‘Active’ regulation of cutaneous exchange by capillary recruitment in amphibians: Experimental evidence and a revised model for skin respiration

Abstract

Oxygen uptake, carbon dioxide elimination, cutaneous and systematic blood flows) measured by microsphere technique) and the numbe of perfused capillaries in the hind foot web have been measured at 25°C in unanaesthetized bullfrods ( Rana catesbeiana) both while breathing air as they float in water and while resting totally out of water in humidified air. The gas exchange ratio, approximately 1 while breathing with both water and air, fell to 0.5 or lower during 4 h of complete air exposure. A concomitant decrease occurred in both cutaneous blood flow and the proportion of perfused to non-perfussed capillaries in the hind foot web. Upon returning to floating at the water surface, cutaneous blood flow and capillary recruitment increased again and the gas exchange ratio increased to above 2 for several hours. These data suggest that a partial inhibition of CO 2 excretion is linked with a decrease in the extent and pattern of blood flow through the skin, which is the major site of CO 2 elimination. Conventional models for cutaneous CO 2 elimination in amphibians reveal major diffusion limitations but minor, even insignificant, perfusion limitations. Consequently, CO 2 elimination is regarded as highly responsive to changes in blood P CO 2 , but nearly insensitive to changes in blood flow. Importantly, however, such models have treated the skin as a single blood compartment ( i.e., single ‘capillary’), through which blood flow is varied. We propose a multi-capillary model of which incorporates changes in capillary recruitment, and thus changes in the surface area cross which CO 2 elimination from the blood can occur. In such a model, changes in the number of perfused capillaries cause major changes in CO 2 elimination. Experimental data on CO 2 elimination agrees well with predicted changes using this new multi-capillary model.