Intestines

The intestines, collectively the small and large intestines, are a major component of the digestive system. They are responsible for digestion, nutrient absorption, and water and electrolyte balance, as well as hosting a diverse microbiota.

The small intestine is approximately 6 meters long, between the stomach and large intestine, and consists of the duodenum, jejunum, and ileum. The large intestine is about 1.5 meters, including the cecum, colon (ascending, transverse, descending, sigmoid), and the rectum. The ileocecal valve separates small and large intestines. The mucosa of the small intestine has villi and microvilli to increase surface area; the colon absorbs water and electrolytes. The internal-external anal sphincters control defecation.

The small intestine receives chyme and enzymes from the pancreas and bile; digestion occurs here; nutrients absorbed include monosaccharides, amino acids, fatty acids, fat-soluble vitamins, calcium, iron, and vitamin B12 in the ileum (with intrinsic factor from the stomach). Bile acids are reabsorbed mainly in the terminal ileum. The large intestine absorbs water and electrolytes; gut microbiota ferment indigestible carbohydrates producing short-chain fatty acids and gas; formation and storage of feces.

Circulation and innervation: arterial supply from the superior mesenteric artery (small intestine and part of the colon) and inferior mesenteric artery (distal colon). Venous drainage via the portal vein to the liver. Innervation by the autonomic nervous system via the vagus nerve (parasympathetic to much of the gut) and pelvic nerves; sympathetic to the mesenteric plexuses.

Clinical notes: disorders include inflammatory bowel disease (Crohn’s disease and ulcerative colitis), celiac disease affecting the small intestine, intestinal obstruction, diverticulosis/diverticulitis, and colorectal cancer. Diagnosis often relies on imaging and endoscopy.