Cellular Aspects of Body Electrolytes and Water 31 



extrusion of Na+ would lead to an elimination of these ions 

 and it could well be that a new steady state would be estab- 

 lished at a lower level of internal K+ and Na+ concentrations. 

 Unfortunately, practically nothing is known of the factors 

 that control the normal activity of the salt-excreting system 

 of the cell. 



The large differences in the amount of extracellular water 

 that take place with age may be, to some extent, associated 

 with differences in the amount of water per cell of the organ- 

 ism; thus, other things being equal, a decrease in cellular 

 water is reflected in a rise in the extracellular water, expressed 

 as a percentage. To prove this, however, it would be necessary 

 to measure not so much the percentage water in the cells as 

 the amount of water per cell, and this might be attempted by 

 relating the water to the deoxyribonucleic acid content of the 

 tissue. It seems more likely, however, that long-term fluctu- 

 ations in the fractions of intra- and extracellular water, 

 especially those taking place during development, will be 

 determined by changes in the number of cells in unit weight 

 of tissue rather than in changes of their size, and this could be 

 achieved by (a) multiplication or reduction of the number of 

 cells; (b) expansion or contraction of the extracellular space, 

 by changes in the quantity of connective tissue and in the 

 ability of this to hold fluid. 



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