Small intestine is the principal site for the absorption of various digested food materials. It is generally folded and ridged to provide an extensive surface for absorption.
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Most active absorption takes place in the upper part of the small intestine called duodenum as its mucous membrane is especially adapted for this function.
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The internal lining or the mucous membrane of the intestine is thrown into folds which again have innumerable finger-like projections tailed villi.
These are highly specialized absorptive organs as they facilitate active absorption even while a flow of intestinal juice is simultaneously coming out of the glands of intestinal mucosa.
These are adequately exposed to the liquid food in the intestine because intestinal contractions keep the liquid food constituents constantly in motion.
The length and diameter of villi vary (in man) from 0’2 to 1 mm. The entire surface of the villi is covered with epithelium, the cells of which contain fibrils, mitochondrial apparatus that changes during absorption.
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Each villus contains a network of capillaries derived from blood vessels in the Wall of the alimentary canal and central lymph capillary or lacteal which begins blindly under the epithelium at the tip of the villus and drains, after forming a large plexus and a submucosa plexus, into the main lymphatic channels of the gut wall.
In the submucosa plexus the lacteals possess valves owing to which the lymph flows only in one direction.
The consensus is that lipids pass primarily into the lacteals, while the sugars and amino-acids are absorbed directly in the capillary blood.
Both the villus and the intestinal fold contain longitudinally arranged smooth muscle fibres, the rhythmic contractions of which bring the villi into contact with the intestinal contents and thus maintain the circulation in the lacteals, lymphatics and small blood vessels.