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 2 produc- 

 tion and blood flow. If the amount of oxygen being 

 taken up and CO2 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- 



