THE NEW PHYSIOLOGY. 65 



of a liquid depends on its hydrogen ion concentration. 

 The accurate measurement of the hydrogen ion con- 

 centration of blood by the electrometric method is 

 attended with great difficulties ; but these have been to 

 a large extent overcome by Hasselbalch of Copenhagen, 

 who has obtained measurements of the effects of satura- 

 tion with different partial pressures of C0 2 on the hydro- 

 gen ion concentration of blood. He has also shown ex- 

 perimentally that when the alveolar C0 2 threshold is 

 lowered or raised by an acid or alkaline diet, this raising 

 or lowering is just sufficient to keep the hydrogen ion 

 concentration of the arterial blood sensibly steady. It 

 is now certain, therefore, that what the respiratory centre 

 is reacting to when it reacts to a slight increase in the 

 alveolar C0 2 percentage is the consequent slight increase 

 in the hydrogen ion concentration of the blood. 



The latter increase is so minute that it can only be 

 detected electrometrically when it is of sufficient extent 

 to produce very gross changes in the breathing. The 

 respiratory centre is enormously more delicate as an 

 index of change in hydrogen ion concentration of blood 

 than any known physical or chemical reaction. 



As already remarked, the alveolar C0 2 percentage is 

 extremely steady under ordinary conditions. This 

 implies that the hydrogen ion concentration of the blood 

 is regulated with almost incredible delicacy, and must 

 be so regulated apart altogether from the breathing. 

 The breathing simply regulates the rapid disturbances 

 in hydrogen ion concentration caused by variations in 

 the production of C0 2 : other disturbances are regulated 

 otherwise than by the breathing. There is clear evidence 

 that both the kidneys and the liver play a part in this 



5 



