Kinetics of inert gas equilibration in an exclusively skin-breathing salamander, Desmognathus fuscus

Characteristics of cutaneous gas exchange in amphibians were studied by analysis of the equilibration kinetics of an inert test gas in salamanders which have neither lungs nor gills. Specimens of the common dusky salamander (Desmognathus fuscus, Plethodontidae, Urodela), average body mass 6.1 g were equilibrated with 20% chlorodifluoromethane (Freon 22) in oxygen. The time course of subsequent elimination of Freon 22 into atmospheric air was more rapid in living than in dead animals. This difference was attributed to convective transport by blood flow. Several alternative models were proposed, providing a basis for quantitative analysis of the data. All models yielded similar values for convective conductance due to blood flow. In order to calculate blood flow therefrom, a simplified circulation model based on anatomical evidence was used: the cardiac output is in part directed to the skin, subserving gas exchange with the environment, and in part to the internal organs; the blood returning from both skin and internal organs is mixed before reaching the heart. Depending on assumptions regarding the model and the partitioning of blood flow to the skin and to internal organs, the following range of values was calculated from the experimental data: cardiac output, 85-195 mul/(min-g body mass); cutaneous blood flow, 27-63 mul/(min-g body mass). Due to inherent assumptions these values must be considered minimum estimates.