50 CRITICISM OF MECHANISTIC THEORY 



ion concentration, or acidity, brought about 

 by the very slight deficiency or excess of 

 carbon dioxide. He has also investigated 

 quantitatively the effects on the hydrogen 

 ion concentration of the blood of varying the 

 partial pressure of carbon dioxide. From his 

 results and ours it follows that the hydrogen 

 ion concentration of the blood during rest 

 is extraordinarily constant, and remains so 

 day by day and year by year. As the amount 

 of acid and alkali passing into the blood from 

 the food and other sources is constantly 

 varying, it follows that the regulation of 

 hydrogen ion concentration is mainly brought 

 about by the kidneys. It has been known 

 for long that the urine varies in acidity or 

 alkalinity according to the diet ; but Hassel- 

 balch has measured the actual variations in 

 hydrogen ion concentration. Putting to 

 gether his conclusions and ours, it appears 

 that during ordinary resting conditions the 

 variations in hydrogen ion concentration of 

 the urine are about a hundred thousand times 

 as great as those of the arterial blood. 



Thus the kidney epithelial cells react so 

 delicately to variations in hydrogen ion con- 



