1206 



PHYSIOLOGY 



same ratio. The results of such experiments are very striking. Even 

 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 respira- 

 tion (Fig. 503). 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 vary- 

 ing percentages of carbon dioxide. 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 respi- 

 ratory centre is so arranged as to react to the slightest increase of 

 C0 2 tension in the blood, any increase in this gas giving at once a 

 compensatory increase in depth and frequency of respiration, so that 

 the alveolar C0 2 content may be maintained almost constant. 



That it is the tension of C0 2 in the alveolar air, and therefore in 

 the blood bathing the centres, and not the percentage amount of this 

 gas, which is the determining factor is shown by a comparison of the 

 composition of the alveolar air under different atmospheric pressures. 

 Thus, when the subject of the experiments, from which the above 

 Table was derived, was placed in an air-chamber compressed to a 

 pressure of 1261 mm., the mean percentage of C0 2 in the alveolar air 



was 3 '42, corresponding, however, to a tension of 3 '42 X 



760 



= 5-6 



