| Ileum | |
|---|---|
 Small intestine | |
 The cecal fossa. The ileum andcecum are drawn backward and upward. | |
| Details | |
| Precursor | Midgut |
| Artery | Ileal arteries,ileocolic artery |
| Vein | Ileal veins |
| Nerve | Celiac ganglia,vagus[1] |
| Identifiers | |
| Latin | ileum |
| MeSH | D007082 |
| TA98 | A05.6.04.001 |
| TA2 | 2959 |
| FMA | 7208 |
| Anatomical terminology | |
 |
| Major parts of the |
| Gastrointestinal tract |
|---|
Theileum (/ˈɪliəm/) is the final section of thesmall intestine in mosthigher vertebrates, includingmammals,reptiles, andbirds. Infish, the divisions of the small intestine are not as clear and the termsposterior intestine ordistal intestine may be used instead of ileum.[2] Its main function is to absorbvitamin B12,bile salts, and whatever products of digestion that were not absorbed by thejejunum.
The ileum follows theduodenum andjejunum and is separated from thececum by theileocecal valve (ICV). In humans, the ileum is about 2–4 m long, and thepH is usually between 7 and 8 (neutral or slightlybasic).
Ileum is derived from the Greek word εἰλεός (eileós), referring to a medical condition known asileus.[citation needed]
The ileum is the third and final part of the small intestine. It follows thejejunum and ends at the ileocecal junction, where theterminal ileum communicates with thececum of the large intestine through theileocecal valve. The ileum, along with the jejunum, is suspended inside themesentery, aperitoneal formation that carries the blood vessels supplying them (thesuperior mesenteric artery andvein), lymphatic vessels and nerve fibers.[3]
There is no line of demarcation between the jejunum and the ileum. There are, however, subtle differences between the two:[3]
The four layers that make up the wall of the ileum are consistent with those of thegastrointestinal tract. From the inner to the outer surface, these are:[4]: 589
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Thesmall intestine develops from themidgut of theprimitive gut tube.[6] By the fifth week ofembryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called theprimary intestinal loop. Theproximal half of this loop will form the ileum. The loop grows so fast in length that it outgrows the abdomen and protrudes through theumbilicus. By week 10, the loop retracts back into the abdomen. Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo. It rotates a further 180 degrees after it has moved back into the abdomen. This process creates the twisted shape of thelarge intestine.[6]
In thefetus the ileum is connected to thenavel by thevitelline duct. In roughly 2−4% of humans, this duct fails to close during the first seven weeks after birth, leaving a remnant calledMeckel's diverticulum.[7]
The main function of the ileum is to absorbvitamin B12,bile salts, and whatever products of digestion were not absorbed by the jejunum. The wall itself is made up of folds, each of which has many tiny finger-like projections known asvilli on its surface. In turn, the epithelial cells that line these villi possess even larger numbers ofmicrovilli. Therefore, the ileum has an extremely large surface area both for theadsorption (attachment) ofenzyme molecules and for theabsorption of products ofdigestion. 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 theprotease andcarbohydrase enzymes responsible for the final stages ofprotein andcarbohydrate digestion into thelumen of the intestine. These enzymes are present in thecytoplasm of theepithelial cells.
The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to thehepatic portal vein and the liver.Lacteals are small lymph vessels, and are present in villi. They absorbfatty acid andglycerol, the products of fat digestion. Layers of circular and longitudinalsmooth muscle enable thechyme (partly digested food and water) to be pushed along the ileum by waves of muscle contractions calledperistalsis. The remaining chyme is passed to thecolon.
The ileum is of importance in medicine as it can be affected in a number of diseases,[8] including:
In veterinary anatomy, the ileum is distinguished from the jejunum by being that portion of the jejunoileum that is connected to thecaecum by theileocecal fold.
The ileum is the short termi of the small intestine and 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.[9] 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 one of the causes of colic in horses. During any intestinal surgery, for instance, during appendectomy, distal 2 feet of ileum should be checked for the presence of Meckel's diverticulum.
