Abdomen:Hollow viscus:Colon

The **colon** has four part with the main function of absorption of water and electrolyte from the digested material prior to excretion.

The **ascending colon** is the first part of the colon

* continuation of the caecum superior to the ileocaecal valve to the right colic flexure.
* Secondarily retroperitoneal, but with its own mesentery in 25% of patients.

The longitudinal muscles form taeniae coli, comparable with the caecum. The circular musculature is exposed underneath. Small pouches of peritoneum with fat, appendices epiploicae project from the serous coat of the colon.

Relations:

* anterior - loops of small bowel, greater omentum, anterior abdominal wall
* posterior  - posterior abdominal wall and the iliac fascia.
* inferior - caecum, terminal ileum
* superiorly - hepatic flexure next to right kidney and liver.
* laterally - paracolic gutter
* medially - right infracolic gutter

Blood supply:

* arterial - colic branch of the ileocolic artery and right colic arteries - branches of superior mesenteric artery
* venous
	+ similarly named veins draining in to the portal venous system
	+ also drains via lumbar veins (as is retroperitoneal) to the inferior vena cava - this is a pathway for portal-systemic collateral pathways

Nerve supply:

* sympathetic - superior mesenteric plexus
* parasympathetic - pelvic splanchnic nerves (from S2-S4)
* afferent pain fibres run with sympathetic fibres - gives periumbilical pain

Lymph supply:

Lymph runs with the arteries to the paracolic lymph nodes, which drain into the superior mesenteric group.

The **transverse colon** is the longest and most mobile part of the large intestine. It measure up to 45cm in length.

* Extending from the hepatic to the splenic flexure in a downward convex path.
* It is almost completely invested by peritoneum with a transverse mesocolon which connect it to the inferior pole of the right kidney, to the second part of duodenum, and pancreas and inferior pole of left kidney. The gastrocolic ligament attaches the transverse colon to the stomach.
* The taeniea coli continues from the ascending colon. The appendices epiploicae are larger and more numerous.

Relations

* superiorly (right to left) - liver, gallbladder, greater curvature of the stomach,spleen
* inferiorly - small bowel
* anteriorly - greater omentum and anterior abdominal wall
* posteriorly (right to left) - 2nd part duodenum, head of pancreas, small bowel

Blood supply

* arterial
	+ middle colic artery (branch of superior mesenteric artery) supplies proximal 2/3
	+ ascending branch of left colic artery (branch of IMA) - supplies distal 1/3
* venous - via similarly named veins to splenic vein to the portal venous system

Nerve supply

* sympathetic
	+ superior mesenteric plexus
	+ inferior mesenteric plexus
* parasympathetic - derived from pelvic splanchnic nerves (S2-S4)
* afferent pain fibres run with sympathetic fibres - gives hypogastric and periumbilical pain

Lymphatic supply

* Lymphatics accompany vessels and rain to paracolic nodes to the superior mesenteric group (proximal two-thirds) and inferior mesenteric group (distal two-thirds).

The**descending colon** is the continuation of the transverse colon after the left colic flexure. It measures up to 25 cm length.

* The colon becomes retroperitoneal.
* Passes into the pelvic brim.
* At the splenic flexure, a fold of peritoneum, the phrenicocolic ligament attach it to the diaphragm at the level of 10-11th ribs.

Relations

* anteriorly - loops of small intestine, anterior abdominal wall, greater omentum
* posteriorly - posterior abdominal wall
	+ distal descending colon crosses over external iliac and gonadal vessels
* laterally - paracolic gutter
* medially - left infracolic gutter
* superior - spleen and left kidney
* inferior - sigmoid colon

Blood supply

* arterial - branches of inferior mesenteric artery:
	+ descending branch of left colic artery - proximal descending colon
	+ sigmoid arteries - distal descending colon
* venous - similarly named veins draining to the splenic vein and portal venous system

Nerve supply

* sympathetic - inferior mesenteric plexus (T10-12)
* parasympathetic - via pelvic splanchnic nerves (S2-S4)
* afferent pain fibres run with sympathetic fibres - gives hypogastric pain.

Lymphatic supply

* Lymphatics travel with vessels to the inferior mesenteric group

The **sigmoid colon** is the continuation of the descending colon and the final part of the colon, ending at the rectum. It curved medially it enters the pelvis, where it gains a mesentery (sigmoid mesocolon) and is then called the sigmoid colon. It measures approximately 15-40 cm in length (great variability in length) with its apex may be as high as the umbilicus.

Similar to the colon, it has three taeniae coli, and these muscular bands are wider. Diverticulosis is frequent in this segment.

Relations

* anteriorly
	+ male - bladder
	+ female - bladder and uterus
* superiorly - loops of small intestine
* posteriorly - left internal iliac artery and vein, left ureter, sacrum, sacral plexus
* laterally
	+ male - pelvic side wall, vas deferens
	+ female - pelvic side wall, ovary

Blood supply

* arterial - two-to-four sigmoid arteries (branch of inferior mesenteric artery)
* venous - similarly named veins drain into the inferior mesenteric vein

Nerve supply

* sympathetic - inferior mesenteric plexus
* parasympathetic - via pelvic splanchnic nerves (from S2-S4)
* afferent pain fibres run with sympathetic fibres - gives hypogastric pain.

Lymphatics

* Lymphatics travel with vessels to the inferior mesenteric group.

All the blood supply to the colon is via branches of the SMA and IMA, forming anastomoses of marginal arteries, which run along the edge of the colon (similar to arcades in small bowel). The weakest area is between middle and left colic branches, and prone to ischaemia.

**Normal variant**

* Primary colonic pneumatosis
* Colonic interposition – hepatic flexure lies between the right diaphragm and transverse colon and may appear as free air
* Cathartic colon
* Intramural pseudodiverticulosis
* Functional megacolon
* Partial duplication