574 THE ENTRANCE OF OXYGEN. [BOOK n. 



partition to the alveolar air. Further, the evidence, so far as it 

 goes, seems to shew that blood absorbs oxygen in the same way 

 as an aqueous solution of haemoglobin of the same concentration ; 

 tlie zone of plasma spoken of above as surrounding each corpuscle 

 behaves as far as regards the passage of oxygen to and from the 

 corpuscles in no essentially different respect from the way the 

 molecules of water, belonging to a molecule of dissolved haemo- 

 globin, behave in regard to the absorption or the giving-off of 

 oxygen by an aqueous solution of haemoglobin. 



355. 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 difference is not very 

 great. At the ordinary atmospheric pressure 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 tempe- 

 rature of the body to a partial pressure 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 oxygen, 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 or of solutions of haemoglobin (for 

 the two as we have just said behave in this respect very much 

 alike) towards an atmosphere in which the partial pressure of 

 oxygen is made to vary. Without going into- detail, we may state 

 that these experiments shew that the partial pressure of oxygen 

 in the lungs is amply sufficient to bring about, at the temperature 

 of the body, the association of that additional amount of oxygen 

 by which venous blood becomes arterial. When a solution of 

 haemoglobin or when blood is successively exposed to increasing 

 oxygen pressures, as the partial pressure of oxygen 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 proportionately 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 oxygen, is not quite saturated. When 

 arterial blood is thoroughly exposed to air it takes up rather more 

 than 1 vol. p.c. of oxygen ; 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 ifc exists in the pulmonary alveoli. The greater relative 



