Ileum

Where is the ileum?

It follows the jejunum with no clear boundary and opens into the cecum through its medial wall, in the right iliac fossa. (OpenStax A&P 2e, 23.5; StatPearls, Large Intestine)

Like the jejunum, it hangs from the posterior abdominal wall on the mesentery, framed by the colon and the rest of the large intestine. (OpenStax A&P 2e, 23.5; StatPearls, Small Intestine and Physiology, Small Bowel)

How is the ileum built?

Clusters of lymphoid follicles called Peyer’s patches are densest in the distal ileum. The small intestine also acts as an immune barrier to gut bacteria. (OpenStax A&P 2e, 23.5; StatPearls, Small Intestine)

The circular folds of the small intestine grow fewer and smaller along the ileum and disappear toward its lower end. (OpenStax A&P 2e, 23.5; Gray’s Anatomy (1918), The Small Intestine)

Segmentation, the mixing contraction that churns the jejunum, slows in the ileum, where peristalsis takes over. (OpenStax A&P 2e, 23.5; StatPearls, Physiology, Small Bowel)

What does the ileum do?

It takes up what the duodenum and jejunum left behind, above all vitamin B12 and bile acids. (StatPearls, Small Intestine)

Most bile salts are absorbed at the end of the ileum and carried in the blood back to the liver, which secretes them again. This loop is the enterohepatic circulation. (OpenStax A&P 2e, 23.6; StatPearls, Physiology, Bile Acids)

Vitamin B12 uptake in the terminal ileum depends on intrinsic factor, made by parietal cells in the stomach. (StatPearls, Stomach)

What supplies the ileum?

Ileal arteries from the superior mesenteric artery run in the mesentery and form arcades and vasa recta, as in the jejunum. Its veins drain to the superior mesenteric vein and on to the portal vein. (StatPearls, Small Intestine)

Its lymph drains to nodes in the mesentery and along the superior mesenteric artery, then to the cisterna chyli and the thoracic duct. This route is a major path for absorbed fat. (StatPearls, Small Intestine)

Which conditions involve the ileum?

A Meckel diverticulum, when present, often lies near the ileocecal valve. The “rule of 2s” puts it about 2 feet from the valve, in about 2% of people. (StatPearls, Small Intestine)

Crohn disease, a chronic inflammatory condition, affects the ileum more often than any other part of the gut. (StatPearls, Small Intestine)

Where the sources differ

The sources disagree on how the ileum compares with the jejunum. OpenStax says the ileum is thicker, more vascular and has more developed mucosal folds; StatPearls says the jejunum has the thicker wall and greater vascularity, and the ileum more mesenteric fat and more Peyer patches. (OpenStax A&P 2e, 23.5; StatPearls, Small Intestine)

Its length differs by source: about 1.8 m (6 ft) in OpenStax, around 3 m in StatPearls. (OpenStax A&P 2e, 23.5; StatPearls, Small Intestine)