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Wednesday, August 2, 2017

ABDOMEN-Gross Anatomy of Colon


The colon (large intestine) is a distal part of the gastrointestinal tract, extending from the cecum to the anal canal. It receives digested food from the small intestine, from which it absorbs water and ions to form faeces
Anatomically, the colon can be divided into four parts – ascending, transverse, descending and sigmoid. These sections form an arch, which encircles the small intestine.
diagram of colon;

Anatomical Course

The colon averages 150cm in length, and can be divided into four parts – ascending, transverse, descending and sigmoid. We shall now examine the course of these four sections.
1.Ascending Colon;
The colon begins as the ascending colon, a retroperitoneal structure which ascends superiorly from the cecum. When it meets the right lobe of the liver, it turns 90 degrees to move horizontally. This turn is known as the right colic flexure (or hepatic flexure), and marks the start of the transverse colon.
2.The transverse colon crosses the abdomen, extending 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 (orsplenic flexure). Here, the colon is attached to the diaphragm by the phrenicocolic ligament.
The transverse colon is the least fixed part of the colon, and is variable is position – it can dip into the pelvis in tall, thin individuals. Unlike the ascending and descending colons, the transverse colon inintraperitoneal.
3.Descending Colon
After the left colic flexure, the colon moves inferiorly, and so is termed 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 colon begins to turn medially, it becomes the sigmoid colon.
4.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 its characteristic “S” shape.
The sigmoid colon is attached to the posterior pelvic wall by a mesentery – the sigmoidmesocolon. The long length of the mesentery permits this part of the colon to be particularly mobile.

Anatomical Relations


Table 1 – Anatomical Relations of the Colon
AnteriorPosterior
Ascending colonSmall intestine
Greater omentum

Anterior abdominal wall
Iliacus and quadratus lumborum
Right kidney

Iliohypogastric and ilioinguinal nerves
Transverse colonGreater omentum
Anterior abdominal wall

Duodenum
Head of the pancreas

Jejunum and ileum
Descending colonSmall intestine
Greater omentum

Anterior abdominal wall

Iliacus and quadratus lumborum
Left kidney

Iliohypogastric and ilioinguinal nerves
Sigmoid colonUrinary bladder
Uterus and upper vagina (females only)
Rectum
Sacrum

Ileum

Anatomical Structure

The large intestine has a number of characteristic features, which allows it to be distinguished from the small intestine:
  • Attached to the surface of the large intestine are omental appendices – small pouches of peritoneum, filled with fat.
  • Running longitudinally along the surface of the large bowel are three strips of muscle, known as the teniae coli. They are called the mesocolic, free and omental coli.
  • The teniae coli contract to shorten the wall of the bowel, producing sacculations known as haustra.
  • The large intestine has a much wider diameter compared to the small intestine
These features cease at the rectosigmoid junction, where the smooth muscle of the teniae coli broaden to form a complete layer within the rectum.
BLOOD SUPPLY;

Arterial Supply

As a general rule, midgut-derived structures are supplied by the superior mesenteric artery, and hindgut-derived structures by the inferior mesenteric artery.
The ascending colon receives arterial supply from two branches of the superior mesenteric artery; the ileocolic and right colic arteries. The ileocolic artery gives rise to colic, anterior cecal and posterior cecal branches – all of which supply the ascending colon.
The transverse colon is derived from both the midgut and hindgut, and so it is supplied by branches of the superior mesenteric artery and inferior mesenteric artery:
  • Right colic artery (from the superior mesenteric artery)
  • Middle colic artery (from the superior mesenteric artery)
  • Left colic artery (from the 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).

Venous Drainage

The venous drainage of the colon is similar to the arterial supply:
  • Ascending colon – ileocolic and right colic veins, which empty into the superior mesenteric vein.
  • Transverse colon – superior mesenteric vein.
  • Descending colon – left colic vein, which drains into the inferior mesenteric vein.
  • Sigmoid colon – drained by the sigmoid veins into the inferior mesenteric vein.
The superior mesenteric and inferior mesenteric veins ultimately empty into the hepatic portal vein. This allows toxins absorbed from the colon to be processed by the liver for detoxification.

Innervation


  • Midgut-derived structures (ascending colon and proximal 2/3 of the transverse colon) receive their sympathetic, parasympathetic and sensory supply via nerves from the superior mesenteric plexus.
  • Hindgut-derived structures (distal 1/3 of the transverse colon, descending colon and sigmoid colon) receive their sympathetic, parasympathetic and sensory supply via nerves from the inferior mesenteric plexus. The parasympathetic innervation is supplied by the pelvic splanchnic nerves, and sympathetic innervation via the lumbar splanchnic nerves.

Lymphatic Drainage

The lymphatic drainage of the ascending and transverse colon is into the superior mesenteric nodes. The descending colon and sigmoid drain into the inferior mesenteric nodes.
Most of the lymph from the superior mesenteric and inferior mesenteric nodes passes into the intestinal lymph trunks, and on to the cisterna chyli – where it ultimately empties into the thoracic duct.
CLINICAL ANATOMY

1.Evaluation of the colon

The colon can be evaluated by colonscopy (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.

2.Surgical considerations

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.