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.
Anatomical Position
The colon averages 150cm in length, and can be divided into four parts (proximal to distal): ascending, transverse, descending and sigmoid.
Ascending Colon
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.
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.
Descending Colon
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.
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.