22 Hugh Davson 



environment, provided that an ion-excreting mechanism is 

 active. Loss of this, by cooUng the tissue or by metabohc 

 poisons, causes a loss of K+ and a gain of Na+, CI" and HCO3-, 

 the net effect being an increase in osmolarity with a consequent 

 swelling of the cells. Re-warming of the tissue may cause a 

 reversal of these changes (see, for example, Steinbach, 1954). 



Thus, in all of the cell types that we have considered, the 

 system can be treated, theoretically at least, as a system that 

 maintains an osmotic equilibrium between the interior and 

 external fluids by virtue of an "effective impermeability" to 

 one or more ionic types ; if the membrane were truly imperme- 

 able to the ions in question, the osmotic equilibrium would 

 be independent of metabolic processes and could be described 

 as a true equilibrium ; in practice, the effective impermeability 

 is the result of a continuous process of active transport. 

 For the purposes of mathematical description this is equivalent 

 to an impermeability, at any rate under normal conditions; 

 under abnormal conditions, on the other hand, the precarious- 

 ness or instability of the equilibrium is shown by the cellular 

 oedema that follows either the failure of the ion-excreting 

 mechanism or such a large increase in the permeability of the 

 membrane that the mechanism can no longer keep pace with 

 the influx of Na+. 



If these considerations are correct, we may expect to find 

 that by adding up the total osmolarities inside and outside 

 the cell the two totals should be equal within the limits of 

 experimental error. Probably the muscle fibre has been 

 studied most carefully from this aspect, and it would seem 

 from Conway's (1957) figures (Table I), that osmotic equili- 

 brium does exist between the cell and its environment. The 

 same is probably true of the erythrocyte and the nerve fibre, 

 but it must be remembered that the analytical techniques for 

 all the constituents of the cell are not so accurate that a dif- 

 ference of one or two per cent would be ascertained. Within 

 this limit, then, it seems quite safe to affirm that these cells 

 are in osmotic equilibrium with their environment. 



Within recent years the possibility that mammalian cells 



