The main function of the colonic mucosa is to concentrate the faeces, i.e. to absorb electrolytes and water. The absorption of Na+ occurs via the Na-H exchangers and ENaC transporters located at the surface epithelium, the latter regulated by aldesterone. Also water absorption is probably located at this part of the epithelium as it contains aquaporin 2 in its apical membranes. Passive absorption of hydrophilic substances is very low as pores in the surface epithelium have a radius around 3 Å, i.e. passage of urea (mol radius 2.3 Å) over the epithelium is heavily restricted.

The crypt epithelium in the colon contains larger pores (50-60 Å in proximal and significantly less in the distal colon) in spite of that the colonic epithelium is considered to be a semi-tight epithelium. However, these pores are not accessible from the lumen in vivo, a phenomenon probably explained by the mucous layer covering the mucosa and the continuous secretion of water occurring from the crypts. This spontaneous fluid secretion is particularly pronounced in the distal colon depending on activity of the enteric nervous system. In contrast to the small intestine several luminal secretagoques, e.g. cholera toxin, toxin A from Clostridium difficile or bile acids do not induce a fluid secretion from the colonic epithelium, an exception being the STa toxin from E Coli. This indicates that colon does not primarily respond to a toxic agent by eliciting a fluid secretion but instead in such cases mainly rely on increased motility.