ALVEOLAR AIR 105 



diffusion at high and secretion at low atmospheric pressure. 

 There are therefore three possibihties ; the process may be — 



1 . Entirely due to secretion ; 



2. Due to chffusion supplemented under special circum- 

 stances by secretion ; or 



3. Due to diffusion under all conditions. 



That the lung should be capable of secreting oxygen is 

 not an unreasonable supposition. Such a process is known 

 to occur in the swim-bladder of the fish, which may con- 

 tain as much as 80 per cent, of oxygen. At the same time, 

 the swim-bladder is not a lung either in structure or in 

 function. 



In order to decide the nature of oxygen absorption two 

 data are required, the partial pressure of oxygen in the 

 alveoh and the tension of oxygen in the arterial blood. 



The Composition of the Alveolar Air 



This is determined by two methods. 



Haldanes Method. — The apparatus consists of a tube 

 one inch in diameter and several feet in length. At one 

 end is fitted a mouthpiece, while two inches from it a short 

 side-tube leads into a gas-receiver which is fitted at each 

 end with a tap. At the beginning of the experiment the 

 receiver is filled with mercury. The subject, after taking a 

 normal inspiration, breathes into the tube as forcibly and 

 as deeply as he can, and then stops the mouthpiece with 

 his tongue. The end of the tube nearest to him now 

 contains air which may be regarded as coming from the 

 alveoh. On opening the taps this flows into the receiver, 

 from which it can be analysed. The experiment is now 

 repeated, but with this difference, that the forcible expira- 

 tion into the tube takes place not after an inspiration, 

 but after a normal expiration. The mean between the 

 two samples is taken as the composition of the alveolar 

 air. The normal oxygen pressure is found to be about 

 100 mm. of mercury. 



Krogh and Lindhard's Method. — The subject breathes 



