Small intestine (jejunum + ileum)

Function

The jejunum and ileum are the body's absorptive workhorse: 6–7 m of suspended coils whose plicae circulares, villi and microvilli amplify its surface area enormously — about 30 m² by modern estimates (older textbooks quoted ~200 m²).

The jejunum (proximal two-fifths) does most nutrient absorption through its thick wall, wide lumen and long vasa recta; the ileum specialises — bile acid recycling, vitamin B12 capture at the terminal ileum, and Peyer's patches standing immune watch. Peristaltic waves and segmentation mix chyme for 3–5 hours as enzymes finish their work on the brush border.

Blood supply

Superior mesenteric artery arcades — numerous for the jejunum, sparse and long for the ileum.

Nerve supply

Vagus (parasympathetic), sympathetic T9–T11 via the SMA plexus; the enteric nervous system's own several hundred million neurons.

Clinical relevance

Crohn's disease

Skip lesions of transmural inflammation favour the terminal ileum — cobblestone ulceration, fistulas and B12 deficiency.

Coeliac disease

Autoimmune villous atrophy triggered by gluten flattens the jejunum's carpet — malabsorption of iron and folate first.

Intussusception

The telescoping of ileum into caecum — the redcurrant-jelly stool and sausage mass of infancy.

Related conditions

• Crohn's disease
• Coeliac disease
• Gastroenteritis
• Inguinal hernia
• Bowel obstruction
• Typhoid fever
• Cholera
• Intestinal worm infections