14 ORGANISM AND ENVIRONMENT 



average percentage of CO 2 in the alveolar air of adult 

 men is 5.6. This is calculated for dry air. Allowing 

 for the moisture present the pressure of CO 2 with 

 normal barometric pressure is (760 47) X 5< %oo 

 39.9, or, in round numbers, 40 mm. 



On observing the alveolar CO 2 percentage at 

 increased or moderately diminished atmospheric pres- 

 sure we found, just as might be expected from Paul 

 Bert's experiments, that it is the pressure, and not 

 the percentage, of CO 2 which remains constant. The 

 percentage is only constant if the barometric pressure 

 remains the same. At five atmospheres' pressure the 

 percentage of CO 2 in moist alveolar air during rest 

 is only a fifth of what it is at normal pressure. At 

 any one position on the earth's surface the changes 

 in barometric pressure from day to day are so slight 

 that the corresponding changes in the alveolar CO 2 

 percentage are not very noticeable; but with con- 

 siderable changes in altitude, or in the case of workers 

 in compressed air, these changes may of course be 

 very great. 



We thus reach the provisional conclusion that the 

 breathing is so regulated as to keep the pressure or 

 concentration of CO 2 in the alveolar air constant 

 within narrow limits. The slightest increase in con- 

 centration of CO 2 causes an increase in the breathing 

 which almost completely neutralises the increase in 

 concentration. The slightest decrease in the con- 

 centration of alveolar CO 2 causes a compensating 

 diminution in breathing. To put the matter some- 

 what differently, the respiratory centre reacts with 



