12 ORGANISM AND ENVIRONMENT 



there follows excessive breathing; and if the alveolar 

 air be then analysed it will be found that the CO 2 

 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 2 . They saturate and desaturate somewhat slowly 

 when brought into connection with varying concen- 

 trations of CO 2 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 2 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 2 

 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 2 is sensibly the same. 

 Increased frequency is compensated for by diminished 



