Microvascular Regulation of Cutaneous Gas Exchange in Amphibians

Abstract

SYNOPSIS. Gas exchange across amphibian skin is regulated by the cutaneous microcirculation. Parameters involved in regulating gas exchange are capillary density, radius and blood flow. Changes in capillary density and radius should affect gas exchange by altering the cutaneous diffusing capacity (D2) while changes in capillary blood flow affect the perfusive conductance of the skin. A simple model predicts that the effect of capillary density changes on D2 will become more pronounced as capillary density and epidermal thickness decrease. Changes in capillary radius should have only a minor effect on D2 Previous analyses have suggested that cutaneous gas exchange is not significantly affected by the perfusive conductance of the skin. Consequently, it has been thought that changes in total capillary blood flow have little impact on cutaneous gas exchange. Earlier analyses, however, may have underestimated the importance of perfusive conductance in amphibian skin, primarily because functional heterogeneities in the microcirculation were not considered. The density of perfused capillaries is regulated in the foot web of Rana esculenta by environmental Po2 and PCO2, and also by lung ventilation. In Rana catesbeiana, capillary density in the web decreases during air exposure. Chronic exposure to environmental hypoxia increases total capillary density in bullfrog tadpole skin. In Rana pipiens, regulation of cutaneous gas exchange by environmental and pulmonary O2 probably involves changes in total capillary blood flow.