Colon Anatomy

Colon Anatomy and Blood Supply

• Length: Colon varies in length, approximately 150-165 cm.
• Structure: The colonic wall contains 3 longitudinal bands (tenia coli), creating haustra (sacculations).
• Regions: The cecum, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon, and rectosigmoid colon are anatomically distinct regions.

Cecum

• Dimensions: ~5-10 cm in length and ~7.5 cm in diameter.
• Wall: Relatively thinner compared to other colon regions.
• Mesentery: No mesentery.
• Jackson's Membrane: A prececal fibrous band containing small blood vessels, clinically insignificant.

Ascending Colon

• Length: ~15-20 cm.
• Location: Extends from cecum to liver.
• Mesentery: No mesentery.
• Position: Overlaps the right kidney and ends under the right liver.

Transverse Colon

• Length: ~45-50 cm (longest colon portion).
• Location: From liver to spleen.
• Redundancy: Most redundant portion of the colon.
• Embryological Origins: Proximal 2/3rd midgut; distal 1/3rd hindgut.
• Mesentery: Has a mesentery (transverse mesocolon).
• Mesocolon Content: The transverse mesocolon contains the middle colic artery and is enveloped by visceral peritoneum of both sides, extending over the pancreas.

Descending Colon

• Length: ~12-30 cm.
• Location: From splenic flexure to left psoas muscle, transitioning to the sigmoid colon.
• Mesentery: No mesentery.

Sigmoid Colon

• Length: ~40 cm (variable).
• Location: Starts at iliac crest, lateral psoas muscle.
• Wall: Relatively more thick-walled.
• Mesentery: Has a mesentery (sigmoid mesocolon).
• Mesocolon Shape: Inverted V shape; inter-sigmoid recess between limbs; left ureter exiting at apex.

Rectosigmoid Colon

• Definition: Segment (~7-8 cm) of colon where taenia coli merge into a single layer, marking the rectum's beginning.
• Mesentery: Shorter, more tethered mesentery compared to the colon.
• Rectum Beginning: Begins variably between the promontory and S3.

Arterial Supply

• Sources: Blood from both superior mesenteric artery (SMA) and inferior mesenteric artery (IMA).
• SMA Branches:
  • Middle colic artery: Constant; arises from SMA inferior border of pancreas; travels through the transverse mesocolon.
  • Ileocolic artery: Constant; terminal SMA branch; runs posterior or anterior to the superior mesenteric vein (SMV); sometimes branches to right colic artery.
  • Right colic artery: Variable (~25% of cases); can branch from the ileocolic artery or occasionally the SMA.
• IMA Branches:
  • Left colic artery: Originates distal to IMA; supplies left colon; collateralizes with middle colic via marginal artery of Drummond.
  • Sigmoidal branches: 2-5 branches to the sigmoid colon, forming a marginal artery near the bowel wall; superior (constant) and inferior (variable) divisions.
• SMA-IMA Collateralization:
  • Marginal artery of Drummond: Constant; within 3cm of bowel wall; important for mesenteric windows.
  • Arc of Riolan: Rare (~5.5-11.4% of cases); collateral vessel; indicative of possible mesenteric stenosis.

Venous Drainage

• Ileocolic vein: Drains into SMV (98%) or Gastrocolic Trunk of Henle (GTH) (~2%); important for ligation during colon cancer surgery.
• Right colic vein: Variable (~50-90%); drains into SMV or GTH.
• Gastrocolic Trunk: Formed by right gastroepiploic, superior right colic, and superior pancreaticoduodenal veins; drains into SMV; present in ~80% of patients.

Nerve Supply

• Superior hypogastric plexus: (L4/L5); important for genitourinary function; high IMA ligation can protect this.

Other Structures

• White line of Toldt: Peritoneal reflection of colon.
• Toldt's fascia: Interface of visceral and parietal layers of the colon, not as thick as a fascia.
• Vascularity: Variable; depends on BMI.

Clinical Significance

• Dissecting along anterior mesocolon maintains avascular plane.
• Avoid caudal or anterior traction on transverse mesocolon to avoid accidental avulsion of Gastrocolic Trunk.