MECHANICS OF RESPIRATORY MOVEMENTS 1171 



the gas lost during the experiment by diffusion into the blood-vessels. By analysis 

 of the gaseous mixture in the bag, it is possible to determine the amount of air 

 in the lungs at the beginning of the experiment. Supposing, for example, the 

 bag held 4000 c.c. hydrogen, after two respirations the total volume is unaltered, 

 but the gas is found to consist of 3000 c.c. hydrogen and 1000 c.c. oxygen, nitrogen, 

 and CO 2 , i.e. pulmonary gases. Since the gas in the lungs must have the same 

 composition and 1000 c.c. hydrogen have disappeared from the bag, it is evident 

 that the lungs will contain 1000 c.c. hydrogen and $~ t i.e. 330 c.c. pulmonary 

 gases. Thus the total volume of gas left in the lungs at the end of the forced 

 expiration was 1330 c.c., which is the residual volume for the individual. 



The above example is purely imaginary. As a result of actual 

 determinations carried out we may assume the residual air in the lungs 

 as something between 600 and 1200 c.c. 



Of the 500 c.c. of tidal air taken in at each inspiration, only a certain 

 part reaches the alveoli, part being required to fill the air-tubes, trachea, 

 bronchi, and bronchioles which lead to the air-cells. The volume of 

 the air-tubes has been reckoned to amount to 140 c.c., so that of the 

 500 c.c. about 360 c.c. reach the alveoli. For the same reason the 

 expired air represents the air from the alveoli (360 c.c.) diluted with 

 140 c.c. of air which has remained in the air-tubes and undergone 

 very little change, other than the elevation of temperature and satura- 

 tion with aqueous vapour. We have therefore to allow for this air 

 contained in the so-called ' dead space ' of the lungs when we 

 seek to arrive at the composition of alveolar air from an analysis of 

 expired air. 



