Water and Electrolyte Homeostasis

Abstract

One needs to appreciate that the milieu intérieur is a dynamic concept and that the observed steady state is in effect the resultant of the continual influx and efflux of those essential elements of which the body is composed. Measuring, or more often estimating, these essential fluxes is as important a part of the study of the maintenance of the milieu intérieur as is the measurement of the changes in those control mechanisms which ultimately moderate these fluxes. Radioactive and stable isotopes are used routinely to estimate these fluxes and there now exists a large body of literature on the subject, especially for reptiles which are ideal subjects for such a study (Nagy 1988). Theoretical considerations demand that the injected or ingested isotopes do not diffuse from the body and that their specific activity be only reduced by the influx of nonlabelled isotope. This limitation renders the technique less than useful with groups such as amphibians and fish in which there is a constant high rate of interchange of elements between the animal’s body and the external environment. In such cases the high loss rate of the isotope may be used to measure the permeability of external membranes, but overall influx via such routes is much greater than the influx of say, water, by feeding and the technique thus cannot be used to estimate feeding rates as in the case of a terrestrial animal such as a lizard.