Absorption and secretion of fluid and electrolytes by the rectum.

Although the amounts of ions and water absorbed and secreted by the rectum are small compared to those of other parts of the colon, rectal epithelium is capable of generating and maintaining considerable ionic gradients. Absorption of Na+ and Cl- and secretion of K+ and HCO-3 leads to relatively low concentrations of the former and high concentrations of the latter within the lumen. Detailed examination of the transport processes indicates that the epithelial mechanisms can be interpreted in terms of ionic movements through transcellular and paracellular pathways. These results show that the epithelium is relatively tight and largely amiloride-sensitive. Active Na+ absorption and apparent active K+ and HCO-3 secretion occur. The secretion of HCO-3 is related to Cl- and organic anion absorption. Mineralocorticoids increase the transepithelial potential difference (p.d.) as well as stimulating Na+ absorption and K+ secretion. Rectal epithelium may, with various physiological or pathological stimuli, become overtly secretory of a predominantly NaCl containing fluid but it remains uncertain whether some secretion is normally present although masked by absorption.