464 THE ENTRANCE OF OXYGEN. [BOOK n. 



through the action of simple physical causes to bring about 

 the association of the additional quantity of oxygen whereby 

 the venous is converted into arterial blood? The reply is as 

 follows. 



286. In man, as we have seen, expired air contains about 

 16 p.c. of oxygen. The air in the pulmonary alveoli must 

 contain less than this, since the expired air consists of tidal 

 air mixed by diffusion with the stationary air. How much 

 less it contains we do not exactly know, but probably the dif- 

 ference is not very great. At the ordinary atmospheric pres- 

 sure of 760 mm. 16 p.c. is equivalent to a partial pressure of 

 122 mm. The question therefore stands thus, Will venous 

 blood, exposed at the temperature of the body to a partial pres- 

 sure of less than 122 mm. (less than 16 p.c.) of oxygen take 

 up sufficient oxygen (from 8 to 12 vols. p.c.) to convert it 

 into arterial blood? Numerous experiments have been made 

 (chiefly but not exclusively on the dog) to determine on the 

 one hand the oxygen-pressure of both arterial and venous blood 

 (i.e. the partial pressure of oxygen in an atmosphere exposed 

 to which the arterial blood neither gives up nor takes in oxy- 

 gen, and the same for venous blood), and on the other hand 

 the behaviour, at the temperature of the body or at ordinary 

 temperatures, of blood towards an atmosphere in which the 

 partial pressure of oxygen is made to vary. Without going 

 into detail, we may state that these experiments seem to shew 

 that the partial pressure of oxygen in the lungs is amply suffi- 

 cient to bring about, at the temperature of the body, the asso- 

 ciation of that additional amount of oxygen by which venous 

 blood becomes arterial. When blood is successively exposed 

 to increasing oxygen pressures, as the partial pressure of oxy- 

 gen is gradually increased, the curve of absorption rises at first 

 very rapidly but afterwards more slowly ; that is to say, the 

 later additions of oxygen at the higher pressures are propor- 

 tionately less than the earlier ones at the lower pressures. 

 And this is consonant with what appears to be the fact that 

 the haemoglobin of arterial blood though nearly saturated with 

 oxygen, i.e. associated with almost its full complement of oxy- 

 gen, is not quite saturated. When arterial blood is thoroughly 

 exposed to air it takes up rather more than 1 vol. p.c. of oxy- 

 gen ; and that appears to represent the difference between 

 exposing blood to pure air, such as enters or ought to enter 

 the mouth in inspiration, and exposing blood to the air as it 

 exists in the pulmonary alveoli. The greater relative absorp- 

 tion at the lower pressures has a beneficial effect in as much 

 as it still permits a considerable quantity of oxygen to be 

 absorbed even when the partial pressure of oxygen in the air 

 in the lungs is largely reduced, as in ascending to great heights. 



Similar observations seem to shew that arterial blood ceases 



