The distribution of pulmocutaneous heart output to lungs and skin was determined in non-anaesthetized, fully recovered bullfrogs (Rana catesbeiana) by application of the microsphere method in order to study the modulation of blood flow to different gas exchange sites in amphibians during environmental air and water hypoxia. The relative perfusion of various skin areas was found to be rather heterogeneously distributed with an over-proportionately high blood flow to the ventral body surface. This distribution of flow among different skin areas remained unaffected by any type of environmental hypoxia. The relative perfusion of lungs and skin, however, was significantly affected by the pattern of environmental oxygen partial pressure. The relative lung perfusion (approximately equal to 80% of pulmocutaneous flow in normoxic control conditions) was increased during water hypoxia, and reduced with lowered inspired PO2. This mechanism could be interpreted as a readjustment of blood flow towards the gas exchange site with higher oxygen partial pressure, but may also represent a mechanism to prevent oxygen loss from the body stores at gas exchange sites of low oxygen tension.

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