REGULATION OF ENVIRONMENT 71 
metabolism act directly on the walls of the small 
arteries, but it is somewhat difficult to imagine how 
this could be brought about. 
Be this as it may, there is no doubt that in some way 
the blood flow through different parts of the body is 
regulated in accordance with the requirements of each 
part, so that during extra activity in any part there is 
a correspondingly greater blood flow. Measurements 
of the circulation through various organs have been 
recently carried out, in particular by Barcroft and his 
associates, in connection with simultaneous measure- 
ments of the oxygen consumption in these organs. 
The general parallelism between increased oxygen 
consumption and increased rate of circulation is evi- 
dent from these measurements. 
To measure the circulation rate of the body as a 
whole by direct means is impossible without opera- 
tive procedures which hopelessly disturb the physio- 
logical conditions. Indirect methods have, however, 
been introduced recently. One of these is to measure 
in the lungs by a rapid method the gas pressures of 
the whole venous blood entering the lungs. From the 
gas pressures the gas contents can be calculated, as 
already seen, and a comparison of the venous with the 
arterial gas contents gives a direct measure of the 
ratio between oxygen consumption or CO, produc- 
tion and blood flow. If the amount of oxygen being 
taken up and COs given off at the time is known, 
the blood flow itself can also be calculated. Using this 
method in man both Dr. Boothby of Boston and I 
have found that the blood flow increases proportion- 
