64 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 Im^s. 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 



