Cellular Aspects of Body Electrolytes and Water 17 



capillary endothelium, and is capable of imposing restrictions 

 on the movements of ions that are very much smaller than the 

 protein ions; as a result, it is conceivable that much larger 

 differences of osmotic pressure could be established, since 

 these smaller ions may be present in vastly higher concen- 

 trations than those of proteins with their large molecular 

 weights. Let us consider the erythrocyte; for simplicity we 

 may choose the cat or dog erythrocyte which shows no 

 accumulation of potassium. The distribution of ions is 

 indicated roughly in Fig. 2; the cell contains the protein 

 haemoglobin which behaves as an anion, so that we may 

 expect to be able to apply the Gibbs-Donnan equilibrium to 

 the diffusible ions. If the Na+, Cl~ and HCOg" ions could 

 diffuse across the membrane, the position would be entirely 



Cell Membrane Plasma 



Na+ Hb- 



Na+ CI- 



Na+ CI- 



Fig. 2. The cat erythrocyte. 

 (Hb=haemoglobin). 



analogous with that already considered, and the contents of 

 the cell would have a higher osmolarity than the surrounding 

 plasma, so that unless the membrane could resist the expan- 

 sion caused by an influx of water, the cell would have to swell, 

 and swell indefinitely since this difference of osmolarity must 

 prevail so long as the cell contains a higher protein concentra- 

 tion than that in the outside medium. Cell membranes are 

 not strong and would certainly not be able to resist the dif- 

 ference of osmotic pressure that would be developed, which 

 in this case would be several times higher than in the case 

 considered earlier, owing to the very high concentration of 

 protein in the red cell. We know that the cat erythrocyte is 

 stable, and we must ask: how? Theoretically, stability could 

 be achieved by making the membrane impermeable to salts, 

 i.e. to all the ions of the system. Alternatively, stability could 

 be achieved by making the cell permeable to anions only and 



