SECRETION AND ABSORPTION OF GAS, ETC. 247 



proportions. An increase or diminution of the tension of 

 one of the two gases, that of the other remaining the same, 

 increases or diminishes in exact proportion the relative 

 share of the haemoglobin combining with the former gas. 

 Thus provided we know the final state of combination of 

 the haemoglobin, and the percentage or tension of one of 

 the two gases (oxygen and carbonic oxide), in a gas 

 mixture we can infer the percentage or partial pressure 

 of the other. The striking difference of tint between 

 oxyhemoglobin and carbonic-oxide haemoglobin in dilute 

 solution renders it easy to estimate colorimetrically the 

 relative proportions of the two compounds present in the 

 haemoglobin of even a single drop of blood. 



It is clear that on the diffusion theory when an animal 

 breathes, say, "07 per cent, of carbonic oxide in air the 

 latter gas must continue to be absorbed bv the haemoglobin 

 of the blood until an equilibrium establishes itself, due 

 to the fact that the partial pressure of oxygen reached 

 by the blood passing through the lungs is sufficient to 

 prevent further absorption. This point will depend, firstly, 

 on the percentage of carbonic oxide in the alveolar air, 

 which, after absorption has ceased, will of course be the 

 same as that of the inspired air, and, secondly, on the 

 oxygen tension reached by the blood leaving the alveolar 

 capillaries. If, for instance, the oxygen tension of the 

 blood in the lungs just reaches that of the alveolar air 

 the proportion of carbonic oxide finally absorbed by the 

 blood will correspond, in the case of man, to what would be 

 absorbed on shaking the blood to saturation with an atmos- 

 phere containing about 14 per cent, of oxygen and "07 per 

 cent, of carbonic oxide. With such an atmosphere the 

 haemoglobin becomes just 60 per cent, saturated with car- 

 bonic oxide. On the diffusion theory of respiratory exchange, 

 therefore, the blood within the body ought finally to become 

 at least 60 per cent, saturated. We have thus a means of 

 testing the diffusion theory by a method which is free from 

 the probable sources of fallacy connected with the aero- 

 tonometer method. Moreover, as the absorption of a 

 moderate proportion of carbonic oxide by the blood causes 



