Jejunum

The jejunum is the proximal two-fifths of the mobile small intestine extending from the duodenojejunal flexure to the ileum. Beginning at the left side of the second lumbar vertebra (L2), it occupies predominantly the upper left part of the infracolic compartment of the abdominal cavity.

The jejunum has a thicker wall, a wider lumen, and a deeper red colour than the ileum due to its richer vascular supply. Its mucosa is highly specialised for absorption and like the duodenum, it contains numerous large, closely packed plicae circulares (valves of Kerckring), which are tall and prominent, particularly in the proximal jejunum. These folds are covered by long, finger-like villi that greatly increase the absorptive surface area.

Histologically, the jejunum lacks Brunner's glands, which are confined to the duodenum, and contains only sparse solitary lymphoid follicles, in contrast to the abundant aggregated lymphoid nodules (Peyer's patches) found in the ileum.

Blood supply and innervation

The jejunum is suspended by a mesentery. Its arterial supply is derived from jejunal branches of the superior mesenteric artery, which form one or two tiers of arterial arcades giving rise to relatively long vasa recta. In comparison, the ileum possesses multiple tiers of arterial arcades and shorter vasa recta. Venous drainage follows corresponding veins into the superior mesenteric vein.

Sympathetic innervation is derived from the thoracic splanchnic nerves via the superior mesenteric plexus, and parasympathetic innervation is supplied by the vagus nerve.

Differentiating features of jejunum and ileum

The jejunum gradually transitions into the ileum without a clear demarcation. Compared with the ileum, the jejunum has a thicker and more vascular wall, a larger diameter, more prominent plicae circulares, longer villi, fewer arterial arcades, longer vasa recta, less mesenteric fat, and fewer lymphoid aggregates.

In contrast, the ileum has a thinner, paler wall, smaller lumen, low and sparse circular folds, shorter villi, numerous arterial arcades, short vasa recta, abundant mesenteric fat extending to the intestinal border, and prominent Peyer's patches.