68 ORGANISM AND ENVIRONMENT 
nite results on this point. In any case we should 
hardly expect to meet with active secretion of CO,, 
considering that the breathing is regulated by the CO, 
pressure in the arterial blood, and that the oxygen 
supply to the lungs is dependent on this regulation. 
During very excessive muscular work it seems to be 
the oxygen supply to the body that first begins to fail. 
This is indicated by the fact that in very hard work the 
alveolar CO, percentage begins to fall, and may even 
become lower than during rest. 
The delicate and exactly regulated organization by 
which CO, is removed from the blood in the lungs, 
and oxygen supplied, would quite clearly be of little 
service to the body if there were not also a regulation 
of the circulation of blood so as to keep the removal of 
CO, from the body tissues and their supply of oxygen 
correspondingly steady. We must now, therefore, 
consider what is known as to the circulatory regula- 
tion. Knowledge on this subject is unfortunately still 
very fragmentary, mainly because physiologists have 
failed to appreciate the delicacy of organic regulation, 
or have even lost sight of it altogether when investi- 
gating various matters of detail. 
The blood brings to the tissues the various sub- 
stances required for their normal life, and removes 
from them substances which are then carried to other 
tissues or to secretory organs. It is also a carrier of 
heat. The carriage of oxygen and CO, is thus only 
one of its many functions. Hence we must not expect 
that the circulation will be solely regulated with refer- 
ence to the carriage of these gases. Bernard noticed 
