Ileum

The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.

The ileum follows the duodenum and jejunum and is separated from the cecum by the ileocecal valve (ICV). In humans, the ileum is about 2-4 m long, and the pH is usually between 7 and 8 (neutral or slightly alkaline).

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.

Differences between jejunum and ileum
There is no line of demarcation between the jejunum and the ileum. There are, however, subtle differences between the two.


 * The ileum has more fat inside the mesentery than the jejenum
 * While the length of the intestinal tract contains lymphoid tissue, only the ileum has abundant Peyer's patches, unencapsulated lymphoid nodules that contain large numbers of lymphocytes and other cells of the immune system.

Embryology
In the fetus the ileum is connected to the navel by the vitelline duct. In roughly 2% of humans, this duct fails to close during the first seven weeks after birth, causing a condition called Meckel's diverticulum.

Veterinary anatomy
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 ileocecal fold.

The ileum is the short terminal part of the small intestine and forms the connection to the large intestine.It is suspended by the caudal part of the mesentery (mesoileum) and is attached, in addition, to the cecum by the ileocecal fold. the ileum terminates at the cecocolic junction of the large intestine forming the ileal orifice. In the dog the ileal orifice is located at the level of the first or second lumbar vertebra, in the ox in the level of the fourth lumbar vertebrae, in the sheep and goat at the level of the caudal point of the costal arch. By active muscular contraction of the ileum, and closure of the ileal opening as a result of engorgement, the ileum prevents the backflow of ingesta and the equalization of pressure between jejunum and the base of the cecum. disturbance of this sensitive balance is not uncommon and is the one of the causes of colicin horses.