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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The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pancreatic duct to aid in digestion. The small intestine is about long and folds many times to fit in the abdomen. Although it is longer than the large intestine, it is called the small intestine because it is narrower in diameter.
Epithelium or epithelial tissue is a thin, continuous, protective layer of compactly packed cells with a little intercellular matrix. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the epidermis, the outermost layer of the skin. Epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue.
Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from lac (gen. lactis), the Latin word for milk, plus the suffix -ose used to name sugars. The compound is a white, water-soluble, non-hygroscopic solid with a mildly sweet taste. It is used in the food industry. Lactose is a disaccharide derived from the condensation of galactose and glucose, which form a β-1→4 glycosidic linkage.
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