The Energetics of Osmotic Regulation in Brackish- and Fresh-water Animals

INTRODUCTION The classical work of Griffiths (1891) and Fredericq (1901) more than half a century ago showed that fresh-water animals are hypertonic to the media in which they live. If the body wall of a fresh-water animal is permeable to water the animal will suffer from an osmotic inflow of water. Overton (1904) first suggested that the excretory organ of a fresh-water animal helped to balance this inflow by producing hypotonic urine. Since this time numerous papers have been published on osmotic regulation confirming and elaborating these ideas. In particular, the work of Krogh (1937, 1938) on the active uptake of ions from dilute solutions has helped to complete a dynamic picture of water and salt balance in freshand brackish-water animals. This dynamic balance can only be maintained by the continuous expenditure of energy. A number of workers have attempted to demonstrate significant variations in the metabolism of brackishand fresh-water animals in response to alterations of the concentrations of the external media (Schwabe, 1933; Peters, 1935). Although large variations were found Peters demonstrated that much of the variation was due to changes in the tissue metabolism as the blood concentration varied. The total mass of the salt-absorbing tissues is generally so small that changes in their metabolism are largely masked by the metabolism of the rest of the tissues. In spite of this the calculation of the minimum thermodynamic work performed in osmotic regulation is of considerable interest, as it makes it possible to examine quantitatively the effect of different variables on the minimum energy requirements, and provides a convenient basis for the correlation of a large amount of data. Qualitative considerations suggest that the energy needed for osmotic regulation will be a function of the animal's permeability, surface area, blood and urine concentrations, and the concentration of the medium in which it is living. If a few simple assumptions are made it is possible to express the total osmotic work in terms of these variables.