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Intestinal villus

Intestinal villi (singular: villus) are small, finger-like projections that extend into the lumen of the small intestine. Each villus is approximately 0.5–1.6 mm in length (in humans), and has many microvilli projecting from the enterocytes of its epithelium which collectively form the striated or brush border. Each of these microvilli are much smaller than a single villus. The intestinal villi are much smaller than any of the circular folds in the intestine. Villi increase the internal surface area of the intestinal walls making available a greater surface area for absorption. An 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.

Structure

Histology

File:Gray1059.png|Vertical section of a villus from the dog's small intestine. X 80. (Simple columnar epithelium labeled at right, third from top.) File:Gray1060.png|Transverse section of a villus, from the human intestine. X 350.a. Basement membrane, here somewhat shrunken away from the epithelium.b. Lacteal.c. Columnar epithelium.d. Its striated border.e. Goblet cells.f. Leucocytes in epithelium.f’. Leucocytes below epithelium.g. Blood vessels.h. Muscle cells cut across.| Enterocytes, along with goblet cells, represent the principal cell types of the epithelium of the villi in the small intestine.http://www.copewithcytokines.org/cope.cgi?key=Paneth%20cells

Function

There, the villi and the microvilli increase intestinal absorptive surface area approximately 30-fold and 600-fold, respectively, providing exceptionally efficient absorption of nutrients in the lumen. 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, 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.

Additional images

Image:Coeliac Disease.png|Different stages of coeliac disease Image:Intestinal villus simplified.svg|Structure of a villus Image:nudemousejejunum_EM.jpg| Microvilli (shaggy hair) show electron dense plaques (open arrow) at their apices.

References

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Intestinal_villus, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Intestinal_villus&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany