Function
The function of the ileum is mainly to absorb vitamin B12 and bile salts and whatever products of digestion that were not absorbed by the jejunum. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. In turn, the epithelial cells that line these villi possess even larger numbers of microvilli. Therefore the ileum has an extremely large surface area both for the adsorption (attachment) of enzyme molecules and for the absorption of products of digestion. The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones (gastrin, secretin, cholecystokinin) into the blood. Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestion into the lumen of the intestine. These enzymes are present in the cytoplasm of the epithelial cells.
The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver. Lacteals are small lymph vessels, and are present in villi. They absorb fatty acid and glycerol, the products of fat digestion. Layers of circular and longitudinal smooth muscle enable the digested food to be pushed along the ileum by waves of muscle contractions called peristalsis. The undigested food (waste and water) are sent to the colon.
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