CHAP. IL] RESPIRATION. 345 



sociation is further especially dependent on temperature, a high 

 temperature favouring dissociation so that at a high temperature 

 less oxygen is taken up than would be taken up (or more given off 

 as the case may be than would be given off) at a lower 

 temperature, the partial pressure of the oxygen in the atmosphere 

 remaining the same. 



Hence the question arises, Are the conditions in which haemo- 

 globin and oxygen exist in ordinary venous blood as it flows to the 

 lungs, of such a kind that the venous blood in passing through the 

 pulmonary capillaries will find the partial pressure of the oxygen 

 in the pulmonary alveoli sufficient to bring about the association of 

 the additional quantity of oxygen whereby the venous is converted 

 into arterial blood ? 



The oxygen of expired air contains (in man) as we have seen 

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

 contain rather 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. The question therefore stands thus, Will venous 

 blood, exposed at the temperature of the body to a partial pressure 

 of 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 experi- 

 ments have been made (chiefly on the dog) to determine on the one 

 hand the oxygen-tension 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 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 haemo- 

 globin 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 comple- 

 ment 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 air as it enters the mouth in inspiration 

 and exposing blood to the air as it exists in the pulmenary alveoli. 



