14. ORGANISM AND ENVIRONMENT 
average percentage of CO, 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, with 
normal barometric pressure is (760—47) X 5-%o00 = 
39.9, or, in round numbers, 40 mm. 
On observing the alveolar CO, 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, which remains constant. The 
percentage is only constant if the barometric pressure 
remains the same. At five atmospheres’ pressure the 
percentage of CO, 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, 
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, in the alveolar air constant 
within narrow limits. The slightest increase in con- 
centration of CO, causes an increase in the breathing 
which almost completely neutralises the increase in 
concentration. The slightest decrease in the con- 
centration of alveolar CO, causes a compensating 
diminution in breathing. To put the matter some- 
what differently, the respiratory centre reacts with 
