Quantitation of counter-current exchange during passive absorption from the dog small intestine: evidence for marked species differences in the efficiency exchange

signed to quantitatively assess the possible influence of countercurrent exchange on passive absorption from the small intestine of the dog. Villus blood flow was measured with a modification of the microsphere method. Simultaneously, the absorption from the gut lumen of five diffusible gases (H2, He, CH4, 133Xe, and CO) was determined. Villus blood flow averaged 0.247+0.03 (SEM) ml/min per g. The ob-served absorption of H2, He, CH4, and 133Xe was only 16.2±+1.8, 12.8+2.3, 12.0±+1.8, and 15.8±1.4%, respec-tively, of what this villus blood flow could carry away if it reached perfect equilibrium with the luminal gases. This low absorption rate could result from diffusion limitation to absorption or counter-current exchange. The diffusive permeability of th