18 



RESPIRATION 



the deep expiration ; and it requires some care to secure this. Under 

 normal resting conditions the depth of expiration needed in order 

 to give a reliable sample at the end of inspiration is at least 800 cc. 

 With less than this the sample is likely to be mixed with air of 



Figure i. 

 Apparatus for obtaining and analysing alveolar air. 



the apparent dead space; for though with normal breathing the 

 volume of the apparent dead space is far less than 800 cc., at least 

 three or four times its volume of alveolar air is needed in order 

 to flush it and the breathing tube out thoroughly. If more than 

 about 800 cc. are expired, the composition of the sample is the 

 same whatever the depth of the expiration, and we designated 

 air of this constant composition as "alveolar air" although, as 

 will be shown later, the composition of the air in the alveoli is by 

 no means such a simple matter as we thought. The following are 

 the averages of results which I obtained on this point when the 

 samples were taken just at the end of inspiration. 6 



Vol. of air expired 

 through tube 



190 cc. 



335 

 5io 

 650 

 950 

 1350 



Per cent of COz in sample 

 taken from tube 



3-03 



4-37 

 5-04 

 5-19 



5-51 

 5.48 



As soon as this method of sampling the alveolar air was applied 

 on ourselves and others it became evident that the alveolar CO 2 

 and O 2 percentage during rest under normal conditions are sur- 



* Haldane, Amer. Journ. of Physiol., XXXVIII, p. 20, 1915. 



