Summary
Intestinal villi (: 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 about 1 μm in length, around 1000 times shorter 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. 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 epbithelium.g. [[Blood vessel]]s.h. [[Muscle cells]] cut across. File:Cross-section histology of small intestinal villi of the terminal ileum.jpg|Cross-section histology of small intestinal villi of the human [[terminal ileum]]. File:MCT Chicken jejunum oblique.jpg|[[X-ray microtomography|MicroCT]]-based volume projection of the jejunal mucosa of a chicken. Virtual volume block with vertically truncated villi in oblique view. Scalebar = 0.
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