Absorption and secretion in the colon is straighforward:
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.