The dehydrating function of the descending colon in relationship to crypt function

Very high pressure is required to generate hard faeces approximately 5-10 atmospheres. This is much more than can be supplied by the mechanical force from the muscular wall of the colon. Osmotic pressure (at least 200 mOsm) can generate the necessary suction forces required to consolidate faeces. The colon has a hypertonic absorbate (net above plasma approximately 500 mOsm) in vivo. Fluorescence imaging of perifused rat descending colonic mucosa shows high steady state Na+ concentrations (600 mM) in the intercryptal extracellular space and low [Na+] present in the crypt lumen. This [Na+] distribution generates an osmotic pressure gradient across the crypt luminal wall resulting in a fluid inflow into the crypt lumen. Direct observation using confocal fluorescence microscopy of FITC dextran (mol. wt. 10,000) shows that there is concentration polarisation of the dextran in the upper 30% of the crypt lumen. The time course and steady state distribution of concentration polarisation of fluorescent dyes within the crypt lumen permit an estimation of the fluid convection rate along the length of the crypt lumen. This is sufficient to account for the majority of fluid absorption by the colon. Observation of the suction force on agarose gels by rat descending colon in vivo shows that the colon generates up to 4,000 cm H2O suction pressure on the stiff gels, this is accompanied by a hypertonic absorbate from the gels of 800 mOsm. Disruption of the colonic mucosa by bile salts reduces the suction pressure to about 40 cm H2O.