40 ORGANISM AND ENVIRONMENT 



neys. Human urine is usually acid in reaction, though 

 it is separated from the alkaline liquid, the blood. As 

 shown clearly by L. J. Henderson of Harvard, the 

 urine, like the blood, contains "buffer" substances, so 

 that the slight acidity of the urine is an index of the 

 separation of much acid from the blood. But the 

 reaction of the urine, and therefore the separation of 

 acid by the kidneys, varies from hour to hour, and 

 depends on whether the diet is more or less acid 

 forming or alkali forming. In herbivorous animals, 

 which live on an alkali- forming diet, the reaction of 

 the urine is normally alkaline ; and in man the urine 

 also becomes alkaline when alkalies are administered. 

 It seems evident, therefore, that the kidneys, as well as 

 the liver, are constantly regulating the alkalinity of 

 the blood, and doing so with an accuracy which no 

 means of direct physical or chemical measurement 

 enables us to measure, but which is shown by the great 

 constancy of the alveolar COg percentage. Neverthe- 

 less, we can be quite certain that it is in response to 

 the stimulus of very slightly altered reaction in the 

 blood that the regulating activity of the liver and 

 kidneys comes into play : for by such means as acid 

 poisoning we can make the stimulus so strong that 

 direct measurements can detect it. 



It has been rightly pointed out by L. J. Henderson 

 that the blood, and the body as a whole, are so full of 

 so-called buffer substances that a considerable amount 

 of acid or alkali might be added without any measur- 

 able disturbance of the blood alkalinity being produced. 

 This is certainly true, and very important, but the 



