Introduction

The acid–base status of an aqueous solution is reflected by the relative activities of hydrogen ({H+}) and hydroxide ({OH−}) ions. These two quantities can be altered by changes in temperature, the concentrations of weak and strong electrolytes, and PCO2 (Stewart, 1978). In biological solutions, {H+} is determined by the strong ion difference (SID) (defined as the sum of all strong base cations minus the sum of all strong acid anions), PCO2, and the total weak acids present. At constant temperature, acid–base status can be changed only by changes in one or more of these independent variables (Stewart, 1978).

The maintenance of a stable acid–base status of the internal body fluids is thought to be of importance to animals primarily because of the need to maintain protein function. Protein structure (and hence function) is dependent on the degree of ionisation of the component amino acids and of the α-imidazole group in particular (Reeves, 1972). The degree of ionisation of amino acids can be altered by changes in the acid–base conditions of the medium. To preserve protein function and maintain the physiological processes to which it contributes, animals must regulate acid–base status and may, for example when faced by changes in temperature, either maintain a constant relative alkalinity (see below), or regulate pH independently of temperature, in order to depress or enhance rates of metabolism (see Whiteley, this volume).

To understand fully how these processes are affected by acid–base conditions and how acid–base status is maintained in living organisms requires a suitable means for representing the acid–base status of a solution.