The Colon

Original Author: Dominic Aubrey-Jones
Last Updated: March 22, 2017
Revisions: 42

The colon (large intestine) is a 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 ions 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 ascending, transverse, descending and sigmoid parts of the colon. We will consider their anatomical structure and relations, neurovascular supply, and lymphatic drainage.


Anatomical Course

The colon averages 150cm in length, and can be divided into four parts – ascending, transverse, descending and sigmoid. We shall now examine the course of these four sections.

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 crosses the abdomen, extending 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 is position – it can dip into the pelvis in tall, thin individuals. Unlike the ascending and descending colons, the transverse colon is intraperitoneal.

Descending Colon

After the left colic flexure, the colon moves inferiorly, and so is termed 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.

Fig 1 - Overview of the four main parts of the colon.

Fig 1 – Overview of the four main parts of the colon.

Paracolic Gutters

The paracolic gutters are two spaces between the ascending/descending colon and the posterolateral abdominal wall.

These structures are clinically important, as they allow infective material that has been released from abdominal organs to accumulate elsewhere in the abdomen.

 


Anatomical Relations

The colon has numerous important anatomical relations in the abdomen, as shown in Table 1:

Table 1 – Anatomical Relations of the Colon
Anterior Posterior
Ascending colon Small intestine

 

Greater omentum

 

Anterior abdominal wall

Iliacus and quadratus lumborum

 

Right kidney

 

Iliohypogastric and ilioinguinal nerves

Transverse colon Greater omentum

 

Anterior abdominal wall

 

Duodenum

 

Head of the pancreas

 

Jejunum and ileum

Descending colon Small intestine

 

Greater omentum

 

Anterior abdominal wall

 

Iliacus and quadratus lumborum

 

Left kidney

 

Iliohypogastric and ilioinguinal nerves

Sigmoid colon Urinary bladder

 

Uterus and upper vagina (females only)

Rectum

 

Sacrum

 

Ileum


Anatomical Structure

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.

Fig 2 - The macroscopic features of the large intestine.

Fig 2 – The macroscopic features of the large intestine.


Neurovascular Supply

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.

Arterial Supply

As a general rule, midgut-derived structures are supplied by the superior mesenteric artery, and hindgut-derived structures by the inferior mesenteric artery.

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.

Venous Drainage

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 – 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.

Fig 3 - The major arteries and veins supplying the colon.

Fig 3 – The major arteries and veins supplying the colon.

Innervation

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. The parasympathetic innervation is supplied by the pelvic splanchnic nerves, and sympathetic innervation via the lumbar splanchnic nerves.

Lymphatic Drainage

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.

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The transverse colon has a dual supply from which two vessels?

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The ileocolic artery is responsible for supplying which region of the colon?

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The left colic flexure is located at which organ?

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The middle colic artery is a branch of which vessel?

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The artery which provides collateral supply to the colon is known as the...

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