RESPIRATION 49 



is diminished in consequence of voluntary or involuntary inter- 

 ference, the depth is correspondingly increased owing to a very 

 slightly increased CO 2 stimulus. This is the explanation of why 

 the mean alveolar CO 2 percentage remains so steady with varying 

 frequency of breathing. It is only, as a rule, when there is very 

 considerable increase in the breathing that there is any material 

 increase in the frequency; and during health the frequency is 

 hardly affected by moderate muscular exertions or moderate 

 stimulation by CO 2 in other ways. The frequency of breathing 

 is thus no measure of the amount of air breathed; but undue 

 frequency of breathing, as will be shown later, is a very important 

 abnormal sympton. 



The response of the breathing to abnormal resistance has re- 

 cently been investigated by Davies, Priestley, and myself. 8 For 

 recording the depth and frequency of breathing we used the 

 recording "concertina" described in Chapter VII (Figure 43). 

 For a resistance to breathing we sometimes used partly closed 

 taps, the effects of which could be thrown in suddenly by closing 

 alternative inspiratory and expiratory air passages. In place of 

 the taps we also sometimes employed cotton wool resistances, as 

 with a cotton wool resistance the driving pressure varies directly 

 as the air flow, while with a tap the pressure varies as the square 

 of the air flow. The pressure was measured with a water ma- 

 nometer connected with the tubing between the mouth and the 

 resistance. 



Figure 15. 



Effects of resistance. In this and subsequent figures inspiration = upstroke. 

 Time marker = 10 seconds. To read from left to right. 



It was found that when a resistance is thrown in the immediate 

 effect is a great slowing of the breathing. After the next breath 

 the respirations become deeper and less slow, and after several 

 breaths the breathing settles down to a rhythm in which the 

 respirations are deeper and correspondingly less frequent. With 

 a considerable resistance the frequency is often reduced to a fourth 

 of the normal rate, while the depth is almost correspondingly in- 



8 Davies, Haldane, and Priestley, Journ. of PAysiol., LIII, p. 60, 1919. 



