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Villi (singular: villus) are tiny, finger-like structures that protrude from the wall of the intestine and have additional extensions called microvilli (singular: microvillus) which protrude from epithelial cells lining villi. In all humans, the villi and microvilli together increase intestinal absorptive surface area 30-fold and 600-fold, respectively, providing exceptionally efficient absorption of nutrients in the lumen. This increases the surface area of the intestine to around the area of a small parking lot or a tennis court. There are also enzymes on the surface for digestion. Villus capillaries collect amino acids and simple sugars taken up by the villi into the blood stream. Villus lacteals collect absorbed fatty acids.
In short, villi are small projections shaped like fingers extending into the interior of the small intestine. They increase the absorptive area of the intestinal wall. Digested nutrients (including sugars and amino acids) pass into the villi. Circulating blood then carries these nutrients away.
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Origins and other meanings
Villus is also used to mean a long bump in the skin that leads to an abnormal opening, or a fluid-filled blister-like opening withing the top later of skin.
Villus is the Latin word for "shaggy hair".
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Other places villi are found
Before babies are born, in their earliest stages of development, villi can be found on the surface of the outermost membrane (the chorion) of the fetus. These type of villi are known as chorionic villi. They help form the placenta.
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Homologue of alveolus
Villi are the homologue of alveoli in the lungs. Both increase surface area, which helps to increase transfer of nutrients into the blood through diffusion, although alveoli also transfer wastes out. Alveoli transfer carbon dioxide out and oxygen into the bloodstream, while villi tranfer digested food in.
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