104 RESPIRATION 



oxygen in the blood leaving the lung is not higher than the 

 partial pressure of oxygen in the alveoh. If, on the other 

 hand, the pulmonary epithehum is capable of actively 

 secreting oxygen into the blood, then the relation between 

 the tension of oxygen in the blood and the partial pressure 

 in the alveoh is of no importance. 



Before proceeding further it is necessary that we should 

 be quite clear as to what we mean by the tension of oxygen 

 in the blood. The tension of a gas in a liquid is the 

 pressure which it exerts in an atmosphere in equihbrium 

 with that liquid, such pressure being independent of the 

 pressure of any other gas present. Suppose that a sample 

 of blood on exposure to air containing oxygen at a pressure 

 of 30 mm. neither loses nor acquires oxygen, the number of 

 molecules which enter the blood and the number which 

 leave it in a given period being equal; then the tension 

 of oxygen in the blood is said to be 30 mm. Such blood 

 can only acquire oxygen by being exposed to a pressure of 

 that gas greater than 30 mm. 



Now suppose that there hes on the surface of the blood 

 a membrane which has the power of absorbing oxygen from 

 the air and passing it into the blood. Then the tension 

 of oxygen in the blood will be higher than it would if no 

 membrane intervened. The question before us is whether 

 the lung behaves actively, hke this membrane, or whether 

 it is merely an inert partition freely permeable to oxygen. 

 It is at once obvious that diffusion, if this occurs, must 

 become more difficult as the pressure of oxygen in the 

 alveoli becomes less. What happens at ten or fifteen 

 thousand feet above sea-level wher© the pressure of oxygen 

 is considerably diminished? Is the oxygen in these 

 circumstances at a higher pressure in the alveoh than in 

 the arterial blood? Further, supposing that the body is at 

 the same time performing strenuous muscular work, will 

 diffusion in a rarefied atmosphere allow of the passage into 

 the blood of the increased amount of oxygen required ? 

 It is of course conceivable that both processes occur, 



