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ESSENTIALS OF PHYSIOLOGY. 



activity, and the amount of air passing into and out of the lungs with 

 each breath may be doubled or trebled. In this way a larger amount 

 of carbonic acid is expelled from the lungs with each expiration, and 

 the mean tension in the lungs is kept at the same level. 



The centre is extremely sensitive to the slightest rise in the tension 

 of carbonic acid in the blood, and a rise of O2 per cent, of an atmo- 

 sphere in the pressure of this gas in alveolar air doubles the ventilation 

 of the lungs, that is to say, the amount of air passing into and out of 



FIG. 103. The effect of C0 2 on the respiratory movements in the normal animal. 

 The inspired air contained 3 per cent. C0 2 during the period marked on the tracing. 



them at each breath. The capacity of the respiratory centre to react 

 to any increase in the tension of carbonic acid in the blood passing to 

 it is very great, as is shown in experiments in which the amount of 

 carbonic acid in the air breathed is gradually increased. As seen in the 

 following table from Haldane, the percentage in alveolar air remains 

 constant until the amount in the inspired air exceeds 3 per cent.; beyond 

 this point the percentage of carbonic acid in the alveolar air rises, and the 

 ventilation of the lungs becomes enormously increased. 



