348 



THE RESPIRATION 



this observation should be discarded. The different observations are 

 then combined in the formula given on page 347. The determination 

 of the C0 2 percentage of expired air is so simple that a number of speci- 

 mens of varying depths of expiration can be taken and thus many points 

 on the curve determined. For the most accurate results it is in general 

 best to compare only those expirations which differ from one another 

 by at' least 0.3 per cent in C0 2 and by at least 200 c.c. in volume. This 

 depends on the fact that the diluting effect of the dead space in reduc- 

 ing the percentage of C0 2 in the expired air from that in the alveolar 

 air is greater in relatively small expirations. If more exact work is de- 

 sired, the 2 content can be determined on each specimen, the respiratory 

 quotient calculated, and only those expirations which show the same 

 respiratory quotient combined. 



In the table each observation is compared with each of the others in 

 all possible combinations. 



General average for CO2 in alveolar air, 5.13. 



General average for dead space, 172. Dead space in valves in this experiment was 

 about 30 c.c. 



Another method which has been suggested for clinical purposes is 

 that of Plesch; this consists in having the subject breathe several times 

 in and out of a small bag. It is assumed that after such respiration 

 the composition of the air in the bag will become the same as that in the 

 alveoli. Although this is no doubt true, it has been shown that the 

 method is fallacious, because the C0 2 tension determined in this way 

 is not that of the arterial blood alone, but is the average between it and 

 that of the venous blood. 



