GI tract - Colon Flashcards
What is the general role of the colon?
1) Absorb water
2) Absorb electrolytes
to then form faeces.
What are the four divisions of the colon?
1) Ascending
2) Transverse
3) Descending
4) Sigmoid
What is the length of the colon?
It is approximately 1.5m in length.
Ascending colon?
The colon begins as the ascending colon, a retroperitoneal structure which ascends superiorly from the caecum.
When it meets the right lobe of the liver, it turns 90 degrees to move horizontally. This is known as the right colic flexure (or hepatic flexure), and marks the start of the transverse colon.
Transverse colon?
The transverse colon is an intraperitoneal structure and is enclosed by the transverse mesocolon.
It 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, an is variable in position (it can dip into the pelvis in tall, thin individuals).
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 collon 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 it characteristic āSā 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.
What are paracolic gutter?
These 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.
Anatomical structure?
The colon has a number of characteristic features, which allows it be distinguished from the small colon:
1) Attached to the surface of the large intestine are omental appendices - small pouches of peritoneum, filled with fat.
2) Running longitudinally along the surgace of the large bowel are three strips of muscles known as teniae coli. They are called the mesocolic, free and omental coli.
The teniae coli contract to shorten the wall of the bowel, produced saculations known as haustra.
3) The large intestine has a much wider diameter compared to the small intestine.
These features cease of the rectosigmoid junction, where the smooth muscle of the teniae coli broaden to form a complete layer within the rectum.
Anatomical relations - ascending colon?
Anterior:
1) Small intestine
2) Greater omentum
3) Anterior abdominal wall
Posterior:
1) Iliacus and quadratus lumborum
2) Right kidney
3) Iliohypogastric and ilioinguinal nerves
Anatomical relations - Transverse colon
Anterior
1) Greater omentum
2) Anterior abdominal wall
Posterior:
1) Duodenum
2) Head of the pancreas
3) Ileum ileum and jejunum
Anatomical relations - descending?
Anterior:
1) Small intestine
2) Greater omentum
3) Anterior abdominal wall
Posterior:
1) Iliacus and quadratus lumborum
2) Left kidney
3) Iliohypogastric and ilioinguinal nerves
Anatomical relations - sigmoid colon?
Anterior:
1) Urinary bladder
2) Uterus and upper vagina
Posterior:
1) Rectum
2) Sacrum
3) Ileum
Neurovascular supply - arterial supply?
The neurovascular supply 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 sigmoid colon - derived from the hindgut.
As a general rule, midgut-derived structures are supplied by the superior mesenteric artery, and hindgut-derived structures by the inferior mesenteric artery.
The transverse colon is derived from both the midgut and the hindgut, and so it is supplied by branches of the superior mesenteric artery and the inferior mesenteric artery:
- Right colic artery (superior mesenteric artery)
- Middle colic artery (superior mesenteric artery)
- Left colic artery (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).
Clinical relevance - marginal artery of Drummond
This artery is clinically significant because it 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 anastamose with each other. These anstomoses form a continuous arterial channel which extends the length of the colon - the marginal artery. Long, straight arterial branches (called the vasa recta) arise from the marginal artery to supply the colon.
Neurovascular supply - venous drainage?
It is similar to the arterial supply:
1) Ascending colon - ileocolic and right colic veins that empty into the superior mesenteric veins.
2) Transverse colon - middle colic vein that empty into the superior mesenteric vein.
3) Descending colon - left colic vein that drain into the inferior mesenteric vein.
4) Sigmoid colon - drained by the sigmoid veins into the inferior mesenteric veins.
The superior and inferior mesenteric veins ultimately drain into the hepatic portal vein. This allows toxins absorbed from the colon to be processed by the liver for detoxification.
