Countercurrent exchange of water in canine jejunum

The possibility of countercurrent exchange of water molecules in canine intestinal villi has been examined. Tritium-labeled water (3H2O) molecules were introduced either into the fluid lavaging the intestinal lumen or into the arterial blood supply for varying periods of time. Quickly frozen samples of intestinal tissue were sectioned such that isotopic concentrations at the villus tip, midvillus, villus base, and underlying submucosa and muscle could be determined. The villus concentration gradients observed were consistent with the existence of a countercurrent exchange but could also be explained by alternative arrangements. More convincing evidence of a countercurrent was obtained from experiments in which [14C]inulin was introduced simultaneously with 3H2O into the intestinal artery. The villus tip-to-base concentration ratio for 3H2O was less than one while the ratio for inulin was greater than one, thus vitiating the alternative explanations and leading to the conclusion that the labeled water molecules must have undergone a countercurrent exchange.