REGULATION OF RESPIRATORY MOVEMENTS 1081 



a slight increase in the percentage of carbon dioxide in the air causes an 

 increase first in the depth and later on in the rhythm of respiration (Fig. 505). 

 This is shown in the following Table by Haldane, which represents the average 

 depth and frequency of the respirations when the subject was breathing 

 normal air and air charged with varying percentages of carbon dioxide. 



FIG. 505. Effect of C0 2 on respiratory movements of rabbit. (ScoTT.) 

 Upper line, tracing of diaphragm slip (Head's method). Lower tracing, carotid 

 pressure. During the first period indicated on the signal line the animal breathed 

 9-6 per cent. C0 2 in air, and during the second period 10 per cent. C0 2 with 33 per 

 cent, oxygen. Time tracing = 2 sees. Scale = mm. Hg. blood-pressure. 



A rise of carbon dioxide in the atmosphere to 2 per cent, increases the depth of 

 respirations by 30 per cent., and the total alveolar ventilation by 50 per cent. 

 A rise of carbon dioxide to 3 per cent, increases the total ventilation of the 

 alveoli by 126 per cent. An amount of carbon dioxide equivalent to 6 

 per cent, increases the depth of each respiration by 272 per cent., and the 

 total alveolar ventilation by 757 per cent. 



If we examine the last column of figures in this Table, representing the 

 percentage of C0 2 in the alveolar air, it will be seen that, in spite of the very 

 large variations in the air breathed, the alveolar content in C0 2 remained 

 practically constant until the C0 2 in the atmosphere was increased to such 

 an extent that the processes of compensation were no longer efficient. We 

 must conclude therefore that the respiratory centre is so arranged as to 



