INTKINSIC ACTIVITY OF SECRETING CELLS 255 



Further proof of the existence of a labile balance of equilibrium 

 is seen in the physiological behaviour of the cell when the appro- 

 priate ions are absent from its circulating fluids. 



As we have seen in the preceding chapter in describing the 

 effects of inorganic salts upon living cells, in order that the physio- 

 logical properties of tissues may be maintained in a normal con- 

 dition, it is necessary that normal amounts of different ions shall be 

 present in the circulating medium. Thus the normal heart-beat 

 cannot be retained unless a certain definite low concentration of 

 potassium ion is maintained in the perfusing fluid. What explana- 

 tion of this can be given on the basis that the active cells are imper- 

 meable to potassium ions ? If the cells of the heart muscle are 

 impermeable to potassium ions, how can the presence, or absence, 

 or variation in concentration, of such ions in the circulating fluid 

 affect the physiological activity of the cells ? Obviously the cells 

 are permeable to the potassium ions and in both directions; for when 

 there is no potassium ion in the circulating fluid, the balance, for 

 potassium ion between circulating fluid and cell contents, becomes 

 upset, and corresponding to the low pressure in potassium ion in the 

 circulating fluid potassium ion must be given out by the cell until 

 a new equilibrium is reached. On the other hand, if potassium ion is 

 present in the circulating fluid at the proper concentration to corre- 

 spond to and balance the concentration in the cell, then exchange 

 will be equal, the concentration of potassium ion in the cell will not 

 change, and the cell will preserve its normal activities. Finally, 

 if the concentration of the potassium ion in the circulating fluid be 

 greater 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 experi- 

 mental facts is not, however, a difficult task. The explanation 

 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 con- 

 centrations for each constituent within and without the cell are never 



