Respiratory gas transport by the incompletely separated double circulation in the bullfrog, Rana catesbeiana.

To investigate respiratory gas transport in the bullfrog, Rana catesbeiana (mean body weight 249 g. ambient temperature 25 degrees C), O2 uptake and CO2 output were determined, and blood gas parameters (PO2, PCO2, pH, O2 content, O2 capacity and hematocrit) were measured in blood samples taken from various heart cavities and blood vessels. Analysis of the data on the basis of a simplified circulatory gas transport model allowed to estimate the cardiac output and its distribution, and to describe the O2 and CO2 exchange in lungs, skin and tissues. The total cardiac output (average 20.5 ml/min) was estimated to be distributed about equally to the pulmocutaneous (56%) and systemic arterial vessels (44%), whereas the systemic venous return (62%) was larger than the pulmonary venous return (38%). The marked difference in oxygenation between aortic and pulmocutaneous arterial blood (average O2 saturation 85% and 47%, respectively) showed a highly effective separation of systemic venous and pulmonary venous blood in the ventricle and conus arteriosus. After enlargement of the ventricle produced by incision of the pericardium, the separation of arterialized and venous blood was markedly reduced, but not abolished.