Large Intestine

Large Intestine

Key Features

• Large intestine begins at the ileocecal valve and ends at the anus.
• It frames the small intestine, with which it is continuous.
• Compared to the small intestine, it is shorter in length, but larger in diameter.
• Teniae coli
  • Three longitudinal ribbon-like bands of muscle fibers that travel the length of the large intestine. The teniae coli represent the muscularis tunic of the large intestine.
  • They act like an elastic band that pulls on the large intestine and causes it to bunch and form haustra.
• Haustra
  • Pouch-like structures.
• Epiploic appendages (aka, omental appendages)
  • Small fat-filled sacs, attach to the tenaie coli.

Key Functions:

• Receives undigested materials from the small intestine.
• Absorbs water and ions from the undigested materials, which converts the remaining materials to feces (the small intestine is the primary place of nutrient absorption).
• Stores and expels feces.

Subdivisions:

• Cecum (appendix attaches, here)
• Ascending colon
• Transverse colon
• Descending colon
• Sigmoid colon
• Rectum
• Anal canal, which opens to external environment via the anus.
  • External and internal anal sphincters regulate passage of feces.
  • External anal sphincter comprises voluntary skeletal muscle
  • Internal anal sphincter comprises involuntary smooth muscle

Key Landmarks:

• Right colic flexure (aka, hepatic flexure)
  Indicates where the ascending becomes the transverse colon inferior to the liver.
• Left colic flexure (aka, splenic flexure)
  Indicates where the transverse colon becomes the descending colon inferior to the spleen.
• Distal sigmoid colon and rectum lie within the pelvis.
• Anal canal lies within the perineum, external to the abdominopelvic cavity.

Clinical correlations:

• In diverticulosis, multiple outpockets form within the mucosa of the large intestine, which can cause inflammation with or without infection (diverticulitis).
• Inflammatory bowel disease (IBD) refers to chronic inflammation of the GI tract
  • Examples include:
    Ulcerative colitis, which causes continuous ulcers, specifically within the lining of the large intestine.
    Crohn's disease, in which infection spreads deep into the walls of the GI tract; it more typically affects the small intestine than the colon.

CT Scans

For more, see our Abdominal CT Scan Atlas on the Course Page