ALVEOLAR AIR. 113 



This sample of alveolar air is discarded by being pressed back 

 into the breathing tube, the object of its collection being to 

 have the side tube and nozzle of the syringe filled with alveolar 

 air (instead of outside air) when the main sample is taken. 

 After several normal breaths have again been taken through 

 the tube, collect another sample (of over 10 c.c.) of alveolar 

 air and remove the syringe meanwhile closing its opening 

 by the finger, and analyse in the gas burette. 



The forced expiration from which the above sample of 

 alveolar air was taken followed a normal inspiration but it is 

 clear that the percentage of CO 2 which it contains will be less 

 than that in a forced expiration made immediately after a 

 normal expiration. To find the average percentage of CO 2 in 

 the alveolar air it is therefore necessary to collect another 

 sample taken from a forced expiration following a normal 

 expiration. This is a more difficult procedure since the person 

 must blow out after a normal expiration for a period of time 

 equal to that of the normal expiration. With a little practice 

 it can, however, be satisfactorily accomplished. Collect and 

 analyse for CO 2 at least two samples of alveolar air following 

 inspiration and expiration. If the results do not check to 

 within 0.2 per cent, repeat the observations until they do. 

 Calculate the average tension of CO 2 in the alveolar air. The 

 chief source of error in the above method depends on the fact 

 that the percentage of CO 2 will progressively increase in each 

 succeeding portion of the air of the forced expiration, because 

 a constant amount of CO 2 is being discharged from the blood 

 into a diminishing volume of alveolar air. It is for this reason 

 that great care must be taken to have the forced expiration 

 of the same duration as a normal one. 



Determination of the percentage of CO 2 which is in equi- 

 librium with the tension of CO 2 in the venous blood of the lungs 

 is much more difficult. To do it with any degree of accuracy 

 it is necessary to determine the percentage of CO 2 in a series 

 of fractions of a deep expiration which immediately follows 

 upon a deep inspiration from a gasometer, or rubber bag, con- 

 taining air with about 10 per cent, of CO 2 . The CO 2 decreases 

 steadily in each succeeding fraction until a level is reached 



