Colon is the part of the large intestine, the final major division of the bowel, between the caecum and rectum. It runs up the right side of the abdomen, across the upper abdomen and down the left side before entering the pelvis, absorbing water and moving contents towards the rectum.
Where is the colon located?
The colon forms a broad frame around much of the small intestine, the bowel between the stomach and large intestine. It begins above the caecum, a blind-ended pouch in the lower right abdomen, and ends where the sigmoid colon becomes the rectum, the bowel segment that continues towards the anal canal.
The ascending colon lies against the posterior abdominal wall, in front of muscles of that wall and near the right kidney. Its upper bend, the right colic or hepatic flexure, lies below the liver. A flexure is a bend in a hollow organ.
The transverse colon crosses the abdomen below the stomach, although its position varies with its mobility. Its left bend, the left colic or splenic flexure, lies near the spleen and is usually higher than the right bend. The descending colon follows the left posterior abdominal wall; the sigmoid colon curves into the pelvis.
What are the parts of the colon?
The parts are named for their course and shape. Their attachment to the abdominal wall helps explain why some are relatively fixed while others can move.
| Part | Course | Main attachment pattern |
|---|---|---|
| Ascending colon | Runs upwards from the caecum to the right colic flexure | Usually fixed against the posterior abdominal wall |
| Transverse colon | Crosses between the right and left colic flexures | Suspended by the transverse mesocolon |
| Descending colon | Runs downwards from the left colic flexure | Usually fixed against the posterior abdominal wall |
| Sigmoid colon | Forms a curved loop between the descending colon and rectum | Suspended by the sigmoid mesocolon |
The peritoneum is the membrane lining the abdominal cavity and covering many abdominal organs. A mesocolon is a double layer of peritoneum that attaches a part of the colon to the posterior wall and carries its vessels and nerves. The transverse and sigmoid mesocolons allow greater mobility than the usual attachments of the ascending and descending colon.
What are the main features of the colon wall?
The taeniae coli are bands of longitudinal smooth muscle, muscle that contracts without voluntary control, on the outer wall of the colon. They are shorter than the bowel wall and help produce haustra, the pouch-like bulges along the colon. Fat-filled peritoneal tags, called omental appendices, project from its external surface.
The bands have distinct names:
- The free taenia has no attachment to a mesocolon or the greater omentum.
- The mesocolic taenia corresponds to the attachment of the transverse mesocolon on the transverse colon.
- The omental taenia corresponds to the attachment of the greater omentum on the transverse colon; the greater omentum is an apron-like peritoneal fold descending from the stomach.
Inside, the mucosa, or inner lining, contains tubular glands and mucus-secreting cells. Unlike the small intestine, it lacks villi, the finger-like projections of the lining. Beneath it lie supporting connective tissue and muscle layers that mix and propel the contents. At the transition to the rectum, the taeniae spread into a continuous longitudinal muscle layer.
What supplies blood to the colon and drains its lymph?
The superior mesenteric artery supplies the ascending colon and proximal two-thirds of the transverse colon through its ileocolic and middle colic branches and, when present, its right colic branch. The inferior mesenteric artery supplies the distal third of the transverse colon, descending colon and sigmoid colon through its left colic and sigmoid branches. Distal means farther along the bowel's course towards the rectum.
These branches generally communicate through the marginal artery, an arterial channel along the mesenteric border of the colon that can be incomplete or poorly developed in places. Veins broadly accompany the arteries. Blood returns through the superior and inferior mesenteric veins to the portal venous system, which carries blood from digestive organs to the liver.
Lymph, fluid collected from tissues, passes through lymph nodes on the bowel wall and beside the colon, then through nodes along the supplying arteries. These pathways lead towards the superior or inferior mesenteric lymph nodes, according to the region drained.
How is the colon innervated?
Innervation means nerve supply. The colon receives autonomic nerves, which regulate involuntary activity, as well as sensory fibres carrying information from its wall. Sympathetic fibres reach it through nerve networks around the mesenteric arteries and generally reduce bowel activity.
Parasympathetic fibres generally promote bowel movement and secretion. The vagus nerve provides parasympathetic fibres to the ascending colon and proximal two-thirds of the transverse colon; pelvic splanchnic nerves provide them to the distal third of the transverse colon and to the descending and sigmoid colon. Local nerve networks within the bowel wall coordinate contractions and secretion.
What does the colon do?
The colon absorbs water and electrolytes, dissolved salts such as sodium and chloride, from intestinal contents. This helps turn the remaining material into faeces. Mucus lubricates the contents and protects the lining.
Muscle contractions mix the contents and move them towards the rectum. The colon also holds material between propulsive movements, allowing time for absorption and gradual onward passage.
How can you explore the colon in the 3D model?
In the 3D model next to this article, rotate and zoom to follow the colon's course, then select and isolate it to examine its overall shape. Switch layers on and off or fade nearby structures to compare its position with surrounding anatomy.
Use the section controls to cut along a sagittal, coronal or transverse plane. Hide selected structures or pull parts apart, then rotate and zoom again to compare the resulting views.