Sigmoid colon

At a glance

English

Sigmoid colon

Latin (TA2)

Colon sigmoideum

System

Digestive

Region

Abdomen

Side

Single, on the midline

About the sigmoid colon

The sigmoid colon, also called the pelvic colon, takes its name from its curved, S-like shape. It starts at the pelvic brim in the left iliac fossa, continuing from the descending colon, and ends opposite the third sacral segment, becoming the rectum. Its length is very variable, on average about 35 to 40 cm. Typically it runs across the pelvis towards the right, loops back on itself to the left and then turns down to join the rectum in the midline.

Unlike the descending colon, it is wrapped in peritoneum and hangs on its own mesentery, the sigmoid mesocolon. That fold is long in the middle and short at both ends, so the loop is fixed where it joins the descending colon and the rectum but moves freely in between; when distended, it can rise into the abdomen. The root of the mesocolon forms an inverted V over the left common iliac vessels, and the left ureter lies at the apex of the V, an important landmark in surgery. Behind the loop lie the external iliac vessels, the left piriformis and sacral plexus, and in front are the bladder and loops of small intestine.

The sigmoid stores faeces before they move into the rectum. Its wall still has taeniae and haustra, but the taeniae begin to spread out at its lower end and merge into a continuous outer muscle layer on the rectum. Its curves allow gas to collect in the upper arch and be passed separately from solid contents.

Raised pressure inside this narrow segment makes it the commonest site of diverticula, small outpouchings of the lining through weak points in the wall, which may bleed or become inflamed (diverticulitis). Because its central part is so mobile, the loop can twist on its mesentery (sigmoid volvulus), especially in older people, blocking the bowel and threatening its blood supply. It is also a common site of cancer and is easily examined with a flexible sigmoidoscope.

Function and supply

Function

Stores faeces and gas before they pass into the rectum, and continues to absorb water.

Nerve supply

Parasympathetic fibres from the pelvic splanchnic nerves and sympathetic fibres from the lumbar splanchnic nerves via the inferior mesenteric ganglion and plexus.

Blood supply

Sigmoid arteries from the inferior mesenteric artery, with veins draining to the inferior mesenteric vein.