RESPIRATION 



301 



represented by that which is not expelled from the lungs of a dead 

 animal when the thorax is opened. In the collapse of the lungs thus 

 produced, the alveoli are not completely emptied of air, because some 

 becomes pocketed within them and is expelled only when the lungs are 

 compressed under water. 



The volume of the residual air can readily be measured during life 

 by causing a person, after a forced expiration, to take two or three 

 breaths in and out of a rubber bag containing a measured quantity of 

 an indifferent gas such as hydrogen. Suppose the bag to contain at 

 the start 4000 c.c. of hydrogen, and after a few breaths 3000 c.c. of 

 this gas and 1000 c.c. of other gases (the total volume of hydrogen and 

 expired air in the bag being still 4000 c.c.) ; then the residual air will 



Maximum inspiration 



Ordinary inspiration 

 TIDAL AIR 



Ordinary expiration 



Supplemental air 



Ifuxirnum expiration 



Residual air 



5QOC.C.Or30Cukin. 



2000 120 



or , 

 c c cub. in 





, Vital capacity 



Capacity of equilibrium 



Fig. 107. Amounts of air contained by the lungs in various phases of ordinary and of forced 



respiration. (From Waller.) 



be 1333 c.c., for it is evident that after a few breaths the composition of 

 the expired air in the bag will be the same as that in the lungs. This 

 calculation is based upon the assumption that no hydrogen is absorbed 

 by the blood during the experiment, which is not strictly the case. 

 The amount absorbed is, however, so small in two or three breaths as to 

 make it permissible to disregard it. The measurement can also be made 

 by taking a few breaths in and out of a bag containing pure 2 . By 

 ascertaining the proportion of nitrogen that collects in the bag, the 

 quantity of residual air can be calculated. We shall see later that the 

 measurement of the residual air during life has some practical impor- 

 tance in connection with the measurement of the bloodflow through the 

 lungs. 



