8 ORGANISM AND ENVIRONMENT 
breathing the ventilation of the lungs is such as to 
keep the percentage of carbon dioxide approximately 
constant in the air which is in close contact with the 
blood in the small airspaces or alveoli inside the lungs. 
If this is so, it is clear that the nearer the percentage of 
carbon dioxide (CO,) in the inspired air approaches 
that in the lung alveoli the greater will be the quantity 
of air which must be breathed in order to keep the 
lung air normal in composition. 
The matter was investigated a few years ago by 
Mr. Priestley and myself. We found that a sample 
of the alveolar air could easily be obtained by catch- 
ing the last parts of a deep breath expired through a 
tube, and that for any individual under normal condi- 
tions, the percentage of CO, in this air remains prac- 
tically constant during rest. On the other hand the 
percentage of oxygen in the inspired and alveolar air 
could be varied within wide limits without affecting 
either the amount of air breathed or the percentage of 
CO, in the alveolar air. It was only when the oxygen 
percentage fell very low that the breathing was 
increased. The percentage of CO, in the alveolar air 
is not quite the same in different individuals, but the 
average is 5.6 per cent for adult men. 
When air containing different percentages of CO, 
was breathed it was found that the volume of air 
breathed was increased to such an extent as to keep 
the percentage of CO, in the alveolar air as nearly nor- 
mal as possible. Nevertheless there was always a 
very slight increase in the alveolar CO, percentage 
with each increase in the breathing. For an increase 
