Ileum

The ileum (/ˈɪliəm/) 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.[2] Its main function is to absorb vitamin B12, bile salts, and whatever products of digestion that were not absorbed by the jejunum.

Not to be confused with Ilium.
Ileum
Small intestine
The cecal fossa. The ileum and cecum are drawn backward and upward.
Details
Precursormidgut
Arteryileal arteries
Veinileal veins
Nerveceliac ganglia, vagus[1]
Identifiers
Latinileum
MeSHD007082
TA98A05.6.04.001
TA22959
FMA7208
Anatomical terminology
Major parts of the
Gastrointestinal tract
Upper gastrointestinal tract
Lower gastrointestinal tract
See also

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 basic).

Ileum is derived from the Greek word εἰλεός (eileós), referring to a medical condition known as ileus.

Structure

The ileum is the third and final part of the small intestine. It follows the jejunum and ends at the ileocecal junction, where the terminal ileum communicates with the cecum of the large intestine through the ileocecal valve. The ileum, along with the jejunum, is suspended inside the mesentery, a peritoneal formation that carries the blood vessels supplying them (the superior mesenteric artery and vein), 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]

Histology
Main article: Gastrointestinal wall

The four layers that make up the wall of the ileum are consistent with those of the gastrointestinal tract. From the inner to the outer surface, these are:[4]: 589 

  • General structure of the gut wall. Brunner's glands are not found in the ileum, but are a distinctive feature of the duodenum.
    General structure of the gut wall. Brunner's glands are not found in the ileum, but are a distinctive feature of the duodenum.
  • Goblet cells in the wall of an ileum vili. At its sides, enterocytes are visible over a core of lamina propria.
    Goblet cells in the wall of an ileum vili. At its sides, enterocytes are visible over a core of lamina propria.
  • Cross section of ileum with a Peyer's patch circled.
    Cross section of ileum with a Peyer's patch circled.

Development

The small intestine develops from the midgut of the primitive gut tube.[6] 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.[6]

In the fetus the ileum is connected to the navel by the vitelline duct. In roughly 2−4% of humans, this duct fails to close during the first seven weeks after birth, leaving a remnant called Meckel's diverticulum.[7]

Function

The main function of the ileum is to absorb vitamin 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 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.

Clinical significance

It is of importance in medicine as it can be affected in a number of diseases,[8] including:

Other animals

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 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.

References
  1. Nosek, Thomas M. "Section 6/6ch2/s6ch2_30". Essentials of Human Physiology. Archived from the original on 2016-03-24.
  2. 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. Retrieved 2009-01-09.
  3. Moore KL, Dalley AF, Agur AM (2013). Clinically Oriented Anatomy, 7th ed. Lippincott Williams & Wilkins. pp. 241–246. ISBN 978-1-4511-8447-1.
  4. Ross M, Pawlina W (2011). Histology: A Text and Atlas. Sixth edition. Lippincott Williams & Wilkins. ISBN 978-0-7817-7200-6.
  5. Santaolalla R, Fukata M, Abreu MT (2011). "Innate immunity in the small intestine". Current Opinion in Gastroenterology. 27 (12): 125–131. doi:10.1097/MOG.0b013e3283438dea. PMC 3502877. PMID 21248635.
  6. 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.
  7. Sagar J.; Kumar V.; Shah D. K. (2006). "Meckel's diverticulum: A systematic review". Journal of the Royal Society of Medicine. 99 (10): 501–505. doi:10.1177/014107680609901011. PMC 1592061. PMID 17021300.
  8. Cuvelier, C.; Demetter, P.; Mielants, H.; Veys, E.M.; De Vos, M (Jan 2001). "Interpretation of ileal biopsies: morphological features in normal and diseased mucosa". Histopathology. 38 (1): 1–12. doi:10.1046/j.1365-2559.2001.01070.x. PMID 11135039. S2CID 28873753.
  9. Nickel, R., Shummer, A., Seiferle, E. (1979) The viscera of the domestic mammals, 2nd edn. Springer-Verlag, New York, USA.
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