THE RESPIRATORY CHANGES IN THE LUNGS. 363 



which keeps the film moist. This film, moreover, is composed of living 

 matter, and the considerations which a little while back ( 265) we urged 

 concerning the diffusion through a living membrane of solid substances in 

 solution, hold good also for the diffusion of gases in solution. 



We have now to consider the question, Are the conditions in which 

 haemoglobin 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 pul- 

 monary capillaries will find the partial pressure of the oxygen in the pul- 

 monary alveoli sufficient to bring about the association of the additional 

 quantity of oxygen whereby the venous is converted into arterial blood ? 



298. In man, as we have seen, expired air contains about 16 per cent, 

 of oxygen. The air in the pulmonary alveoli must contain less than this, 

 since the expired air consists of a 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 per cent, 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 pressure of less than 122 mm. (less 

 than 16 per cent.) of oxygen take up sufficient oxygen (from 8 to 12 vols. 

 per cent.) 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 behavior at the temperature of the body or at ordi- 

 nary temperatures of blood or of solutions of haemoglobin (for the two, as we 

 have just said, behave in this respect very much alike) toward an atmos- 

 phere in which the partial pressure of oxygen is made to vary. Without 

 going into detail, we may state that these experiments show 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 oxy- 

 gen 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 absorp- 

 tion rises at first very rapidly but afterward 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. per cent, 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 it exists in the pulmonary alveoli. The greater relative 

 absorption at the lower pressures has a beneficial effect, inasmuch 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. 



Observations made both with dog's blood and ox's blood seem to show 

 that arterial blood ceases to take up oxygen and begins to give off oxygen ; 

 in other words, that dissociation begins to take place when the partial 

 pressure of the oxygen in the atmosphere to which it is exposed sinks to 

 about 60 mm. of mercury ; that is to say, when the whole atmospheric 

 pressure is reduced from 760 mm. to about 300 mm., or when the percentage 

 of oxygen in the atmosphere is reduced by decidedly more than half. And 



