READJUSTMENTS OF REGULATION 41 
disturbances which physiology has to deal with are far 
more minute than those which are appreciable by 
chemical methods, so that exact regulation of the 
reaction of the blood is indispensable. 
We have seen above that the composition of the 
blood is so regulated that not only is its reaction 
practically constant, but the volume of CO, taken up 
by a given volume of blood at a given pressure of CO, 
remains also the same under ordinary normal condi- 
tions. It is easy, however, to disturb this regulation 
temporarily. One means of doing so is by violent 
muscular exertion. Douglas and I found that a few 
minutes after violent exertion the volume of CO, 
taken up by a given volume of human arterial blood 
was reduced to about half. An hour later, however, 
the blood was again normal. The reduction was 
probably due to excessive discharge of lactic acid into 
the blood: for not only was the resting alveolar CO, 
pressure diminished, but Ryffel succeeded in showing 
that after similar violent exertion the proportion of 
lactic acid in the blood and urine is greatly increased. 
Ryffel showed also that this excess disappears in about 
an hour, which is the same time, as we had observed, 
that the alveolar CO, pressure requires to rise again to 
normal after a violent exertion. It is clear, however, 
that the capacity of the blood for taking up CO, can- 
not depend merely on its reaction, and must depend on 
the presence in regulated amount of all the various sub- 
stances including albuminous substances, which enter 
into chemical reaction when CO, is present. Their 
amount must therefore be regulated—probably by the 
