Jejunum
Jejunum
The jejunum is the proximal two-fifths of the mesenteric small intestine. It is wide, thick-walled and very vascular, and it is the main site of nutrient absorption.
Overview
The jejunum begins at the duodenojejunal flexure (L2, left of the midline) and gradually becomes the ileum. There is no sharp boundary, but the two ends differ clearly. Its name means "empty", because it is usually found empty at autopsy owing to rapid transit. Its loops lie mainly in the left upper and central abdomen, suspended by the mesentery.
Compared with the ileum, the jejunum has a wider lumen (about 2.5–4 cm), a thicker wall and a deeper red colour from its rich blood supply. Its plicae circulares are tall and closely packed, which gives a "feathery" pattern on barium studies. Its mesenteric vessels form only one or two rows of large arcades that give off long vasa recta. Its mesentery contains less fat, so clear "windows" can be seen between the vessels. It has few lymphoid follicles. Surgeons use these features to orient a loop of bowel.
Most carbohydrates, amino acids, fatty acids, water-soluble vitamins (including folate) and much of the water are absorbed here.
- Location
Left upper quadrant and umbilical region, from the duodenojejunal flexure onwards.
Parts & subdivisions
- Jejunum vs ileum: diameter and wallJejunum: about 2.5–4 cm, thick and heavy. Ileum: about 2–3 cm, thinner and lighter.
- Jejunum vs ileum: colour and vascularityJejunum: deep red and more vascular. Ileum: paler pink.
- Jejunum vs ileum: arcades and vasa rectaJejunum: 1–2 rows of large arcades with long vasa recta. Ileum: up to 4–5 rows of short arcades with short vasa recta.
- Jejunum vs ileum: mesenteric fatJejunum: less fat, with translucent windows. Ileum: more fat throughout the mesentery.
- Jejunum: large, tall and closely packed. Ileum: low and sparse, absent distally.
- Jejunum: few solitary follicles. Ileum: Peyer patches on the antimesenteric border.
Anatomy
- Innervation
Vagus nerve (parasympathetic). Sympathetic fibres from T8–T10 via the superior mesenteric plexus. Pain is referred periumbilically.
- Arterial supply
Jejunal branches of the superior mesenteric artery (arcades → long vasa recta, which are end arteries).
- Venous drainage
Jejunal veins → superior mesenteric vein → hepatic portal vein.
- Lymphatic drainage
Lacteals → juxta-intestinal and mesenteric nodes → superior mesenteric nodes → cisterna chyli.
- Histology
Tall, finger-like villi; prominent plicae circulares; no Brunner glands and no Peyer patches. Goblet cells are fewer than in the ileum.
Function
- Main site of absorption of monosaccharides, amino acids and peptides, fatty acids and monoglycerides, and water-soluble vitamins.
- Brush-border digestion (disaccharidases, peptidases).
- Bulk water absorption; the "leaky" epithelium allows iso-osmotic uptake.
Clinical significance
Coeliac disease and jejunal biopsy
Villous atrophy is most marked in the duodenum and proximal jejunum. Combined with jejunal folate absorption and duodenal iron absorption, this explains why folate and iron deficiency are typical presenting features.
Test yourself
Which feature favours jejunum rather than ileum at laparotomy?
- Abundant Peyer patches
- Many short arterial arcades with short vasa recta
- Long vasa recta, few arcades and translucent fat-free mesenteric windows
- Thin wall and narrow lumen
Answer: Long vasa recta, few arcades and translucent fat-free mesenteric windows
The jejunum is wider, thicker and redder, with one or two arcades, long vasa recta and little mesenteric fat. The ileum has many arcades, short vasa recta, more fat, Peyer patches and a thinner wall.
Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.