694 PHYSIOLOGY OF RESPIRATION. 



upon the activity of the respiratory center. The trend of recent 

 work favors the view that the normal stimiihis to the respiratory 

 center is the carbon dioxid, or perhaps it would be more accurate 

 to say the carbonic acid, H2CO3, formed by the reaction between 

 the carbon dioxid and the water. When carbon dioxid is present 

 above a certain amount or tension it acts as a stimulus, directly or 

 indirectly, and gives rise to the moderate movements of normal in- 

 spiration. If the tension of the carbon dioxid is increased, the 

 stimulus becomes stronger and leads to the production of a condi- 

 tion of hj^perpnea and dyspnea. On the other hand, if for any 

 reason, such as active ventilation of the lungs, the tension of the 

 carbon dioxid in the blood falls below a certain value, estimated by 

 Zuntz as lying between 19 and 24 mms., no stimulation occurs, the 

 center is in a condition of apnea and respiratory movements cease. 

 Haldane* has laid great emphasis upon the delicacy of the reaction 

 of the respiratory center to any change in the carbon dioxid press- 

 ure. The reaction, in fact, is so sensitive that under quite extreme 

 conditions the concentration of carbon dioxid in the alveolar air is 

 kept nearly constant. In muscular exercise, for example, there may 

 be a large output of carbon dioxid to the blood, but the resulting 

 increase in carbon dioxid pressure so stimulates the respiratory 

 center that there is a corresponding augmentation in the breathing, 

 the alveolar space is more thoroughly ventilated, and the concen- 

 tration of CO2 in the alveolar air, and consequently in the arterial 

 blood, shows but little change. The average concentration of CO2 

 in the alveolar air is 5.5 per cent. An increase of as little as 0.2 

 per cent, will be attended by a doubling of the volume of respira- 

 tion. Accepting the view that the carbon dioxid of the blood con- 

 stitutes the normal stimulus to the respiratory center and that the 

 activity of this center varies proportionately to the concentration of 

 the carbon dioxid in the blood, the question has arisen whether the 

 carbon dioxid acts directly as a stimulus or in virtue of its effect 

 in increasing the acidity or hydrogen ion concentration of the 

 blood. The CO2 in solution forms carbonic acid, H2CO3, which dis- 

 sociates to yield hydrogen ions. Acids added to the blood experi- 

 mentally or formed in the body, as in conditions of acidosis, stimu- 

 late the center, and a number of observersf have concluded, there- 

 fore, that the stimulus to the center is, in reality, the hydrogen-ion 

 content of the blood. An increase in carbon dioxid concentration 

 ;such as occurs in muscular exercise would tend to increase the hy- 

 drogen-ion concentration and thus its stimulating effect is explained. 

 The mechanism works automatically in the direction of keeping the 

 reaction of the blood normal, since an increase of acidity by stimu- 

 lating the center and causing an increased ventilation of the lungs 



* Haldane, "Organism and Environment," 1917. 



t Haldane, loc. cit.; Winterstein, "Pfluger's Archiv," 138, 167, 1911. , 



