To a first approximation, absorption and secretion in the colon is straighforward:

• Absorption: water, sodium ions and chloride ions
  
  
• Secretion: bicarbonate ions and mucus

Water, as always, is absorbed in response to an osmotic gradient. The mechanism responsible for generating this osmotic pressure is essentially identical to what was seen in the small intestine - sodium ions are transported from the lumen across the epithelium by virtue of the epithelial cells having very active sodium pumps on their basolateral membranes and a means of absorbing sodium through their lumenal membranes. The colonic epithelium is actually more efficient at absorbing water than the small intestine and 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.

Next page > Gross and Microscopic Anatomy > Absorption, Secretion and Formation of Feces > Large Intestinal Motility > Microbial Fermentation