EESPIEATION 317 



the tension may be as low as 20 mm. Hg and the CO, 

 tension as high as 40 mm. Hg. And that in blood coming 

 from the lungs the tension is generally well over 100 mm. 

 Hg, while the C0 2 tension varies enormously, but is, on an 

 average, about 20 mm. Hg. 



II. Partial Pressure of Gases in the Air Vesicle. The air in 

 the alveoli is not renewed by direct ventilation from without, 

 but by a process of diffusion (p. 303). For this reason the 

 amount of oxygen in the alveoli must be much smaller, the 

 amount of carbon dioxide much larger, than in the respired air. 



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 



Sampling Tube 



FIG. 145. Haldane's Apparatus for determining the Composition of 

 Alveolar Air. 



end of the tube near the bulb is put in the mouth and the per- 

 son under observation breathes through it. At the end of an 

 ordinary inspiration he expires deeply and quickly through the 

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

 the expired air. A second sample is taken in the same w r ay at 

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

 represents the average composition of the alveolar air. 



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

 pressure .of the varies within wide limits, while the partial 

 pressure of the C0 2 remains very constant. 



Thus at the top of Ben Nevis the pressure of oxygen was 

 76 mm. Hg ; at the bottom of a mine it was 111 ; while 

 in both places the pressure of carbon dioxide was about 

 42 mm. 



