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 (C0 2 ) 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 C0 2 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 

 C0 2 in the alveolar air. It was only when the oxygen 

 percentage fell very low that the breathing was 

 increased. The percentage of C0 2 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 C0 2 

 was breathed it was found that the volume of air 

 breathed was increased to such an extent as to keep 

 the percentage of C0 2 in the alveolar air as nearly nor- 

 mal as possible. Nevertheless there was always a 

 very slight increase in the alveolar C0 2 percentage 

 with each increase in the breathing. For an increase 



