20 Hugh Davson 



as before, but once again studies with isotopes have shown 

 that both Na+ and K+ can pass across the membrane and an 

 active transport of Na+ out of the cell and of K+ into the cell 

 must be postulated to account for the osmotic stability of the 

 system. 



It was considered at one time that a mere extrusion of Na + would 

 account for the osmotic stabiHty and high concentration of K + in the 

 cell, i.e. that the extrusion of Na+ would demand a replacement by K+. 

 It was pointed out, however (Davson, 1951), that this would lead simply 

 to an excretion of NaCl and NaHCOg from the cell, with a resultant 

 shrinkage. Extrusion of Na + will only lead to accumulation of K + if 

 exchange of K+ for Na+ is obligatory on the system in order to pre- 

 serve electrical neutrality. If anions can accompany the excreted Na + 

 then exchange for K + is not obligatory. In nerve and muscle, where the 

 concentration of non-permeable anions in the cell is very high, such a 

 sodium-excreting mechanism would cause accumulation of K +. 



Cell Membrane Plasma 



K+ Hb- Na+ Cl- 



K+Cl- 



FiG. 3. The human erythrocyte. 

 ( Hb = haemoglobin). 



Once again, the water content of such a system will be 

 determined by the osmolarity of the plasma and the activity 

 of the metabolic ionic pumps; thus, over-activity of the Na+- 

 excreting mechanism would lead to a shrinkage; over- 

 activity of the K+-accumulating mechanism would lead to a 

 swelling. It is interesting that the two processes show some 

 degree of linkage, in that Harris (1954) has shown that 

 accumulation of the one ion is associated with a nearly 

 equivalent excretion of the other; the linkage is not complete, 

 however, since on cooling erythz'ocytes swell as a result of 

 gaining more Na+ than they lose K+; when they are re- warmed 

 the extra Na+ is excreted and they return to their original 

 volume. The fact that the cell maintains its characteristic 

 water content and proportions of Na+ to K+ within fairly 

 narrow limits indicates that there is some homeostatic 

 mechanism controlling the rates of accumulation of K+ and 



