Regulation of acid-base status in ectothermic vertebrates: the consequences for oxygen pressures in lung gas and arterial blood.

Extensive literature reports a negative delta pHa/delta t in ectothermic vertebrates, but data are scarce as to its consequences for O2 transport. In reptiles, the negative delta pHa/delta t results from an elevated lung gas PCO2 (PACO2) at higher temperatures, implying a corresponding fall of PAO2. In parallel, arterial PO2 rises with temperature, due to a combination of central vascular shunt and decreasing Hb.O2 affinity. As a result, the PO2 gradient between lung gas and blood (PA-aO2) becomes reduced at higher temperatures. In amphibians, the negative delta pHa/delta t results from combined cutaneous and pulmonary CO2 elimination. We propose that this leads to a rather temperature-independent lung gas PO2. Moreover, our calculations suggest that resting reptiles and amphibians maintain a relatively large PA-aO2 also at high temperatures. The negative delta pHa/delta t in teleost fish is generally considered to be a result of modulated plasma [HCO3-]. Recent data from our laboratory suggest that acute pH adjustments at high temperatures may involve alterations of PaCO2 through gill ventilation, leading to a decrease of PaO2 with rising temperature.