RESPIRATION 319 



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 

 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. 



Alveolar and Dead Space Air 



In addition to these moieties of respired air, we have to consider the 

 division of the air in the lungs into what is called alveolar air and 

 dead space air. The former is the air which comes in contact with the 

 epithelium through which gas diffusion between the blood and the air 

 occurs ; the latter being the air which fills the respiratory passages. The 

 dead space can not be defined anatomically with exactitude ; it is func- 

 tional rather than morphologic. 



Measurement of the volume of the alveolar and dead space air can be made 

 by taking advantage of the fact that, while it is in the lungs, the air has 

 added to it C0 2 gas, which is present in the inspired air only in negligible 

 traces. The necessary data are : (1) the volume of the tidal respiration ; (2) 

 the percentage of C0 2 in alveolar air ; (3) the percentage of C0 2 in the tidal 



air. Suppose the values to be 500 c.c., 6 per cent and 4 per cent, re- 



4 



spectively; then the volume of alveolar air must be 500 x = 333 c.c., 



b 



and the dead space 167 c.c. The measurement so made is accurate only 

 when certain precautions are taken. Because of the practical impor- 

 tance of this part of our subject we shall, however, defer its further 

 consideration until we have become familiar with the general features 

 of pulmonary physiology. Since the first air to move into the alveoli 

 at the beginning of inspiration is that present in the dead space, the 

 last air expelled from the alveoli on the previous expiration, it is of 



