Introduction

In recent years, studies of the effects of environmental variables on the physiology and biochemistry of different animal species have increasingly included an analysis of acid–base status and regulation. pH values in different body compartments are widely accepted to play a key role in the maintenance of physiological function or its limitation under functional or environmental stress. pH affects protein function in metabolism and O2 transport. Also, acid–base and metabolic regulation are interdependent processes such that changes in pH may affect metabolic rate, the mode of catabolism and energetic parameters. Ideally, these analyses should not only describe correlated changes in the different processes under investigation, but should also provide a quantitative picture of the changes involved and the processes responsible for them (Heisler, 1989b).

However, acid–base regulation not only means adjustment or defence of pH, which is traditionally seen as being the key acid–base parameter determining regulatory processes, but it may, under certain conditions and with the help of the respective membrane carriers (see below), also give priority to the regulation of the levels of base (carbonate, bicarbonate) or acid (carbonic acid, proportional to PCO2) in the respective body fluids. In that sense, pH would become a dependent variable. Also, for some treatments it is not pH which is of interest but rather the activity of protons (pH = –log aH+), when protons contribute to some biochemical reactions in a concentration-dependent manner (see equation 1, p. 73, as an example).