54 ORGANISM AND ENVIRONMENT 
pressure of CO if the haemoglobin of human blood 
at body temperature is to be divided equally between 
the two gases. Clearly, therefore, if the pressure of 
CO is known, and also the percentage saturation of 
the blood after equilibrium has occurred, the pressure 
of oxygen can be calculated very exactly. We there- 
fore breathed an exactly known small percentage of 
CO until the blood ceased to take up any more CO. 
We then determined the percentage saturation of the 
haemoglobin with CO, and the pressure of CO in the 
alveolar air. From these data we calculated the pres- 
sure of oxygen in the blood leaving the lungs. CO, as 
already mentioned, is, apart from its action on haemo- 
globin, a physiologically indifferent gas like nitrogen or 
hydrogen. It is not oxidised in the body, and it 
appears to pass freely by simple diffusion, like nitro- 
gen or hydrogen. We could therefore assume that it 
diffuses freely into the blood and finally reaches a 
pressure which is the same in the blood of the lungs 
as in the alveolar air. 
In the human experiments we reached the appar- 
ently unmistakable result that the oxygen pressure in 
the blood leaving the lungs is considerably higher than 
in the alveolar air, and that there is therefore active 
secretion inwards. Experiments with animals showed, 
further, that when the percentage of CO was in- 
creased so as to produce symptoms of oxygen want 
the evidence of active secretion became much more 
striking. 
On repeating the human experiments at a later date 
we could not get the same results. Douglas and I then 
