The colon (large intestine) is the distal part of the gastrointestinal tract, extending from the cecum to the anal canal. It receives digested food from the small intestine, from which it absorbs water and electrolytes to form faeces.
Anatomically, the colon can be divided into four parts – ascending, transverse, descending and sigmoid. These sections form an arch, which encircles the small intestine.
In this article, we shall look at the anatomy of the colon – its anatomical structure and relations, neurovascular supply, and clinical correlations.
The colon averages 150cm in length, and can be divided into four parts (proximal to distal): ascending, transverse, descending and sigmoid.
The colon begins as the ascending colon, a retroperitoneal structure which ascends superiorly from the cecum.
When it meets the right lobe of the liver, it turns 90 degrees to move horizontally. This turn is known as the right colic flexure (or hepatic flexure), and marks the start of the transverse colon.
The transverse colon extends from the right colic flexure to the spleen, where it turns another 90 degrees to point inferiorly. This turn is known as the left colic flexure (or splenic flexure). Here, the colon is attached to the diaphragm by the phrenicocolic ligament.
The transverse colon is the least fixed part of the colon, and is variable in position (it can dip into the pelvis in tall, thin individuals). Unlike the ascending and descending colon, the transverse colon is intraperitoneal and is enclosed by the transverse mesocolon.
After the left colic flexure, the colon moves inferiorly towards the pelvis – and is called the descending colon. It is retroperitoneal in the majority of individuals, but is located anteriorly to the left kidney, passing over its lateral border.
When the colon begins to turn medially, it becomes the sigmoid colon.
The 40cm long sigmoid colon is located in the left lower quadrant of the abdomen, extending from the left iliac fossa to the level of the S3 vertebra. This journey gives the sigmoid colon its characteristic “S” shape.
The sigmoid colon is attached to the posterior pelvic wall by a mesentery – the sigmoid mesocolon. The long length of the mesentery permits this part of the colon to be particularly mobile.
The paracolic gutters are two spaces between the ascending/descending colon and the posterolateral abdominal wall.
These structures are clinically important, as they allow material that has been released from inflamed or infected abdominal organs to accumulate elsewhere in the abdomen.
The large intestine has a number of characteristic features, which allows it to be distinguished from the small intestine:
- Attached to the surface of the large intestine are omental appendices – small pouches of peritoneum, filled with fat.
- Running longitudinally along the surface of the large bowel are three strips of muscle, known as the teniae coli. They are called the mesocolic, free and omental coli.
- The teniae coli contract to shorten the wall of the bowel, producing sacculations known as haustra.
- The large intestine has a much wider diameter compared to the small intestine.
These features cease at the rectosigmoid junction, where the smooth muscle of the teniae coli broaden to form a complete layer within the rectum.
The colon has numerous important anatomical relations in the abdomen, as shown in Table 1:
The neurovascular supply to the colon is closely linked to its embryological origin:
- Ascending colon and proximal 2/3 of the transverse colon – derived from the midgut.
- Distal 1/3 of the transverse colon, descending colon and sigmoid colon – derived from the hindgut.
The ascending colon receives arterial supply from two branches of the superior mesenteric artery; the ileocolic and right colic arteries. The ileocolic artery gives rise to colic, anterior cecal and posterior cecal branches – all of which supply the ascending colon.
The transverse colon is derived from both the midgut and hindgut, and so it is supplied by branches of the superior mesenteric artery and inferior mesenteric artery:
- Right colic artery (from the superior mesenteric artery)
- Middle colic artery (from the superior mesenteric artery)
- Left colic artery (from the inferior mesenteric artery)
The descending colon is supplied by a single branch of the inferior mesenteric artery; the left colic artery. The sigmoid colon receives arterial supply via the sigmoid arteries (branches of the inferior mesenteric artery).
Marginal Artery of Drummond
The marginal artery (of Drummond) is a clinically important vessel that provides collateral supply to the colon – thereby maintaining arterial supply in the case of occlusion or stenosis of one of the major vessels.
As the terminal vessels of the superior mesenteric and inferior mesenteric artery approach the colon, they split into many branches, which anastomose with each other. These anastomoses form a continuous arterial channel which extends the length of the colon – the marginal artery. Long, straight arterial branches (called vasa recta) arise from the marginal artery to supply the colon.
The venous drainage of the colon is similar to the arterial supply:
- Ascending colon – ileocolic and right colic veins, which empty into the superior mesenteric vein.
- Transverse colon – middle colic vein, which empties into the superior mesenteric vein.
- Descending colon – left colic vein, which drains into the inferior mesenteric vein.
- Sigmoid colon – drained by the sigmoid veins into the inferior mesenteric vein.
The superior mesenteric and inferior mesenteric veins ultimately empty into the hepatic portal vein. This allows toxins absorbed from the colon to be processed by the liver for detoxification.
The innervation to the colon is dependent on embryological origin:
- Midgut-derived structures (ascending colon and proximal 2/3 of the transverse colon) receive their sympathetic, parasympathetic and sensory supply via nerves from the superior mesenteric plexus.
- Hindgut-derived structures (distal 1/3 of the transverse colon, descending colon and sigmoid colon) receive their sympathetic, parasympathetic and sensory supply via nerves from the inferior mesenteric plexus:
- Parasympathetic innervation via the pelvic splanchnic nerves
- Sympathetic innervation via the lumbar splanchnic nerves.
The lymphatic drainage of the ascending and transverse colon is into the superior mesenteric nodes. The descending colon and sigmoid drain into the inferior mesenteric nodes.
Most of the lymph from the superior mesenteric and inferior mesenteric nodes passes into the intestinal lymph trunks, and on to the cisterna chyli – where it ultimately empties into the thoracic duct.