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 CO, 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 
