Ileum

Overview

The ileum is the final and longest segment of the small intestine. It plays a vital role in the absorption of nutrients that were not absorbed by the jejunum, particularly vitamin B12 and bile salts. The ileum also contributes to the immune system via its abundant lymphoid tissue. It connects to the large intestine through the ileocecal valve, regulating the transition of intestinal contents.

Location

The ileum is located in the lower abdomen, primarily in the right lower quadrant. It:

• Begins at the duodenojejunal junction and extends from the end of the jejunum

• Terminates at the ileocecal valve, where it enters the cecum of the large intestine

• Occupies a central and lower position in the abdominal cavity, often forming loops in the pelvis

It is intraperitoneal and suspended by the mesentery, allowing it considerable mobility within the abdominal cavity.

Structure

The ileum is about 3–4 meters in length and shares general features of the small intestine, with specific differences from the duodenum and jejunum:

• Mucosa: Lined with simple columnar epithelium with abundant goblet cells; villi are shorter and sparser than in the jejunum

• Peyer’s patches: Aggregated lymphoid nodules in the mucosa and submucosa; prominent in the ileum and critical for gut immunity

• Plicae circulares: Circular folds are fewer and smaller than in the jejunum

• Muscularis externa: Contains inner circular and outer longitudinal smooth muscle layers for peristalsis

• Serosa: Covered by visceral peritoneum and connected to the posterior abdominal wall via the mesentery

Function

The ileum is responsible for absorbing several key nutrients and substances:

• Vitamin B12 absorption: Absorbed in the terminal ileum via intrinsic factor binding

• Bile salt reabsorption: Recycles bile acids back to the liver via enterohepatic circulation

• Residual nutrient absorption: Absorbs remaining amino acids, fatty acids, and simple sugars not absorbed in the jejunum

• Electrolyte and water absorption: Complements jejunal reabsorption to maintain fluid balance

Physiological Role(s)

The ileum contributes significantly to digestive physiology and immune surveillance:

• Completing digestion: Finishes enzymatic breakdown of nutrients and ensures efficient absorption

• Enterohepatic circulation: Reabsorbs bile acids, conserving resources for fat digestion

• Immune defense: Peyer’s patches monitor gut pathogens and help stimulate mucosal immunity

• Transit regulation: Works with the ileocecal valve to control entry of chyme into the colon and prevent reflux

Clinical Significance

The ileum is implicated in several important clinical conditions:

• Crohn’s disease: A chronic inflammatory condition that frequently affects the terminal ileum, leading to ulcers, strictures, and malabsorption

• Ileal resection: Surgical removal (e.g., for cancer, trauma, or Crohn’s) can result in vitamin B12 deficiency and bile salt-induced diarrhea

• Ileocecal tuberculosis: A form of abdominal TB that commonly affects the terminal ileum and ileocecal junction, mimicking malignancy or IBD

• Short bowel syndrome: Loss of a large portion of the ileum leads to severe malabsorption and nutritional deficiencies

• Ileal ulcers: May be caused by NSAID use, infections, or systemic diseases like Behçet’s or lymphoma

• Meckel’s diverticulum: A congenital anomaly of the distal ileum resulting from incomplete obliteration of the vitelline duct

The ileum is evaluated using imaging (CT, MRI enterography), endoscopy (ileoscopy), and histopathology when ileal pathology is suspected.