302 THE RESPIRATION 



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 in an animal breathing under normal conditions by taking ad- 

 vantage 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 500x^=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 

 no value in purifying the air already present in the alveoli. If we take 

 a tidal inspiration as amounting to 500 c.c. and the functional dead space 

 as 150 c.c., it is plain that only 350 c.c. of the outside air gains the 

 alveoli, and that the subsequent expiration is composed of 150 c.c. of 

 outside air that had lodged in the dead space plus 350 c.c. of alveolar air. 



These facts deserve a certain amount of emphasis because of their 

 practical importance in many phenomena connected with respiration. 

 One seldom thinks, for example, that out of the 500 c.c. of air inspired 

 with each breath, only 350 c.c. reaches the alveoli, where it comes in 

 contact with the 2500-3000 c.c. of air already present in this part of the 

 lungs. 



There must therefore be a sort of interface somewhere in the alveoli 

 between the fresh outside air that comes in with each breath through 

 the bronchioles and the air which is more or less stagnant in the alveoli. 

 This interface must move backward and forward somewhat with each 

 breath, and a rapid diffusion of oxygen and of C0 2 must take place 



