A frame of gut that ascends on the right, crosses beneath the liver and stomach, and descends on the left before an S-shaped loop
The colon: ascending, transverse, descending and sigmoid.
Anatomy · Organs & viscera
Colon
About 1.5 m of large bowel
Abdomen and pelvis · gastrointestinal tract
The colon is the long, wide tube of the large bowel that frames the small intestine on three sides, running from the caecum in the right iliac fossa to the rectum. It is divided into ascending, transverse, descending and sigmoid parts, with sharp bends at the hepatic and splenic flexures. It reabsorbs water and salts and houses the gut microbiome.
How to spot them
- ▸A wide, sacculated tube forming a frame around the coils of small bowel: up the right, across the top, down the left
- ▸Puckered into pouches (haustra) by three longitudinal ribbons of muscle, the taeniae coli
- ▸Hanging fatty tags called appendices epiploicae along its surface
- ▸Wider and more peripheral than the small intestine; the ascending and descending parts are fixed to the back wall, the transverse and sigmoid parts hang on mesocolons
What to know
Three flat bands of longitudinal muscle that pucker the wall of the large bowel into pouches
The taeniae coli, which create the haustra, the sacculations of the colon.
Small fat-filled tags that dangle from the outer wall of the large bowel
Appendices epiploicae of the colon.
The bend beneath the right lobe of the liver, and its higher counterpart tucked hard against the spleen
The hepatic and splenic flexures of the colon.
An S-shaped loop in the left iliac fossa that leads on into the rectum, prone to twisting in the elderly and to sac-like outpouchings
The sigmoid colon, the site of sigmoid volvulus and most diverticular disease.
The mobile stretch of the large bowel that hangs from the back wall on its own mesentery, with the greater omentum draped down from it
The transverse colon, on the transverse mesocolon.
A continuous arterial arcade running along the inner margin of the large bowel that links the superior and inferior mesenteric supplies
The marginal artery of Drummond, running alongside the colon.
Key ideas
- Water, sodium and some vitamins are absorbed as the contents pass along, turning liquid chyme into formed faeces.
- The colon holds a huge community of bacteria that ferment fibre and produce vitamin K and short-chain fatty acids.
Quick-fire facts
- •The large intestine is about 1.5 m (5 feet) long and wider than the small intestine, with the widest part in the caecum.
- •The ascending and descending colon are retroperitoneal; the transverse and sigmoid colon are intraperitoneal.
- •The splenic flexure lies higher and further back than the hepatic flexure and is tethered to the diaphragm by the phrenicocolic ligament.
- •The three taeniae coli converge at the base of the appendix, a useful landmark for finding it in surgery.
- •Its blood supply changes at the distal transverse colon: the superior mesenteric artery supplies the right side and the inferior mesenteric artery the left.
- •The splenic flexure is a 'watershed' area between the two arterial territories and is vulnerable to ischaemic colitis.
- •The sigmoid colon is named from the Greek letter sigma, in its old crescent-like form.
- •The colon and rectum are the commonest site of bowel cancer, and diverticula most often form in the sigmoid.
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