CELLULAR ASPECTS OF THE ELECTROLYTES 

 AND WATER IN BODY FLUIDS 



Hugh Davson 



Medical Research Council, Department of Physiology, 

 University College, London 



The water and electrolyte contents of a complex organism 

 are almost entirely determined by the activities of the kidneys, 

 which operate primarily on the blood plasma and, through 

 that, on the extracellular fluid of the organism. Casual 

 fluctuations in the water and electrolyte contents of the 

 organism are therefore usually the consequence of fluctuations 

 in the composition of these two compartments of the body 

 plasma and extracellular fluid. The electrolytes and water of 

 the cells of the body are affected secondarily to these primary 

 fluctuations in the composition of the extracellular fluid and 

 plasma, and, for practical purposes at any rate, the factors 

 that can influence them primarily are usually ignored. Never- 

 theless, since the cells occupy a considerable fraction of the 

 total volume of the organism, and since there must be some 

 reciprocity between the electrolyte and water content of cells 

 and extracellular fluid, it is of some importance that we 

 understand the physical and chemical factors that determine 

 the electrolyte concentrations and volumes of the cells of the 

 body. 



The Gibbs-Donnan Equilibrium. The application of the 

 Gibbs-Donnan equilibrium to the problem of the water and 

 electrolyte distribution between the plasma and extracellular 

 fluid is familiar to all who have concerned themselves with 

 the water balance of the organism. It will be recalled that the 

 most important consequence of the Gibbs-Donnan distribu- 

 tion of ions between the two fluids separated by the capillary 

 membrane that is supposed to be impermeable to the protein 

 molecules of plasma, is that the osmolarity of the plasma is 



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