Ileum

Introduction

Introduction to the Ileum (Midgut Segment of the Abdomen)

The ileum is the distal portion of the small intestine, extending from the jejunoileal junction to the ileocecal valve. It occupies the lower right quadrant of the abdominal cavity and is suspended by the terminal mesentery. Functionally, it is specialized for absorption of vitamin B12, bile salts, and remaining nutrients. Its length averages 3.5 meters, making it the longest segment of the small bowel.

Embryologic Origin and Position

During embryogenesis, the ileum arises from the midgut loop, which undergoes a 270-degree counter‑clockwise rotation around the superior mesenteric artery. This rotation positions the ileum in the right lower abdomen and fixes it by the mesentery. Abnormal rotation can lead to malrotation or volvulus, underscoring the importance of its developmental pathway.

Study

Anatomical Boundaries and Relations

The ileum begins at the duodenojejunal flexure and ends at the ileocecal valve, where it communicates with the cecum. Anteriorly it is covered by peritoneum and posteriorly it lies against the retroperitoneal structures such as the right psoas muscle. It is tethered to the posterior abdominal wall by the terminal mesentery, which contains its vascular and neural elements.

Mucosal and Wall Structure

The mucosa of the ileum features prominent Peyer’s patches, which are lymphoid aggregates important for immune surveillance. Villi are slightly shorter than in the jejunum, and the crypts of Lieberkühn are deeper, facilitating absorption of bile salts. The muscularis externa consists of inner circular and outer longitudinal layers, with the latter forming the characteristic three‑tailed (taeniae coli) pattern.

Vascular Supply and Lymphatics

Arterial blood is delivered by the ileal branches of the superior mesenteric artery, which form a series of arterial arcades and vasa recta. Venous drainage mirrors the arterial pattern, emptying into the superior mesenteric vein. Lymph from the ileum drains into mesenteric lymph nodes and ultimately into the cisterna chyli.

Innervation and Motility

Parasympathetic innervation is provided by the vagus nerve, while sympathetic fibers arise from the thoracic splanchnic nerves. These autonomic inputs regulate peristalsis and sphincter control. The ileum exhibits segmental contractions that mix luminal contents and propulsive waves that move chyme toward the colon.

Clinical Significance and Common Pathologies

The ileum is a frequent site of Crohn's disease, which can cause transmural inflammation and stricturing. Meckel's diverticulum, a persistent vitelline duct, is typically located within 60 cm of the ileocecal valve and may contain ectopic gastric mucosa. Ileal malabsorption of vitamin B12 can lead to megaloblastic anemia if the terminal ileum is resected.

Summary

Key Takeaways

The ileum is the distal small‑bowel segment, anchored by the terminal mesentery and supplied by ileal branches of the superior mesenteric artery. Its mucosa contains abundant Peyer’s patches and is specialized for absorption of vitamin B12 and bile salts. Vascular, lymphatic, and neural networks follow the characteristic arterial arcade pattern, and its motility is regulated by autonomic innervation.

Clinical Correlate

Pathologies such as Crohn's disease, Meckel's diverticulum, and terminal ileal resection have direct implications for nutrient absorption and systemic health. Recognizing the ileum’s anatomical landmarks and blood supply is essential for surgical planning and for interpreting imaging studies of the right lower abdomen.