Anatomy of Colon and Rectum

The entire colon is about 5 feet (150 cm) long, and is divided into five major segments. The 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 cecum, ascending colon, hepatic 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

See Figure 2