12 ORGANISM AND ENVIRONMENT 
there follows excessive breathing; and if the alveolar 
air be then analysed it will be found that the CO, 
percentage has fallen below normal. The breathing 
is, as it were, making up for lost time. 
This is easy to understand. Not only does it take 
an appreciable time for the blood to flow from the 
lungs to the respiratory centre, but both the blood 
and the lymph surrounding the tissue elements in the 
respiratory centre have a large capacity for absorbing 
CO,. They saturate and desaturate somewhat slowly 
when brought into connection with varying concen- 
trations of CO, in the alveolar air. Consequently the 
respiratory centre only responds gradually to these 
variations. Were it not so the breathing would be 
very jerky, and it would be difficult to interrupt it in 
speaking or singing or swallowing. Momentary varia- 
tions in the alveolar CO, percentage have thus no 
appreciable influence on the breathing, and it is only 
the average that counts. But this average is regu- 
lated with an accuracy which is extraordinary. 
It is evident that the average percentage of CO, 
in the alveolar air can be kept constant either by 
shallow and frequent or by deep and infrequent 
breathing. We can voluntarily set the breathing to 
very different frequencies, letting the depth take care 
of itself. For instance we can breathe three times or 
fifty times a minute. If, however, samples are taken 
of the alveolar air when once these different rates 
have been properly established, it is found that the 
average percentage of CO, is sensibly the same. 
Increased frequency is compensated for by diminished 
