Large intestine

Abstract

Gross anatomy

Overview

Function

  • Colon: passage of food content and absorption of water and nutrients
  • Rectum and anal canal: defecation and fecal continence

Structure and location

Large intestine includes:

  • Cecum:
    • Proximal end of the ascending colon; located in the right iliac fossa
    • Includes the vermiform appendix, a tubular structure connected to the cecum 2 cm below the ileocecal junction
  • Colon:
    • Ascending: from right iliac fossa to underneath the liver; retroperitoneal
    • Transverse: from underneath the liver to under the spleen; intraperitoneal
    • Descending: from underneath the spleen to the left iliac fossa; retroperitoneal
    • Sigmoid: from left iliac fossa to rectum; intraperitoneal
  • Rectum: posterior to the bladder; becomes the anal canal at the coccyx level; partially retroperitoneal
  • Anal canal: divided by the dentate (pectinate) line into upper two-thirds and lower third

Related structures

Vasculature and innervation

Arteries Veins Lymphatics Innervation

Proximal ⅔ of large intestine

Ascending and transverse colon

  • Branches of the superior mesenteric veinportal vein
  • Visceral: superior mesenteric plexus

Distal ⅓ of large intestine

Descending colon, sigmoid

  • Left colic vein and sigmoid veins that drain into the inferior mesenteric veinportal vein
  • Visceral: inferior mesenteric plexus
Rectum and anal canal Above pectinate line
Below pectinate line
  • Middle and inferior rectal veininternal pudendal veininternal iliac vein → common iliac vein → inferior vena cava
  • The arc of Riolan: anastomosis between SMA and IMA that provides collateral flow in the setting of arterial occlusion or significant stenosis

The rectum is a major site of porto-systemic anastomoses. In cases of portal hypertension (e.g., in cirrhosis), venous blood is shunted from the portal system through the portosystemic anastomosis between the superior rectal vein (portal system) and the middle and inferior rectal veins into the systemic venous system. Chronic volume overload may result in formation of rectal varices.

The splenic flexure (Griffith's point) and the rectum rectosigmoid region (Sudeck's point) are watershed areas. Limited collateral vascularity means these areas are at highest risk of ischemia if there is hypoperfusion.

References:[1][2]

Microscopic anatomy

Layers of the cecum, colon, and sigmoid

  • Mucosa (innermost):
  • Submucosa:
    • Contains blood vessels and nerves from the Meissner nerve plexus
  • Muscularis propria:
    • Inner circular and outer longitudinal muscles
    • Myenteric Auerbach nerve plexus
    • In the colon, the outer longitudinal muscles form the tenia coli and the haustra.
  • Serosa (outermost):

Histology of the rectum and anal canal

Whereas the part of the rectum above the pectinate line with columnar epithelium is the site of adenocarcinoma, the lower third of the anal canal is the site of squamous cell carcinoma.

References:[3]

Embryology

References:[4]

Clinical significance

Colon

Rectum and anal canal

Internal hemorrhoids occur above the pectinate line (superior rectal vein) and are not painful (visceral innervation). External hemorrhoids occur below the pectinate line and are painful because the pudendal nerve provides this area with somatic innervation.