RESPIRATION 385 



the even regulation of temperature in the body would apparently 

 become impossible, and in other ways the physiological inter- 

 connection between different parts of the body would be less close 

 and rapid. 



Although water and salts are by ordinary measurements neither 

 absorbed by nor given off from most living tissues, it is evident 

 that this only means that passage of them into the tissues is bal- 

 anced by passage outwards. A liquid, like a gas, consists of mole- 

 cules in rapid movement and diffusing in all directions. We can- 

 not follow the movements of individual molecules, and can only 

 detect gain or loss when either the relative proportions of different 

 kinds of molecules alter, or the total number increases or dimin- 

 ishes. When as many molecules or ions of any one substance 

 are passing in as are passing out there appears to be neither ab- 

 sorption nor giving off of the substance. Nevertheless there is 

 continuous molecular or ionic exchange, and the blood is in con- 

 stant and active physiological connection with the surrounding 

 tissues. As is shown by the immediate effects of altering the 

 diffusion pressure of salts, water, or other blood constituents, the 

 exchange of molecules continues during life, whether a tissue is 

 "active" or "resting." In reality there is constant physiological 

 activity, and the conventional sharp distinction between conditions 

 of rest and activity is extremely misleading. 



From the standpoint of physical chemistry life depends upon 

 the maintenance of a balance of molecular exchanges between the 

 tissue elements and their environments. If the balance is disturbed, 

 so that, for instance, too many or too few water molecules or 

 potassium, calcium, or sodium ions are passing from the blood 

 to the tissues or vice versa, life is imperiled. The case is exactly 

 similar with oxygen molecules, or with hydrogen and hydroxyl 

 ions. If the oxygen diffusion pressure in the plasma falls so low 

 that the proportion of oxygen molecules passing in is abnormally 

 low as compared with that passing out there is physiological dis- 

 turbance; and similarly, as shown in Chapter XII, when too much 

 oxygen is passing inwards. 



Hitherto the supply of oxygen has not been regarded from this 

 standpoint. It has been generally assumed that the oxygen mole- 

 cules are all passing in one direction and that an irreversible re- 

 action occurs in the living tissues by which oxygen is fixed so that 

 no free oxygen molecules are returned to the environment. The 

 facts indicating the great importance of a certain definite dif- 

 fusion pressure of oxygen in the immediate environment of the 



