CHAPTER XXXVI 

 THE MECHANICS OF RESPIRATION (Cont'd)* 



VARIATIONS IN THE DEAD SPACE, THE RESIDUAL AIR AND 

 MID-CAPACITY, AND THE VITAL CAPACITY IN VARI- 

 OUS PHYSIOLOGICAL AND PATHOLOGICAL 

 CONDITIONS 



Dead Space 



Under ordinary conditions of breathing the dead space is fairly con- 

 stant in volume. Haldane 5 and Henderson 6 believe that it may be in- 

 creased by 400 per cent in maximal deep breathing, and that the in- 

 crease is due to the passive stretching of the lower air sacs. Although 

 such large variations in the capacity of the dead space has not been ob- 

 served by Krogh and Lindhard 7 or by R. G. Pearce, 8 it is undoubted 

 that moderate rhythmic variations may occur. Even in deeper breath- 

 ing (1500 c.c. or over), a slight increase, which with maximum breaths 

 may amount to 100 c.c., can be demonstrated. This is not surprising 

 when we remember that the walls of the bronchi and bronchioles are 

 made up largely of readily expansible tissue (elastic and smooth-muscle 

 fibers). As the respirations become deeper and the expanding force of 

 the inspiratory movements of the thorax becomes more pronounced, the 

 diameter of the bronchi and bronchioles will enlarge proportionately 

 that is, the diameter or circumference will increase in direct proportion 

 to this force; but the area of the cross section of the bronchi (i. e., the 

 capacity) will increase as the square of the diameter. This depends on 

 the fact that the area of a circle is increased by 125 per cent when the 

 diameter is increased by 50 per cent, and by about 300 per cent when 

 the diameter is increased by 100 per cent. 



The capacity of the dead space has a certain clinical significance. 

 Siebeck 9 has estimated that the dead space may increase by 100 c.c. in 

 asthma, but others believe that the increase may be greater. One rea- 

 son for the discordant results lies in the fact that the percentage of 

 C0 2 found in the alveolar air obtained by the Haldane-Priestley method 

 has been used as one of the basic figures in the determination of the 



'Most of this chapter was written by R. G. Pearce, M. D. 



327 



