1174 PHYSIOLOGY 



left open or connected with a spirometer. About 2 inches from the mouthpiece 

 is fixed a gas sampling-bulb, which is provided with three-way taps at the upper 

 and lower ends. Before an experiment the bulb is filled with mercury, if the 

 lower end is open, or else it is completely exhausted. The subject of the experi- 

 ment, after breathing normally a few times, at the end of a normal inspiration 

 puts his mouth to the tube, expires quickly and deeply, and closes the mouth- 

 piece with his tongue. The tap of the sampling-bulb is then turned, and 

 the air last expelled from the lungs (which is therefore pure alveolar 'air) 

 rushes into the bulb. The tap of the bulb is then turned off, and the gas 

 may be removed for analysis. A similar sample is then taken, in which 

 the subject expires deeply at the end of a normal expiration. This sample 

 will, of course, contain more C0 2 and less O 2 than that obtained at the end 



S/1MPL/NG TUBE. 



FIG. 490. 



of inspiration. The mean of the two samples is taken as the average composi- 

 tion of alveolar air. 



The difference between the composition of expired air and alveolar 

 air is determined by the dilution of the alveolar air with that contained 

 in the dead space. Hence with shallow breathing there will be a large 

 difference, but this will decrease with increased depth of respiration. 

 Thus, if the alveolar air contained 6 per cent. C0 2 and the dead space 

 amounted to 150 c.c., the expired air would only contain 3 per cent. 

 COo when the person was taking in only 300 c.c. at each respiration. 

 If, however, he was breathing slowly and deeply so as to raise the tidal 

 air to 1500 c.c., only one-tenth of this would be represented by the 

 dead space, and the expired air would contain nine-tenths as much 

 C0 2 as the alveolar air, i.e. 5'4 per cent. 



The changes in the composition of alveolar air with respiration 

 are by no means so marked as those produced in the tidal air, since 

 the latter forms only a small proportion of the total air in the lung 

 alveoli. Thus at the end of a normal expiration the alveoli still contain 

 2500 c.c. of gases. In inspiration 360 c.c. atmospheric air is taken 

 into this space and mixed with the 2500 c.c. already there. The 

 ' ventilation coefficient ' in quiet breathing is therefore only one-seventh, 

 and the change in the oxygen and carbon dioxide content of the 

 alveolar air produced by this access of 360 c.c. will amount to less 

 than one-half per cent. This is illustrated by the following figures 

 from Haldane, giving the alveolar content in carbon dioxide at the 

 end of inspiration and at the end of expiration respectively. 



