Large Intestine — MRCS Part B Anatomy

A barium enema and the anatomy of the colon, with the appendix and its blood supply as the clinical focus.

The parts of the colon

The caecum is the blind pouch below the ileocaecal valve in the right iliac fossa, about 7 cm across, covered by peritoneum but usually without a mesentery. The ascending colon runs up the right side, retroperitoneal, to the hepatic flexure under the liver. The transverse colon hangs on its own mesentery, the transverse mesocolon, from the hepatic to the splenic flexure, which is higher and more fixed, tethered by the phrenicocolic ligament. The descending colon is retroperitoneal down the left side to the pelvic brim. The sigmoid colon has a mesentery again, loops into the pelvis and becomes the rectum in front of S3. The taeniae coli, haustra and appendices epiploicae mark it out from small bowel; the taeniae converge on the base of the appendix, which is how it is found at operation.

The appendix

A blind tube of gut opening into the posteromedial caecum about 2 cm below the ileocaecal valve, 6–10 cm long, with a complete longitudinal muscle coat and lymphoid tissue in its wall. Its mesentery, the mesoappendix, carries the appendicular artery from the ileocolic. The base is constant, at McBurney's point, two-thirds of the way from the umbilicus to the anterior superior iliac spine; the tip is not. Retrocaecal is commonest (about two-thirds), then pelvic, and less often subcaecal, pre-ileal or post-ileal. A retrocaecal appendix gives few abdominal signs and pain on extending the hip; a pelvic one irritates the bladder and rectum.

The appendicular artery is an end artery. When the lumen obstructs and the wall swells, the artery thromboses and the tip becomes gangrenous quickly, often within a day or two, which is why perforation is common in delayed presentations.

Blood supply

The superior mesenteric artery supplies the midgut colon through the ileocolic, right colic and middle colic arteries, to two-thirds along the transverse colon. The inferior mesenteric artery supplies the rest through the left colic, sigmoid and superior rectal arteries. The marginal artery of Drummond links the two along the mesenteric border; it is weakest at the splenic flexure (Griffiths' point) and at the rectosigmoid (Sudeck's point), which is where ischaemic colitis shows first and why the splenic flexure is the danger area when the inferior mesenteric artery is tied during an aneurysm repair.

Why the caecum perforates

It has the widest diameter and the thinnest wall, so by Laplace's law the tension in its wall for a given pressure is the greatest in the colon. In a distal obstruction with a competent ileocaecal valve it becomes a closed loop; a caecal diameter beyond about 10–12 cm on a radiograph is the point at which it is expected to burst.

Descending and sigmoid colon

The descending colon stores faeces and absorbs the last of the water before the sigmoid, which propels stool into the rectum by mass movement. The sigmoid colon is hindgut: sympathetic supply from the lumbar splanchnic nerves through the inferior mesenteric plexus, parasympathetic from the pelvic splanchnic nerves (S2–4), which is why hindgut pain is felt in the suprapubic region and why sacral nerve injury disturbs bowel control.