REGULATION OF BREATHING 11 



There is no doubt that it is through the blood that 

 slight changes in the C0 2 percentage of the alveolar 

 air affect the respiratory centre. The effects of these 

 changes are equally rapid and marked when all the 

 nervous connections between the lungs and the respir- 

 atory centre are severed. 



To most persons it must come as a surprise that the 

 breathing is so exactly regulated. Common observa- 

 tion shows us that the breathing is often more or less 

 interrupted temporarily, and varies in frequency or 

 depth at different times, as if the regulation were only 

 rough. We also know that breathing is under volun- 

 tary control, and there is a popular idea that by spe- 

 cial forms of training in breathing we can improve 

 the aeration of the blood and the supply of oxygen to 

 the body. 



If samples of the alveolar air are taken it is/ found 

 that they only give a constant percentage oiC0 2 if 

 the breathing is quite regular at the time, and they 

 are taken at the same phase of the respiratory act — 

 say at the end of inspiration or of expiration. Ac- 

 tually the percentage is varying distinctly from 

 moment to moment round the average; and it is only 

 the average that is constant. If, moreover, the per- 

 centage of C0 2 in the air inspired is suddenly 

 increased, it takes some little time before the breath- 

 ing increases to the new average. There is thus a 

 considerable lag between changes in the alveolar C0 2 

 percentage and the response of the respiratory centre. 

 This lag may be in either direction. If, for instance, 

 the breathing is voluntarily held for a short time, 



