In anatomy of the digestive system, the ileum (not to be confused with the ilium, a pelvic bone), is the final section of the small intestine. It is about 4m long in humans, follows the jejunum and is separated from the cecum by the iliocecal valve (ICV). The pH in the ileum is usually between 7 and 8 (neutral or slightly alkaline).
Its function is to absorb vitamin B12 and bile salts. The inside surface is covered in folds, each of which has many tiny finger-like projections known as villi, on its surface. In turn, the epithelial cells which line these villi possess large numbers of microvilli. The ileum, therefore, has an extremely large surface area both for the adsorption (attachment) of enzyme molecules and for the absorption of products of digestion. The cells that line the ileum contain the protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestion. These enzymes are present in the plasma membrane of the cytoplasm of the epithelial cells. The villi contain large numbers of capillaries which 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 contraction called peristalsis.
In the fetus the ileum is connected to the navel by the vitelline duct . In roughly 3% of humans, this duct fails to close during the first seven weeks after birth, causing a condition called Meckel's diverticulum.
In veterinary anatomy, the ileum is distinguished from the jejunum by being that portion of the jejunoileum that is connected to the caecum by the ileocaecal fold.
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