256 RESPIRATION 



employed by Barcroft, all the other observations referred to in 

 the present chapter tend to show that except, perhaps, when physi- 

 cal training or acclimatization is very effective, the arterial oxygen 

 saturation during such work is lower than during rest. 



Clear evidence is brought forward by Barcroft and his associ- 

 ates that no appreciable loss of dissociable oxygen occurs in ar- 

 terial blood which is allowed to stand for a short time. In the 

 Pike's Peak report we concluded that such a loss probably occurs. 

 The chief reason for this conclusion was that the aerotonometer 

 always gives a lower oxygen pressure than that deduced on the 

 diffusion theory from the alveolar oxygen pressure, or indicated 

 by the carbon monoxide method during rest under ordinary baro- 

 metric pressure. As explained above, however, there is now an- 

 other and very clear explanation for this ; and since the investiga- 

 tion by Meakins, Priestley, and myself on the effects of shallow 

 breathing I have altogether ceased to believe in the presence, to 

 any extent which would upset a blood-gas or aerotonometer de- 

 termination, of "reducing substances" in blood. I am in entire 

 agreement with Barcroft's criticism of the old experiments by 

 which Pfliiger believed that he had demonstrated the existence 

 of reducing substances in fresh arterial blood. It may also be men- 

 tioned here that in some unpublished experiments Douglas and I 

 were unable to obtain any evidence by blood-gas analysis of the 

 presence of reducing substances, even in blood which was com- 

 pletely reduced by prolonged stoppage of the circulation in the 

 arm. 



