Structure of the Colon
This blind sac is about 8 cm wide and 8 cm long and is continuous with the ascending colon.
It is situated in the right iliac fossa, completely invested in peritoneum.
The taeniae coli converge on the appendix which is attached to the posteromedial wall of the caecum.
The caecumlies on the iliacus and psoas (iliopsoas) and the lateral cutaneous nerve.
Anteriorly is the anterior abdominal wall.
The femoral nerve and external iliac vessels lie posteriorly and to the left.
The Ascending Colon
The ascending colon extends from the ileocaecal valve to the undersurface of the right lobe of the liver, where it turns medially, forming the hepatic flexure. It is about 15 cm long.
Peritoneum covers it anteriorly and on both sides, fixing the ascending colon to the posterior abdominal wall thus forming a paracolic gutter to its right. At its upper end, this gutter leads into a subphrenic space.
The iliacus, quadratus lumborum and lower pole of the right kidney lie posteriorly. The peritoneal surfaces of the ascending colon are in contact with loops of small intestine.
The Transverse Colon
The transverse colon extends from the hepatic flexure to the splenic flexure across the abdomen suspended by the transverse mesocolon. It is about 50 cm long.
Initially it lies directly on the descending part of the duodenum and the head of pancreas, but in its subsequent course it is attached by its mesentery to the body of the pancreas.
What are its anterosuperior and posterior relations?
The transverse colon is related anterosuperiorly to the liver, gallbladder, stomach, greater omentum and spleen.
Posteriorly it lies on the descending part of the duodenum, head of pancreas, small intestine and the left kidney.
The Descending Colon
The descending colon extends from the splenic flexure to the pelvic brim where it continues as the sigmoid colon. It is about 30 cm long.
Peritoneum covers it anteriorly and on both sides, fixing it to the posterior abdominal wall and forming a paracolic gutter on its left side.
The splenic flexure lies higher than the hepatic flexure and is attached to the diaphgram by a fold of peritoneum, the phrenico-colic ligament. The lower pole of the spleen and the greater curvature of the stomach with greater omentum attached lie superior to the splenic flexure.
Posteriorly the descending colon lies on the lower pole of the left kidney and the diaphragm, quadratus lumborum, iliacus and psoas. Its peritoneal surfaces are in contact with loops of small intestine.
The Sigmoid Colon
The sigmoid colon lies in the left iliac region and extends from the pelvic brim to the front of the third sacral segment where it becomes continuous with the rectum. Its length (usually about 40 cm) and position are variable.
It is attached by the sigmoid mesocolon to the pelvic wall. The apex of the mesentery overlies the left ureter anterior to the bifurcation of the left common iliac artery and the left sacroiliac joint.
Posteriorly the sigmoid colon lies on the left ureter and common iliac vessels, superiorly it is covered by coils of small intestine and inferiorly it lies on the bladder in males or the uterus and bladder in females.
The arterial supply of the large intestine is from the superior and inferior mesenteric arteries. Each arises from the front of the aorta and supplies the abdominal midgut and the hindgut and their derivatives respectively.
Superior Mesenteric Artery
The arterial supply of the right colon – that is, the caecum, ascending colon, hepatic flexure and right third or half of the transverse colon – is derived from the superior mesenteric artery, through its ileocolic, right colic and middle colic branches.
In this section we will look at the colonic branches of the superior mesenteric artery.
- Ileocolic artery – descends to the right and divides into anterior and posterior branches; the former supplies the lower part of the ascending colon and anastomoses with the right colic artery; the latter gives anterior and posterior caecal branches. The appendicular artery is a branch of the posterior caecal artery.
- Right colic artery – ascends to the right behind the peritoneum of the posterior abdominal wall, across the right psoas and ureter, and divides into ascending and descending branches. It supplies the ascending colon and anastomoses with the ileocolic and middle colic arteries.
- Middle colic artery – passes upwards on the body of the pancreas to reach the transverse mesocolon within which it divides to supply the right two-thirds of the transverse colon.
Inferior Mesenteric Artery
The arterial supply of the left colon – that is, the distal one third of the transverse colon, descending colon and sigmoid colon – is derived from the inferior mesenteric artery, through its left colic, sigmoid and superior rectal branches.
The inferior mesenteric artery – arises from the aorta behind the duodenum and descends to the left across the posterior abdominal wall. It crosses the left psoas and left common iliac vessels. The inferior mesenteric vein and the left ureter are on its left side. It continues beyond the brim of the pelvis as the superior rectal artery.
In this section we will look at the colonic branches of the inferior mesenteric artery:
- Left colic artery – ascends to the left across the left psoas and ureter as far as the splenic flexure. Its branches supply the transverse and descending colon, and anastomose with the middle colic and sigmoid arteries.
- Sigmoid arteries – enter the sigmoid mesocolon and supply the sigmoid colon and the lower descending colon.
- Superior rectal artery – is the continuation of the inferior mesenteric artery in the pelvis. It descends in the lower part of the sigmoid colon and reaches the posterior aspect of the upper rectum. It supplies the rectum and proximal two-thirds of the anal canal and anastomoses with the inferior rectal branch of the internal pudendal artery.
Venous Drainage of the Colon
The venous drainage of the gastrointestinal system from the lower oesophagus to the upper rectum is by way of the portal venous system
The portal vein is formed by the superior mesenteric vein and the splenic vein. The inferior mesenteric vein drains into the splenic vein.
The intestinal mucosa is richly supplied with lymph vessels which drain via submucosal and subserous plexuses. These drain into intermediate groups of nodes arranged along arteries which in turn drain into preaortic nodes arranged along the origins of the superior and inferior mesenteric arteries.
The superior mesenteric group drains the caecum and appendix, the ascending and proximal transverse colon.
The inferior mesenteric group drains the distal one third of the transverse colon, the descending and sigmoid colon, and the upper part of the rectum.
How can we distinguish the colon from the small intestine?
Taeniae coli, appendices epiploicae and the paler external appearance of the colon help to distinguish it from small bowel macroscopically.
The longitudinal muscle of the colon is restricted to three bands, taeniae coli.
Taeniae coli are shorter than the rest of the wall and cause it to be sacculated.
Fatty tags, appendices epiploicae, project outwards from the wall of the large intestine.
The greater omentum, a pad of fat, is attached to the free edge of the transverse colon.
The primary difference between the small bowel and colon based upon histology is that the small bowel mucosa consists mainly of enterocytes and is evaginated to form villi. Colonic mucosa consists of columnar cells and numrous goblet cells in crypts without villi being present.