Double Circulation in the Giant Toad, Bufo paracnemis

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Abstract
Gas exchange in the skin and lungs of the giant toad, Bufo paracnemis has been studied in an effort to evaluate the relative importance of cutaneous and pulmonary respiration and the status of a double circulation through the uni-ventricular anuran heart. Anaerobic blood samples were taken from the inflow vessels to the heart: the sinus venosus and the pulmonary vein. Simultaneous samples were taken from the right and left side of the ventricle, and the pulmo-cutaneous and carotid arterial arches, as well as from veins draining the skin. All samples were analyzed for O2 and CO2 content. O2 capacity and hemoglobin content were measured on all animals. Some animals showed a clear respiratory function of the skin with O2 saturation up to 80% in cutaneous effluent blood. Average values for O2 saturation were 65. 0% and CO2 content 26 vol%. In mixed venous blood in the sinus venosus the O2 saturation was 14% and the CO2 content 31. 3 vol. %. The evidence suggests a capacity for a respiratory function of the skin. The contribution of cutaneous respiration to the overall respiration in Bufo paracnemis was however very modest. The pulmonary vein showed 24. 2 vol CO2 and 13. 8 vol% O2. Blood from the left side of the ventricle showed 24.1 vol% CO2 and 13. 3 vol% compared to 27. 5 vol% and 8.9 vol% for the right side. In the carotid arch the values were 22. 4 vol. % CO2 and 14. 0% vol. O2 against 26. 9 vol. % CO2 and 11. 3 vol. % O2 in the pulmo-cutaneous arch. The evidence indicates a high level of functional separation between the O2 rich and O2 depleted blood in their passage through the uni-ventricular toad heart. The O2-rich blood is only slightly mixed with O2 depleted blood while there is a sizeable and progressive shift of blood in the opposite direction.