540 VETERINARY PHYSIOLOGY 



of determining the partial pressure or tension of a gas in the 

 atmosphere has been described on p. 492, and it has been shown 

 that at sea-level, with an atmospheric pressure of 760 mm. Hg, 

 the tension of oxygen is about 152 mm. Hg. 



The air in the vesicles or alveoli is renewed partly by 

 direct ventilation from without, and partly by a process of 

 diffusion (p. 492). For this reason the amount of oxygen in 

 the vesicles must be smaller, the amount of carbon dioxide 

 larger, than in the air respired. 



Haldane has devised a method of procuring samples of the 

 alveolar air for analysis. A wide tube is fitted with a measured 

 glass bulb near one end, and this bulb is made a vacuum. The 



Mouthpiece 



Fig. 219. — Haldane's Apparatus for Determining the Composition of 



Alveolar Air. 



end of the tube near the bulb is put in the mouth or fitted, 

 to a mask, and the person under observation breathes through 

 it. At the end of an ordinary inspiration he expires deeply 

 through the tube, closes the mouthpiece with his tongue, 

 and by opening the upper stop-cock collects a sample of 

 the expired air. A second sample is taken in the same way 

 at the end of a normal expiration. The mean of these 

 samples represents the average composition of the alveolar 

 air (fig. 219). 



By the use of this method, it has been found that the 

 partial pressure of the 0„ varies within wide limits, while the 

 partial pressure of the COo remains very constant. 



Thus, at the top of Ben Nevis the tension of oxygen in 

 the air vesicles Avas 76 mm. Hg, at the bottom of a mine 

 it was 111 mm.; while in both places the tension of carbon 



