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The importance of the intestinal countercurrent exchanger for 85Kr absorption from the feline gut.

The rate of 85Kr absorption from the feline gut was studied at varying intestinal blood flows induced by i.a. infusions of a vasodilator drug or by lowering arterial inflow pressure. The effects on rate of absorption of distending the intestine by increasing intraluminal pressure from 0-1 to 5-7 cm H2O as well as of augmenting the rate of luminal perfusion of the 85Kr solution were also investigated. Distending the small bowel increased rate of 85Kr absorption at all levels of intestinal blood flow except at the very low and the very high blood flow rates. Decreasing blood flow by lowering of arterial perfusion pressure decreased the rate of absorption from the distended gut while it had no effect in the collapsed small intestine. Increasing the rate of luminal perfusion enhanced the rate of 85Kr absorption at all blood flow levels except at the lowest ones. The results are discussed with regard to villous hemodynamics, intestinal countercurrent exchange and intraluminal concentration gradients in the lumen. It is concluded that the countercurrent exchanger represents, under physiological conditions, the major limiting factor for the absorption of lipophilic solutes such as 85Kr.