The purpouse of the villi is to increase the surface area of the small intestine in humans, in order to allow more absorbtion of nutrients. In order to further increase this surface area, each epithelial cell on the outside of each villus is further folded into microvilli. Nutrients are moved into the cell by facilitated diffusion. Na+ ions are pumped out of the cell by means of a Na+/K+ pump, into the blood supply to the villus. A carrier protein on the outside edge of the cell then tries to pull a Na+ ion into the cell, as there is a concentration gradient present. This carrier protein will only function if it can pull in a glucose molecule as well, and so glucose is pulled into the cell faster than diffusion would normally allow. The glucose is sent out of the cell into the blood supply to the villus and to the rest of the body. Similar processes happen for the other nutrients absorbed during digestion, such as proteins and lipids, although lipids will not be carried away by the blood supply, but by the lacteal in the villus, a vessel of the lymph system responsible for carrying lipids around the body.

Vil"lus (?), n.; pl. Villi (#). [L., shaggy hair, a tuft of hair.]

1. Anat.

One of the minute papillary processes on certain vascular membranes; a villosity; as, villi cover the lining of the small intestines of many animals and serve to increase the absorbing surface.

2. pl. Bot.

Fine hairs on plants, resembling the pile of velvet.

 

© Webster 1913.

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