30 RESPIRATION 



or by 26.9 liters. This corresponds very closely to the estimate by 

 Campbell, Douglas, and Hobson of an increase of 10 liters per 

 minute in the breathing for every .26 per cent of increased alveolar 

 CO 2 at normal barometric pressure. 



When, however, the CO 2 production was increased still further, 

 the alveolar CO 2 percentage, instead of continuing to increase, 

 began to diminish, and was only 6.10 per cent with the maximum 

 CO 2 production (2386 cc.) and volume of air breathed (60.9 

 liters). Quite clearly, an additional factor or factors besides mere 

 increase in the alveolar CO 2 percentage was coming into play; 

 for with the higher rates of CO 2 production the lung ventilation 

 is not merely increasing in the same fixed proportion as before to 

 the increased production of CO 2 , but at a slightly higher rate. 

 What this additional factor is will be discussepl later; but mean- 

 while we may rest content with the broad fact that the increased 

 ventilation is almost in proportion to the increased production of 

 CO 2 , just as we should expect from the other facts already dis- 

 cussed with regard to the regulation of breathing. 



It was shown by Paul Bert 14 that the physiological actions of 

 CO 2 , oxygen, and other gases present in the air breathed depend 

 on their partial pressure. It is only when the barometric pressure 

 is constant that their action depends on the percentage proportions 

 in which they are present in the air. The method of calculating 

 the partial pressure of the CO 2 in the alveolar air may be illus- 

 trated by an example. Let us suppose that the barometric pressure 

 is 760 mm., and that 5.6 per cent of CO 2 is found in the alveolar 

 air. In the first place allowance must be made for the aqueous 

 vapor present in the alveolar air, which in the living body must be 

 saturated with aqueous vapor at the body temperature. The pres- 

 sure exercised by this aqueous vapor is 47 mm. Hence the remain- 

 ing gas pressure is 760 47=713 mm. Of this pressure 5.6 per 

 cent is due to CO 2 (the results of the gas analysis being always in 

 terms of dry air) . Hence the pressure of CO 2 is 



760 47 



5-6 x = 39.9 mm., or 5.25 per cent of an atmosphere, 



i oo 



since 39.9 is 5.25 per cent of 760. 



From Paul Bert's results it might be confidently predicted that 

 it is not the mere percentage but the pressure of CO 2 in the alve- 

 olar air which regulates the breathing, and our experiments left 

 no doubt on this point. On descending one of the deepest mines, 

 and ascending the highest hill in Great Britain, we found that the 



"Paul Bert, La Pression barometrique, Paris, 1878. 



