Acid-base regulation and temperature in selected invertebrates as a function of temperature

The pH of the hemolymph of selected invertebrates decreases as their body temperature increases. The magnitude of this change (ΔpH/Δ°C) is very similar to the change of the pH of water with temperature (ΔpN/Δ°C) and suggests that these invertebrates, like poikilothermous vertebrates, regulate the pH of their extracellular fluid so that its degree of alkalinity relative to the pH of water remains constant. The degree of alkalinity (pH_blood-pN) varies between species, but seems to be fixed for any given species. In Limulus pH-pN was essentially the same for in vivo samples, measured after the whole animal had been acclimated to different temperatures, as it was for in vitro samples in which the hemolymph was cooled or warmed anaerobically, suggesting that the CO₂ content of the extracellular fluid is constant as the temperature changes. The P_CO2 of the hemolymph is invariably lower in animals breathing water than in those breathing air. In the invertebrates, as in the vertebrates, manipulation of P_CO2 and HCO₃- is probably the major mechanism in the regulation of the relative alkalinity of the extracellular fluid.