Anatomy of Colon and Rectum

 is the last anatomic segment before the anus.

The ascending and descending colon are supported by peritoneal folds called mesentery.

The right colon consists of the cecumascending colonhepatic flexure and the right half of the transverse colon. The left colon consists of the left half of the transverse colon, splenic flexure, descending colon, and sigmoid. See Figure 1.

Parts of Colon and Rectum

  • Cecum (proximal right colon)
    6 x 9 cm pouch covered with peritoneum
  • Appendix
    A vermiform (wormlike) diverticulum located in the lower cecum
  • Ascending colon
    20-25 cm long, located behind the peritoneum
  • Hepatic flexure
    Lies under right lobe of liver
  • Transverse colon
    Lies anterior in abdomen, attached to gastrocolic ligament
  • Splenic flexure
    Near tail of pancreas and spleen
  • Descending colon
    10-15 cm long, located behind the peritoneum
  • Sigmoid colon
    Loop extending distally from border of left posterior major psoas muscle
  • Rectosigmoid segment
    Between 10 and 15 cm from anal verge
  • Rectum
    12 cm long; upper third covered by peritoneum; no peritoneum on lower third which is also called the rectal ampulla. About 10 cm of the rectum lies below the lower edge of the peritoneum (below the peritoneal reflection), outside the peritoneal cavity
  • Anal canal
    Most distal 4-5 cm to anal verge

Layers of Bowel Wall

  • Lumen (interior surface of colon "tube")
  • Mucosa
  • Surface epithelium
  • Lamina propria or basement membrane—dividing line between in situ and invasive lesions
  • Muscularis mucosae
  • Submucosa—lymphatics; potential for metastases increases
  • Muscularis propria
  • Circular layer
  • Longitudinal layer—in three bands called taenia coli
  • Subserosa—sometimes called pericolic fat or subserosal fat
  • Serosa—present on ascending, transverse, sigmoid only (also called the visceral peritoneum)
  • Retroperitoneal fat (also called pericolic fat)
  • Mesenteric fat (also called pericolic fat)
Illustration of the layers of the bowel wall.

Regional Lymph Nodes

There are between 100 and 150 lymph nodes in the mesentery of the colonRegional lymph nodes are the nodes along the colon, plus the nodes along the major arteries that supply blood to that particular colon segment.

SegmentRegional Lymph Nodes
CecumPericolic, anterior cecal, posterior cecal, ileocolic, right colic
Ascending colonPericolic, ileocolic, right colic, middle colic
Hepatic flexurePericolic, middle colic, right colic
Transverse colonPericolic, middle colic
Splenic flexurePericolic, middle colic, left colic, inferior mesenteric
Descending colonPericolic, left colic, inferior mesenteric, sigmoid
Sigmoid colonPericolic, inferior mesenteric, superior rectal, superior hemorrhoidal, sigmoidal, sigmoid mesenteric
RectosigmoidPerirectal, left colic, sigmoid mesenteric, sigmoidal, inferior mesenteric, superior rectal, superior hemorrhoidal, middle hemorrhoidal
RectumPerirectal, sigmoid mesenteric, inferior mesenteric, lateral sacral, presacral, internal iliac, sacral promontory (Gerota's) superior hemorrhoidal, inferior hemorrhoidal
AnusPerirectal, anorectal, superficial inguinal, internal iliac, hypogastric, femoral, lateral sacral

Lymph nodes along a "named vascular trunk" (as defined by the fourth edition of the AJCC staging manual) are those along a vein or artery that carries blood to a specific part of the colon, for example, the inferior and superior mesenteric arteries, sigmoidal artery, left or right colic artery. In the fifth and sixth editions, the location of the nodes does not affect assignment of the N category.

Illustration of colorectal lymph nodes.

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