The last coils of the small bowel, lying in the right lower abdomen and pelvis and ending at a valve into the large bowel
Ileum — it ends at the ileocaecal valve.
Anatomy · Organs & viscera
Ileum
Lower three-fifths of the free small intestine
Right lower abdomen and pelvis · small intestine
The ileum is the final and longest section of the small intestine, running from the jejunum to the ileocaecal valve in the right iliac fossa. It lies mainly in the right lower abdomen and pelvis, and is narrower, thinner-walled and paler than the jejunum. It absorbs vitamin B12 and bile salts and holds the Peyer's patches, clusters of lymphoid tissue that monitor the gut.
How to spot them
- ▸Coils in the right lower abdomen and pelvis, ending where the ileum joins the caecum at the ileocaecal valve
- ▸Narrower (about 3 cm), thinner-walled and paler than the jejunum, with lower, sparser circular folds
- ▸Mesentery with many arterial arcades (three to five layers) and short vasa recta, and more fat
- ▸Aggregated lymphoid follicles (Peyer's patches) on the side opposite the mesentery
What to know
Clusters of lymphoid tissue in its wall, on the side opposite the mesentery, that sample the contents of the gut
Peyer's patches of the ileum.
The only site where vitamin B12 bound to intrinsic factor is absorbed, along with the recycling of bile salts
The terminal ileum absorbs B12 and bile salts.
A blind pouch found on the wall about 60 cm from the ileocaecal valve in about 2 per cent of people
Meckel's diverticulum — a remnant of the vitelline duct on the ileum, 'the rule of twos'.
The part of the small bowel whose mesentery has several tiers of arterial arcades and short straight vessels
The ileum — short vasa recta and multiple arcades.
The segment most often inflamed in one form of chronic bowel disease that runs from mouth to anus
Crohn's disease often involves the terminal ileum.
Thinner, narrower and paler than the coils above, with lower and sparser circular folds
The ileum compared with the jejunum.
Key ideas
- Bile acids used in fat digestion are reabsorbed in the terminal ileum and returned to the liver, so surgical removal of that segment causes fat malabsorption and diarrhoea.
- Peyer's patches are the main gut-associated lymphoid tissue, with M cells that sample luminal antigens.
Quick-fire facts
- •It makes up about the lower three-fifths of the free small intestine, but the border with the jejunum is gradual.
- •The terminal ileum enters the caecum through the ileocaecal valve, which stops backflow of colonic bacteria.
- •Its walls are thinner and its lumen about 3 cm wide, narrower than the jejunum.
- •The ileum has fewer, lower circular folds than the jejunum, and they disappear in its lower part.
- •Meckel's diverticulum, described by Johann Meckel in 1809, is 2 feet from the valve, 2 inches long, in 2 per cent of people, and often contains gastric or pancreatic tissue.
- •The ileum is supplied by ileal branches of the superior mesenteric artery, and the ileocolic artery supplies its terminal end.
- •A surgical 'ileostomy' brings the ileum out through the abdominal wall as a stoma.
- •The name may come from the Greek eilein, 'to twist', or from Latin ilium, 'groin' or 'flank'.
Drill it — 8 cards
1 / 8new card