256 INTRINSIC ACTIVITY OF SECRETING CELLS 



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 dissolved substance in the 

 plasma, 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 concentra- 

 tion of the particular constituent in the plasma ; just as the existence 

 of the compound oxyhsemoglobin in the red blood- corpuscles 

 depends upon the partial osmotic pressure of oxygen in the plasma. 



In the case of oxyhaemoglobin but little oxygen is given off until 

 the pressure of oxygen in the plasma has fallen to the level of com- 

 mencing dissociation of oxy haemoglobin similarly; in the case 

 of the tissue cells in general but little potassium ion, for example, is 

 given of! 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 diminution 

 of potassium ionic pressure in the plasma begins to become evident. 



On the other hand, with increasing osmotic pressure of oxygen 

 in the plasma above the point at which oxyhaemoglobin has been 

 completely formed, there is but little further uptake of oxygen 

 by the red blood-corpuscle; and similarly in the case of the potas- 

 sium ion, or any other active ion, in the plasma above the concen- 

 tration at which the protoplasm of the tissue cell has been saturated, 

 the uptake of potassium ion by the cell will be small and inappreciable 

 to chemical investigation, so that even in an isotonic or somewhat 

 hypertonic solution of potassium salt alone the amount of potassium 

 ion taken up by the cell will not be appreciably greater to ordinary 

 chemical analysis than that taken up from normal plasma where 

 the osmotic pressure of potassium ion is many times lower, but still 

 sufficiently high to cause almost complete association between the 

 protoplasm and the potassium ion. Although the difference in 

 uptake of either oxygen or potassium ion is so small as to escape 

 chemical determination, it may, however, produce in both cases 

 profound physiological effects, probably from the rapid increase in 

 osmotic pressure of the constituent concerned in the cell after the 



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

 limit. 



