Table of Contents
General Structure
Anatomy of the colon. Author: https://www.cdc.gov/cancer/colorectal/basic_info/what-is-colorectal-cancer.htm. CC BY-SA 4.0. Wikimedia Commons.
Approximately 1.5m long, the colon, or the large bowel, is the distal end of the GI tract running from the ileo-caecal junction to the anal canal. The main function of the colon is to form stool and absorb remaining water in the GI tract.
It is separated into four main components including (proximally to distally): Caecum, Ascending colon, Transverse colon, Descending colon, Sigmoid colon. Distally and completing the GI tract are the Rectum and Anal canal. Importantly, the Transverse colon is Intraperitoneal (enveloped within visceral peritoneum), whereas the Ascending and Descending colon are Retroperitoneal (located behind a layer of parietal peritoneum, covering their anterior surface).
At the junction between the Ascending and Transverse colon is the Hepatic flexure, closely related to the right lobe of the liver. Opposite at the junction of the Transverse and Descending colon is the Splenic flexure, closely related to the spleen.
The Mesentery is a reflection (double fold) of Parietal Peritoneum that tethers the colon and small intestine to the posterior abdominal wall and provides neurovascular and lymphatic supply and drainage. The prefix ‘Meso-’ denotes the mesentery relating to certain GI structures, e.g. Mesocolon or Mesoappendix. The Mesocolon is the mesentery of the colon that contains the neurovascular supply to the colon and is largely adipose tissue. Notably, the Transverse and Sigmoid Mesocolon (Mesosigmoid) permit relative mobility of the colon at these points. Clinically this mobility is significant as it can predispose to twisting of the colon i.e. Sigmoid volvulus. By contrast the Ascending and Descending colon are more strictly tethered and lie flat against the posterior abdominal wall.
Mesentery. Author: https://www.scientificanimations.com/. CC BY-SA 4.0. Wikimedia Commons.
Colon-Specific Features
- Omental appendices – small fatty peritoneal pouches found along the length of the colon.
- Teniae coli – longitudinal smooth muscular bands running along the colon, contracting to shorten the bowel. The teniae converge proximally at the base of the appendix, and distally they broaden and fuse the form a muscular layer completely surrounding the rectum.
- Haustra – sacculations produced by contractions of the teniae coli.
Features of the colon. Author: OpenStax College. CC BY 3.0. Wikimedia Commons.
Relations
As mentioned, the Hepatic (right) and Splenic (left) flexures of the colon are closely related to the right lobe of the liver and the spleen, respectively. This can be clinically significant: for example, during mobilisation of the splenic flexure in surgery, excessive traction or tension on the spleen can cause significant bleeding from iatrogenic tears or lacerations.
Anatomical relations are highly significant especially in the context of rectal cancer and potential invasion of other pelvic organs and structures:
| Relation | |
|---|---|
Anteriorly | Denonvilliers’ fascia separates the rectum from anterior structures, which in males include the prostate, seminal vesicles, bladder base and rectovesical pouch. In females the vagina lies anteriorly with the uterus and pouch of Douglas more superiorly. |
Posteriorly | The sacrum, coccyx and middle sacral artery lie posteriorly to the rectum. Rectal cancers advancing posteriorly may involve the lower sacral nerves and cause severe sciatic pain. |
Laterally | The Levator ani muscle of the pelvic floor supports either side of the rectum, with the ischiorectal fossae just inferiorly, containing fat with neurovascular and lymphatic structures. |
Blood supply
Visceral blood supply to the foregut, midgut and hindgut (4). Permission granted.
Arterial
The Caecum, Ascending colon, and proximal 2/3 of the Transverse colon form the distal end of the Midgut, supplied by the Superior Mesenteric Artery (SMA – L1 level from Aorta). This in turn gives off the Ileocolic, Right colic and Middle colic artery branches, supplying the Caecum, Ascending colon and Transverse colon (proximal 2/3), respectively.
The distal 2/3 of Transverse colon, Descending colon, Sigmoid colon and rectum form the Hindgut, and are supplied by the Inferior Mesenteric Artery (IMA – L3 level from Aorta). This in turn gives off the Left colic, Sigmoid and Superior rectal artery branches, supplying the Descending colon, Sigmoid colon and Rectum, respectively.
The Splenic flexure (a.k.a Griffith’s point) is a watershed area located at the junction between the midgut and hindgut. It receives collateral blood supply from both the SMA and IMA via the Marginal artery (of Drummond). Clinically this significant as an area susceptible to ischaemia e.g. in ischaemic colitis.
Venous
The venous drainage closely follows the arterial supply:
- The Ascending colon is drained by the Ileocolic and Right colic veins, which drain into the Superior mesenteric vein (SMV).
- The Transverse colon is drained by the Middle colic vein, which drains into the SMV.
- The Descending colon is drained by the Left colic vein, which drains into the Inferior mesenteric vein (IMV).
- The SMV, IMV and Splenic vein drain into the Hepatic portal vein to pass through the liver.
Portal venous system. Author: OpenStax College. CC BY 3.0. Wikimedia Commons.
Innervation
Sympathetic
- The Thoracolumbar Splanchnic nerves (Greater, Lesser and Least) originate from the Sympathetic chain, and synapse at the Prevertebral ganglia (coeliac, superior mesenteric, inferior mesenteric).
- Postganglionic sympathetic fibres supply the abdominal viscera via plexuses (e.g. coeliac, superior and inferior mesenteric, superior and inferior hypogastric).
- Inhibit secretomotor activity (reduces peristalsis, inhibits defecation and urination)
Autonomic Nervous system schematic. Sympathetic (red) and Parasympathetic (blue) systems. Original source: Henry Gray (1918) Anatomy of the Human Body. Public domain. Wikimedia Commons.
Parasympathetic
- Vagus nerve (cranial outflow) and pelvic splanchnic nerves (sacral outflow)
- Postganglionic fibres are relayed from tiny ganglia that lie in the walls of the abdominal viscera
- Increase secretomotor activity (promotes peristalsis, defecation and urination)
Parasympathetic nervous system. Author: OpenStax College. Original Source: Anatomy and Physiology, Connexions Website. CC BY 3.0. Wikimedia Commons.
Enteric
- Intrinsic innervation of the GI tract including the bowel
- Myenteric plexus of Auerbach lies between the circular and longitudinal muscle layers. Mainly responsible for peristaltic movement of the bowels and can act independently of the autonomic nervous system. Abnormalities in the Myenteric plexus can lead to motility disorders such as Achalasia in the oesophagus or Hirschprung’s in the colon
- Submucosal plexus of Meissner lies in the submucosal layer. Has a role in gut secretion, absorption and blood flow
Auerbach’s and Meissner’s plexuses. Original Source: Own work based on Ens.png by User:Cayte. Wikimedia Commons. Author: Rehua. CC BY-SA 3.0.
Lymphatics
The lymphatics are contained within the mesentery alongside the arteries and veins.
- Lymphatic drainage of the Ascending and Transverse colon is to the Superior mesenteric nodes.
- Lymphatic drainage of the Descending and Sigmoid colon is to the Inferior mesenteric nodes.
- Lymphatic drainage of the Rectum is via the Pararectal lymph nodes (which drain into the Inferior mesenteric nodes), and the Internal iliac nodes.
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- Kenhub [Internet]. [cited 2025 Apr 17]. Colon. Available from: https://www.kenhub.com/en/library/anatomy/the-colon
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- Theodore S, Xia T, Saillant N. Intestinal Ischemia — Etiology and Foundational Concepts. NEJM Evidence. 2024 Feb 27;3(3):EVIDra2300266.