ileum

—ileal, adj.

/il"ee euhm/, n., pl. ilea /il"ee euh/.

1. Anat. the third and lowest division of the small intestine, extending from the jejunum to the cecum. See diag. under intestine.

2. Zool. the anterior portion of the hindgut of an insect or other arthropod.

[1675-85; < NL, ML ileum, var. of L ilia (neut. pl.) side of the body between hips and groin, guts, appar. by confusion with L ileus ILEUS]

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Final and longest segment of the small intestine.

It is the site of absorption of vitamin B₁₂ (see vitamin B complex) and reabsorption of about 90% of conjugated bile salts. It extends about 13 ft (4 m), from the jejunum (middle section of the small intestine) to the ileocecal valve, where it joins the large intestine. Disorders produce vitamin B₁₂ deficiency and extensive diarrhea (since bile salts in the large intestine interfere with water absorption).

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▪ anatomy

 the final and longest segment of the small intestine. It is specifically responsible for the absorption of vitamin B₁₂ and the reabsorption of conjugated bile salts. The ileum is about 4 metres (13 feet) long and extends from the jejunum (the middle section of the small intestine) to the ileocecal valve, which empties into the colon ( large intestine). The ileum is suspended from the abdominal wall by the mesentery, a fold of serous (moisture-secreting) membrane.

      The smooth muscle of the ileum's walls is thinner than the walls of other parts of the intestines, and its peristaltic contractions are slower. The ileum's lining is also less permeable than that of the upper small intestine. Small collections of lymphatic tissue (Peyer patches (Peyer patch)) are embedded in the ileal wall, and specific receptors for bile salts and vitamin B (vitamin B12)₁₂ are contained exclusively in its lining; about 90 percent of the conjugated bile salts in the intestinal contents is absorbed by the ileum.

      Two percent of all humans are born with a congenital ileum malformation, called Meckel diverticulum, that consists of a side channel up to 5 cm (2 inches) long extending from the intestinal wall. The malformation occurs when the duct leading from the navel to the small intestine in the fetus fails to atrophy and close. A small number of cases require surgical removal because of intestinal bleeding and inflammation.

      Injury or disease affecting the terminal ileum produces vitamin B₁₂ deficiency and extensive diarrhea, the latter resulting from the interference of bile salts on water absorption in the large intestine.

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