Intestinal 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. They increase the absorptive area of the intestinal wall. It is important that the food is absorbed at a considerably fast rate so as to allow more food to be absorbed.(If the process is too slow, the concentration of the blood in the blood vessels and the food will be equal, thus, diffusion will not occur.)Digested nutrients (including sugars and amino acids) pass into the villi through diffusion. Circulating blood then carries these nutrients away. 
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.
Homologue of alveolus
Villi serve the same purpose as the alveoli in the lungs. Both increase surface area, which helps to increase the transfer rate 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 transfer digested food in.
- Histology image: 11504loa – Histology Learning System at Boston University - "Digestive System: Alimentary Canal: pyloro/duodenal junction, duodenum"
- Histology image: 11513loa – Histology Learning System at Boston University - "Digestive System: Alimentary Canal: pyloro/duodenal junction"
- Histology image: 11609loa – Histology Learning System at Boston University - "Digestive System: Alimentary Canal: duodenum, plicae circularis"
- Histology at University of Southern California gi/c_36
- Histology at University of Southern California gi/c_39
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