THE CONTROL OF RESPIRATION (Cont'd) 



THE NATURE OF THE RESPIRATORY HORMONE 



The practical importance of the observations described in the foregoing 

 chapters in the investigation of the relationship between C H of the 

 blood and respiratory activity will now be plain, and it remains for us 

 to consider the physiologic evidence that such a relationship exists. In 

 the first place, let us consider the behavior of the acid-base equilibrium 

 'during conditions of abnormal breathing- hyperpnea and dyspnea.* 



As C0 2 accumulates and 2 becomes used up in a confined space, the 

 breathing becomes intensified. In searching for the exact cause of this 

 effect, we must first of all ascertain whether the hyperpnea is due to the 

 deficiency of 2 or to the accumulation of C0 2 . Many of the experi- 

 ments bearing on these problems can be more satisfactorily performed on 

 man than on laboratory animals, because anesthesia is not necessary and 

 the subjective symptoms experienced are of great value in the inter- 

 pretation of the results. If an individual is placed in a large air-tight 

 chamber (2000 liters' capacity), and the depth and rate of breathing ob- 

 served as the C0 2 accumulates and the 2 becomes used up in the air of 

 the chamber, no distinct change in respiration will be observed until the 

 C0 2 percentage of the air has risen to almost 3. Above this point, how- 

 ever, the hyperpnea becomes more and more pronounced, until finally, 

 when the C0 2 percentage has risen to about 6 and the 2 percentage has 

 fallen to 13.5, it becomes unbearable (dyspnea). From the results of the 

 foregoing observation alone we could not, however, decide whether the 

 excitation of the respiratory center is due to the deficiency of 2 or .to 

 the increase of C0 2 . If the experiment is repeated with the difference 

 that the C0 2 as it accumulates is absorbed by soda lime, no hyperpnea 

 will develop even when the 2 is as low as in the previous experiment. 

 We may conclude, therefore, that in the first experiment C0 2 accumulation 

 must have acted as the respiratory stimulus. 



The same conclusion is arrived at as a result of observations on indi- 

 viduals caused to breathe in a more confined space as into a rubber bag 

 of about 225 liters' capacity. Under these conditions hyperpnea de- 



*Hyperpnea means slightly increased breathing; dyspnea, labored breathing, but yet with suffi- 

 cient ventilation to maintain life; asphyxia, the results of insufficient breathing. 



349 



