Ileum

Ileum

The ileum is the distal three-fifths of the mesenteric small intestine, ending at the ileocaecal valve. It specialises in absorbing vitamin B12 and bile salts and is rich in Peyer patches.

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Overview

Share of mesenteric small bowel≈3/5 (≈3.5 m at autopsy)

Diameter≈2–3 cm

Bile salts reabsorbed≈95%

The ileum lies mainly in the right lower abdomen and pelvis. Compared with the jejunum it is narrower (about 2–3 cm), has a thinner wall and is paler. Its plicae circulares are low and sparse, disappearing in the terminal ileum. Its mesentery carries more fat, and its vessels form many short arcades with short vasa recta. Aggregated lymphoid follicles, the Peyer patches, are scattered along its antimesenteric border.

The terminal ileum has transport functions found nowhere else in the gut. It takes up the vitamin B12–intrinsic factor complex through the cubilin–amnionless receptor, and it absorbs about 95% of conjugated bile salts through the apical sodium-dependent bile acid transporter (ASBT). Its L cells secrete GLP-1 and peptide YY. These hormones form the "ileal brake": when nutrients reach the ileum, gastric emptying and upper gut motility slow down. The ileum ends by invaginating into the caecum at the ileocaecal valve.

Location: Right lower quadrant, hypogastrium and pelvis. It ends at the ileocaecal junction in the right iliac fossa.

Parts & subdivisions

Terminal ileum
The last few tens of cm. The site of B12–IF and bile salt absorption and the commonest site of Crohn disease.
Peyer patches
Aggregated lymphoid follicles in the mucosa and submucosa on the antimesenteric border. The overlying M cells sample luminal antigens.
Ileocaecal valve
Where the ileum invaginates into the caecum.

Anatomy

Innervation: Vagus nerve and sympathetic fibres (T8–T10) via the superior mesenteric plexus.
Arterial supply: Ileal branches of the superior mesenteric artery (multiple arcades → short vasa recta) and the ileocolic artery for the terminal ileum.
Venous drainage: Ileal and ileocolic veins → superior mesenteric vein → hepatic portal vein.
Lymphatic drainage: Mesenteric and ileocolic nodes → superior mesenteric nodes.
Histology: Shorter villi, many goblet cells, abundant Paneth cells in the crypts, and Peyer patches extending into the submucosa.

Function

Absorbs vitamin B12 bound to intrinsic factor.
Reabsorbs bile salts for the enterohepatic circulation.
Absorbs residual nutrients, water and electrolytes (electroneutral NaCl uptake).
Immune sampling and IgA induction (Peyer patches).
Ileal brake via GLP-1 and PYY.

Clinical significance

Meckel diverticulum

A remnant of the vitelline (omphalomesenteric) duct, forming a true diverticulum on the antimesenteric border. It follows the "rule of 2s": about 2% of people, about 2 feet (60 cm) from the ileocaecal valve, about 2 inches long, 2 types of ectopic tissue (gastric, pancreatic), usually symptomatic before age 2. Ectopic gastric acid causes painless rectal bleeding (detected by a technetium-99m pertechnetate scan). It can also cause obstruction, intussusception or diverticulitis that mimics appendicitis.

Terminal ileal disease or resection

In Crohn disease or after resection, three problems follow: B12 deficiency (megaloblastic anaemia), bile-acid diarrhoea, and, with larger losses, steatorrhoea. Bile-salt loss makes bile lithogenic, causing cholesterol gallstones. Free fatty acids bind calcium, so dietary oxalate is absorbed in the colon and forms calcium oxalate kidney stones.

Intussusception and typhoid

Ileocolic intussusception is most common between 6 months and 3 years. Hypertrophied Peyer patches often act as the lead point. Children have colicky pain, a sausage-shaped mass and "red-currant jelly" stools. In typhoid, Salmonella Typhi invades the Peyer patches and can cause ileal ulceration and perforation.

Mnemonics

Meckel: rule of 2s

2% · 2 feet from the ileocaecal valve · 2 inches long · 2 ectopic tissues · presents before age 2 · about 2:1 male.

Test yourself

Which feature favours jejunum rather than ileum at laparotomy?

Abundant Peyer patches
Many short arterial arcades with short vasa recta
Long vasa recta, few arcades and translucent fat-free mesenteric windows
Thin wall and narrow lumen

Answer: Long vasa recta, few arcades and translucent fat-free mesenteric windows

The jejunum is wider, thicker and redder, with one or two arcades, long vasa recta and little mesenteric fat. The ileum has many arcades, short vasa recta, more fat, Peyer patches and a thinner wall.

Vitamin B12 bound to intrinsic factor is absorbed mainly in the:

Duodenum
Jejunum
Terminal ileum
Ascending colon

Answer: Terminal ileum

The IF–B12 complex binds the cubilin–amnionless receptor in the terminal ileum. Ileal resection or Crohn disease therefore causes B12 deficiency, together with bile-salt malabsorption.