Early studies on reptiles demonstrated that plasma pH increases as body temperature falls (Robin, 1962). Rahn (1967) proposed that plasma pH in all poikilothermic vertebrates is regulated as body temperature changes so as to maintain a constant relative alkalinity, i.e. a constant [OH-]/[H+] ratio, and Reeves (1972) suggested a way in which this could be achieved. Known as the ‘imidazole alphastat hypothesis’, it postulates that PCO2 is regulated (by way of ventilation) so that the fractional dissociation (alpha) of the imidazole moiety of histidine is kept constant. As the pK' of imidazole changes with temperature in about the same manner as the neutral pH of water (Heisler, 1986), the alphastat hypothesis is consistent with that of constant relative alkalinity.

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