150 INTRINSIC ACTIVITY OF SECRETING CELLS 



than that required to balance the concentration within the cell, 

 then more potassium ion must enter the cell than leaves it, and 

 the effect becomes evident in a change in the action of the cell. 



But how, it may be asked, is such a statement to be correlated 

 with that upon which the supposed impermeability of the cell 

 for potassium ions is based, with the fact, namely, that the cell 

 does not appear, as far as chemical investigations go, to take up, 

 for example, potassium ion from a solution of a potassium salt 

 in which it is immersed ? The correlation of the two sets of 

 experimental facts is not, however, a difficult task. The explana- 

 tion lies in the fact that the cell possesses different affinities for 

 the different ions and other dissolved constituents of its circulating 

 fluids, so that at the equilibrium point for normal conditions, the 

 concentrations for each constituent within and without the eel] 

 are never equal but bear a definite ratio to each other, and further 

 that these constituents enter into unstable physical or chemical 

 relations with the protoplasm, so that there is a more or less 

 definite minimal concentration for each constituent ion or dis- 

 solved substance in the plasma, which might be termed the 

 " dissociation pressure or concentration " for that particular ion 

 or substance at which the protoplasm becomes combined with 

 it. There is an unstable chemical or physical combination formed 

 between the protoplasm and each of the active constituents of 

 the plasma, the existence of which depends upon the osmotic 

 pressure or concentration of the particular constituent in the 

 plasma ; just as the existence of the compound oxy-haemoglobin 

 in the red blood-corpuscles depends upon the partial osmotic 

 pressure of oxygen in the plasma. 



Just as in the case of oxy-haemoglobin but little oxygen is 

 given off until the pressure of oxygen in the plasma has fallen 

 to the level of commencing dissociation of oxy-ha3moglobin, so 

 in the case of the tissue cells in general but little potassium ion is 

 given off until the osmotic pressure of that ion has fallen in the 

 plasma below a certain limit, when the range of dissociation of 

 potassium ion commences. 1 Accordingly it is only at this limit 

 that the change in physiological action of the cell due to diminu- 

 tion of potassium ionic pressure in the plasma begins to become 

 evident. 



1 The concentration of potassium ion in Ringer's solution lies above this 

 limit. 



