Colon

Major Functions of the colon:

  • mixing the contents to promote absorption of water and electrolytes

  • maintaining an appropriate intraluminal bacterial mass

  • transporting contents in a net distal direction

  • storing fecal material until defecation

  • rapid emptying of colonic contents during defecation
    • ceacum, ascending colon and rectum act as reservoirs for the
      storage of feces
    • the rest (transverse, descending and sigmoid colon) acts to propel
      the feces from the first to the second reservoir
  • Absorption and secretion in the colon is straighforward:

    • Absorption: water, sodium ions and chloride ions

    • Secretion: bicarbonate ions and mucus

    Water absorbed in response to an osmotic gradient. Mechanism identical to what was seen in the small intestine - sodium ions are transported from the lumen across the epithelium due to active sodium pumps on their basolateral membranes. Sodium absorption in the colon is enhanced by the hormone aldosterone.

    Chloride is absorbed by exchange with bicarbonate. The resulting secretion of bicarbonate ions into the lumen aids in neutralization of the acids generated by microbial fermentation in the large gut.

    Goblet cells are abundant in the colonic epithelium, and secrete mucus in response to tactile stimuli from lumenal contents, as well as parasympathetic stimuli from pelvic nerves. Mucus is an important lubricant that protects the epithelium, and also serves to bind the dehydrated ingesta to form feces.

    Normal feces are roughly 75% water and 25% solids. The bulk of fecal solids are bacteria and undigested organic matter and fiber. The characteristic brown color of feces are due to stercobilin and urobinin, both of which are produced by bacterial degradation of bilirubin. Fecal odor results from gases produced by bacterial metabolism, including skatole, mercaptans, and hydrogen sulfide.