Descending colon

At a glance

English

Descending colon

Latin (TA2)

Colon descendens

System

Digestive

Region

Abdomen

Side

Single, on the midline

About the descending colon

The descending colon starts at the left colic (splenic) flexure, high up near the lower pole of the spleen and in front of the left kidney, and runs down the left flank to the region of the iliac crest, about 25 cm in all. In the left iliac fossa it continues as the sigmoid colon, the boundary being placed at the pelvic brim. It is narrower than the ascending colon and lies deeper, further back in the abdomen.

Like the ascending colon it is retroperitoneal. Peritoneum covers its front and sides, while its back lies against the posterior abdominal wall: the diaphragm at its top, the lateral border of the left kidney, then quadratus lumborum, the iliacus and psoas. Nerves crossing that wall, such as the subcostal, iliohypogastric and ilio-inguinal nerves, lie behind it. The left paracolic gutter runs along its outer side. In front lie loops of jejunum and the greater omentum.

Its wall shows the taeniae coli, haustra and omental appendices typical of the colon. By the time contents reach this segment much of the water has been absorbed, so they are firmer and are stored here until a mass movement carries them on into the sigmoid colon and rectum. Its embryological origin from the hindgut means its blood comes from the inferior mesenteric artery and its pain is felt low in the abdomen, below the umbilicus.

Tumours of the left colon tend to narrow the bowel and cause a change in bowel habit or obstruction rather than silent bleeding, because the lumen is narrower and the contents more solid. Diverticular disease, common in the sigmoid, can also involve the descending colon. In surgery the colon is freed by cutting the peritoneum along its outer border, a bloodless plane that lets the segment be lifted forwards together with its vessels.

Function and supply

Function

Absorbs remaining water and salts, stores formed faeces and passes them on to the sigmoid colon.

Nerve supply

Parasympathetic fibres from the pelvic splanchnic nerves (S2-S4) and sympathetic fibres from the lumbar splanchnic nerves via the inferior mesenteric plexus.

Blood supply

The left colic artery and upper sigmoid arteries, both from the inferior mesenteric artery, joined by the marginal artery; veins drain to the inferior mesenteric vein.