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Large Intestine
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
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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.
  • Megacolon - dilation of the colon
    • There are different types/causes of megacolon. Two examples:
    • Toxic megacolon is the result of inflammation, as in inflammatory bowel disease (ulcerative colitis, Crohn's disease, Salmonella, Cytomegalovirus in HIV patients...).
    • Hirsprung's disease (congenital aganglionic megacolon) is a congenital disease caused by the absence of enteric ganglion cells fail to develop; as a result, stool cannot progress through the colon. As the intestines become blocked, infants/children experience constipation, abdominal pain, and vomiting.

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