The cecal fossa. The ileum and cecum are drawn backward and upward.
|celiac ganglia, vagus|
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).
Ileum is derived from the Greek word eilein, meaning "to twist up tightly."
The ileum is part of the small intestine that is a continuation from the jejunum. The ileum is surrounded by mesentery. It connects with the cecum, the first part of the large intestine, at the ileocecal junction.
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 jejunum
- The ileum has a mesenteric and an anti-mesenteric vascular supply, while the jejunum has only a mesenteric vasculature
- 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.
|This section requires expansion. (September 2014)|
Goblet cells in the wall of the ileum
The small intestine develops from the midgut of the primitive gut tube. By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the primary intestinal loop. The proximal half of this loop will form the ileum. The loop grows so fast in length that it outgrows the abdomen and protrudes through the umbilicus. 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 the large intestine.
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.
The function of the ileum is mainly to absorb vitamin B12 and 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 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 chyme (partly digested food and water) to be pushed along the ileum by waves of muscle contractions called peristalsis. The remaining chyme is passed to the colon.
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 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.
- Physiology at MCG 6/6ch2/s6ch2_30
- Guillaume, Jean; Praxis Publishing; Sadasivam Kaushik; Pierre Bergot; Robert Metailler (2001). Nutrition and Feeding of Fish and Crustaceans. Springer. p. 31. ISBN 1-85233-241-7. ISBN 9781852332419. Retrieved 2009-01-09.
- Robertson, John (1991). Robertson's Words for a Modern Age: A Cross Reference of Latin and Greek. Germany: Senior Scribe Publications. p. 75. ISBN 0-9630919-0-5.
- Schoenwolf, Gary C.; Bleyl, Steven B.; Brauer, Philip R.; Francis-West, Philippa H. (2009). "Development of the Urogenital system". Larsen's human embryology (4th ed.). Philadelphia: Churchill Livingstone/Elsevier. p. 237. ISBN 9780443068119.
- 9-Nickel,R., Shummer,A., Seiferle,E. (1979) The viscera of the domestic mammals, 2nd edn. Springer-Verlag, New York, USA.
|Wikimedia Commons has media related to Ileum.|
- Anatomy photo:37:11-0101 at the SUNY Downstate Medical Center – "Abdominal Cavity: The Jejunum and the Ileum"
- Anatomy image:7787 at the SUNY Downstate Medical Center
- Anatomy image:8755 at the SUNY Downstate Medical Center
- Histology image: 12001oca — Histology Learning System at Boston University
- Ileal Villi at endoatlas.com
- Ileum Microscopic Cross Section at nhmccd.edu
- Ileum 20x at deltagen.com