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The arterial pH decreases with rising body temperature in ectothermic vertebrates. We report on how this regulation was achieved in relation to bimodal respiration in the toad Bufo paracnemis. Gas exchange was measured for the lung and also for the whole body (skin and lung). In addition, lung gas pressures (PLO2 and PLCO2) and arterial blood gases (pH and PO2) were measured at 17, 27 and 35oC. Arterial pH fell from 7.85 at 17–7.64 at 35oC. Concomitantly, {PLCO2} increased from 5.6 mmHg at 17oC to 10.4 mmHg at 35oC. Regardless of temperature, {PLO2} remained virtually constant at 125–126 mmHg, whereas arterial {PO2} increased significantly with rising temperature (50 mmHg at 17oC; 79 mmHg at 35oC). The pulmonary gas exchange ratio (RE) increased from 0.28 at 17oC to 0.51 at 35oC, while the ratio for whole body gas exchange (lung+skin) was close to 0.8 irrespective of temperature. Since {CO2} conductance of the skin increased little with temperature, the lung eliminated a larger fraction of total {CO2} output. This shift caused the increase of {RE} for pulmonary exchange with rising temperature, which increased {PLCO2} and contributed to a negative ΔpHa/ΔT. Therefore, bimodal respiration in Bufo accounted for part of the temperature-dependent pH regulation, while the final adjustments depend on pulmonary ventilation.
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