The colon is a 5-6–ft long, inverted, U-shaped part of the large intestine (lower gastrointestinal tract). By definition, the cecum (and appendix) and ano-rectum, which are parts of the large intestine, are not included in the colon.
Embryologically, the colon develops partly from the midgut (ascending colon to proximal transverse colon) and partly from the hind gut (distal transverse colon to sigmoid colon).
On plain abdominal radiographs, the colon is seen to be filled with air and some fecal material. The colon is identified with haustra (irregular incomplete sacculations confer regular complete valvulae conniventes in jejunum).
The colon anatomy is displayed in the image below.
The ascending (right) colon lies vertically in the most lateral right part of the abdominal cavity, occupying the right iliac fossa, right lumbar region and right hypochondrium. The proximal blind end (pouch) of the ascending colon is called the cecum. The ascending colon takes a right-angled turn just below the liver (right colic or hepatic flexure) and becomes the transverse colon, which has a horizontal course from right to left, occupying the right hypochondrium, epigastrium, and left hypochondrium.
The transverse colon again takes a right-angled turn just below the spleen (left colic or splenic flexure, which is attached to the diaphragm by the phrenocolic ligament) and becomes the descending (left) colon, which lies vertically in the most lateral left part of the abdominal cavity, occupying the left hypochondrium, left lumbar region, and left iliac fossa. Splenic flexure is higher (cranial) to hepatic flexure. The descending colon leads to the inverted V-shaped sigmoid colon, which then becomes the rectum at the S3 level; the sigmoid colon is so called because of its S-shape. [1, 2, 3, 4, 5]
Lateral to ascending and descending colon are the right and left paracolic gutters of the peritoneal cavity, through which fluid/pus in the upper abdomen can trickle down into the pelvic cavity. The ascending and descending colon are related to the kidney, ureter, and gonadal vessels of the corresponding side that lie behind them in the retroperitoneum; the ascending colon is also related to the C loop (second part) of the duodenum.
Transverse colon and sigmoid colon
The transverse colon and the sigmoid colon have a mesentery (ie, transverse mesocolon and sigmoid mesocolon, respectively), but the ascending colon and descending colon are retroperitoneal, while the cecum is intraperitoneal but uses the mesentery of the ileum. The base of the transverse mesocolon lies horizontally across the duodenum and pancreas. The greater omentum has several parts, including the 4-layered omental apron hanging down off of the transverse colon and the 2-layered gastrocolic ligament connecting the greater curvature of the stomach and the transverse colon.
Three longitudinal teniae coli are present in the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon; they are not present in the rectum. In the ascending and descending colon, they are present anteriorly and on the posterolateral and posteromedial aspects. Appendages of fat, containing small blood vessels, called omental appendages (appendices epiploicae) are attached to colon.
The colon is supplied by the superior mesenteric artery through its right colic and middle colic branches and by the inferior mesenteric artery through its left colic and multiple sigmoid branches. The terminal branches of these arteries entering the colonic wall are called vasa recta.
A continuing series of anastomoses between the distal branch of the proximal artery and the proximal branch of the distal artery runs along the mesenteric (inner) border of the colon and is called the marginal artery of Drummond. The marginal artery allows a long length of colon to be mobilized (eg, to be taken up into the chest to replace the esophagus after esophagectomy).
The arc of Riolan or the meandering mesenteric artery is a communication between the middle colic artery (or its left branch) and the left colic artery (or its ascending branch).
The junction of the proximal two thirds and distal one third of the transverse colon, where the terminal branches of the superior and inferior mesenteric arteries meet, is the watershed area, which is prone to ischemia.
The superior mesenteric vein accompanies the superior mesenteric artery, but the inferior mesenteric vein drains higher than the origin of the inferior mesenteric artery; it runs vertically upward to the left of the duodenojejunal junction (flexure) and enters the splenic vein or its junction with the superior mesenteric vein to form the portal vein.
Lymphatics of the colon drain into the epicolic (on the surface of colon), paricolic (next to colon), intermediate (along branches of named vessels), and main or mesocolic (along the named colic vessels) lymph nodes.
The colon has the same 4 layers that are present in most parts of gastrointestinal tract: the mucosa, submucosa, muscularis propria, and serosa.
The mucosa includes a columnar epithelium with a large number of mucus-secreting goblet cells (villi, which are present in the small intestine, are absent in colon), lamina propria, and muscularis mucosa.
The submucosa contains the blood vessels and Meissner nerve plexus.
The muscularis propria contains the inner circular and outer longitudinal muscles and myenteric (Auerbach) nerve plexus; teniae coli are formed by outer longitudinal muscles.
The serosa of the colon is visceral peritoneum.
The transverse colon may be long and redundant and may descend down into the umbilical region or even into the pelvis.
The tip of sigmoid loop may cross the midline and lie in the right iliac fossa.
Diverticula (false diverticula containing mucosal herniation through defects in muscles) may be present in the colon, especially the left colon. They can get infected (diverticulitis) or bleed.
Vascular malformations or angiodysplasias can occur most commonly in the right colon; they are a common cause of lower gastrointestinal bleeding.
Malrotation of the gut results in the small intestine being in the right half of the abdominal cavity and the large intestine lying in the left half; the cecum lies in the epigastrium.
Colonic atresia is narrowing or even complete obliteration of the intestinal lumen, resulting in neonatal intestinal obstruction.
Hirschsprung disease involves an absence of ganglia (aganglionosis) in a segment of the (rectum and) colon.
Evaluation of the colon
The colon can be evaluated by colonoscopy (lower GI endoscopy) and lower GI series using radiologic contrast media (eg, barium, Gastrografin). Late films of a barium follow-through study may also reveal the colon.
The entire colon can be visualized at colonoscopy. The sigmoid colon (because of its S shape) and the splenic and hepatic flexures (because of their acute angles), however, are difficult to negotiate at colonoscopy.
The colonic wall and masses in the colon can be evaluated with contrast-enhanced (intravenous and rectal contrast) computed tomography. Reconstruction of CT images (CT colonography; virtual colonoscopy) provides as good an inside view of the colon as colonoscopy does. Ultrasonography is not useful for evaluation of the colon.
Surgical issues to consider include the following:
The lesser sac (behind the stomach and in front of the pancreas) can be approached through the gastrocolic omentum or through the transverse mesocolon
Hepatic flexure and splenic flexure are lowered to mobilize the ascending and descending colon, respectively
Right hemicolectomy includes the removal of a few centimeters of terminal ileum, cecum (with appendix), ascending colon, and proximal transverse colon with ileotransverse anastomosis
Left hemicolectomy includes the removal of the distal transverse colon, descending colon, and sigmoid colon with colorectal anastomosis
Total colectomy includes removal of the cecum (with appendix), ascending colon, transverse colon, descending colon, and sigmoid colon with ileorectal anastomosis
A part of the greater omentum is usually removed along with the colon during operations for cancer
Side-to-side colonic anastomosis should be performed at the teniae, where all the layers are present
Injury can occur to the duodenum during right hemicolectomy and to the ureter during right hemicolectomy or left hemicolectomy
Injury can occur to the spleen during left hemicolectomy
The Cattell Braasch maneuver is downward (inferior) mobilization of the hepatic flexure of the colon and the right transverse colon before mobilization (kocherization) of the duodenum
Other considerations include the following:
Transverse mesocolon and transverse colon can be involved (infiltrated) in cancers of the stomach and pancreas and may require en bloc resection
The sigmoid colon has a narrow-based but long mesentery, which is prone to volvulus (torsion)