244 RESPIRATION 



On the other hand the carbon monoxide method gives, during 

 rest under normal conditions, exactly the same oxygen pressure 

 in the arterial blood as in the alveolar air. This difference in the 

 results by the two methods used to be rather a puzzle, and was 

 explained by me as probably due, either to a process of rapid but 

 slight oxidation in the blood itself, or to a little blood getting 

 through the lungs without exposure to alveolar air. Our shallow 

 breathing experiments, and the neurasthenia cases, showed 

 clearly enough why the mixed arterial blood is not fully saturated 

 to the alveolar pressure; but why does the carbon monoxide 

 method not show this? A little consideration will show the reason. 

 The carbon monoxide method gives the average arterial oxygen 

 pressure of all the portions of arterial blood leaving the lung 

 alveoli, just as the "alveolar air" gives the average oxygen pres- 

 sure of all the portions of air in the alveoli of the air-sac system. 

 But the oxygen pressure of the mixed arterial blood cannot be 

 deduced, as fully explained in Chapter IV, from the average of 

 the oxygen pressures in the blood leaving the alveoli. It is this 

 average that the carbon monoxide method gives. Hence for the 

 purpose of deducing the oxygen pressure of the mixed arterial 

 blood the carbon monoxide method has exactly the same defects 

 as the method of inferring this value from the oxygen pressure of 

 the alveolar air on the assumption (perfectly valid for resting 

 conditions at ordinary atmospheric pressure when pure air is 

 breathed) that diffusion equilibrium is established between al- 

 veolar air and blood. For the purpose, however, of deciding 

 whether or not active secretion of oxygen is occurring, the carbon 

 monoxide method is perfectly valid. It gives just the information 

 needed; and for this purpose it is far more reliable than the 

 aerotonometer method, which has always given misleading in- 

 formation on the question of diffusion equilibrium for oxygen, 

 and made it appear as if diffusion equilibrium is never attained, 

 even during complete rest. 



To those who pin their faith, as regards the secretion question, 

 to the aerotonometer results, I may perhaps point out that if they 

 were accepted as evidence they would completely wreck the dif- 

 fusion theory. For if diffusion equilibrium is not even obtained 

 under resting conditions under normal barometric pressure it 

 would be quite inconceivable on the diffusion theory that anything 

 approaching to diffusion equilibrium would be obtained during 

 muscular work, and particularly at high altitudes. Yet on Pike's 



