THE CONTROL OF THE RESPIRATION 357 



B 



last column; B' figures in first column; A = aqueous tension of alveolar air; P' = figures of 

 second column; B = barometric pressure at sea level. A is obtained from tables giving the aqueous 

 tension at different temperatures. 



Changes in the frequency of breathing that are within physiologic 

 limits have no influence on the tension of alveolar C0 2 , provided that 

 exactly the same time is taken in performing the forced expirations 

 during which the samples of alveolar air for analysis are removed. 



The Degree of Sensitivity of the Respiratory Center to Changes in the 

 C0 2 Tension of the Alveolar Air 



This can be determined by observing the alterations produced in the 

 volume of air that actually enters the alveoli (alveolar ventilation) dur- 

 ing breathing in atmospheres containing different percentages of C0 2 . 

 In man an increase of from 0.2 to 0.3 per cent in the alveolar C0 2 is 

 sufficient to double approximately the alveolar ventilation ; or, more pre- 

 cisely, an increase of ten liters in the air entering and leaving the alve- 

 oli per minute is caused by raising the alveolar C0 2 tension by from 2.2 

 to 3.1 mm. Hg (Douglas, etc.) 24 . 



THE ALVEOLAR C0 2 TENSION DURING BREATHING IN A 

 CONFINED SPACE 



We have already employed similar experiments in ascertaining whether 

 C0 2 accumulation or 2 depletion is responsible for the hyperpnea pro- 

 duced under these conditions. We concluded for the former, but now 

 on closer examination we shall see that, although our conclusion was 

 correct, the deficiency in 2 also has an indirect effect on the respiratory 

 center. This is revealed by the fact that the tension of the C0 2 in the 

 alveolar air does not increase in proportion to the observed increase in 

 pulmonary ventilation. We must conclude that the decrease in 2 has 

 some effect. How may this be explained? Two possibilities exist: (1) 

 that the 2 want has caused organic acids to accumulate in the blood 

 and so raise the C H ; and (2) that in the absence of a certain tension of 



