreases the average distance travelled by nutrient molecules, so effectiveness of diffusion increases. The villi are connected to the blood vessels so the circulating blood then carries these nutrients away.
Transverse section of a villus, from the human intestine. X 350.
a. Basement membrane, here somewhat shrunken away from the epithelium.
c. Columnar epithelium.
d. Its striated border.
e. Goblet cells.
f. Leucocytes in epithelium.
f’. Leucocytes below epithelium.
h. Muscle cells cut across. |
In all humans, the villi together increase intestinal absorptive surface area approximately 30-fold and 60-fold, respectively, providing exceptionally efficient absorption of nutrients in the lumen. This increases the surface area so there are more places for food to be absorbed.
There are also enzymes (Enterocyte digestive enzyme) on the surface for digestion. Villus capillaries collect amino acids and simple sugars taken up by the villi into the blood stream. Villus lacteals (lymph capillary) collect absorbed chylomicrons, which are lipoproteins composed of triglycerides, cholesterol and amphipathic proteins, and are taken to the rest of the body through the lymph fluid.
Villi are specialised for absorption in the small intestine as they have a thin wall, about one cell thick, which enables a shorter diffusion path. They have a large surface area so there will be more efficient absorption of fatty acids and glycerol into the blood stream. They have a rich blood supply to keep a concentration gradient.
Villi also help the intestines to move food along the digestive pathways.
Microvilli (shaggy hair) show electron dense plaques (open arrow) at their apices.
- "villus". tiscali.co.uk. Retrieved 2009-06-19.
- Histology image: 11504loa - Histology Learning System at Boston University – "Digestive System: Alimentary Canal: pyloro/duodenal junction, duodenum"
- Histology image: 11513loa - Histology Learning System at Boston University – "Digestive System: Alimentary Canal: pyloro/duodenal junction"
- Histology image: 11609loa - Histology Learning System at Boston University – "Digestive System: Alimentary Canal: duodenum, plicae circularis"
- Intestinal+villi at eMedicine Dictionary