THE RESPIRATORY SYSTEM. 267 



The effect of carbonic acid in stimulating the respiratory centre is 

 also seen during muscular exercise. During exercise the muscles form 

 a large amount of carbonic acid, which passes into the blood and raises 

 the tension of this gas in the blood. This rise of tension stimulates 

 the respiratory centre, and the respiratory movements become deeper 

 and more frequent, with the result that a larger amount of carbonic 

 acid is carried off in the expired air. Most observers have found a 

 considerable rise in the tension of carbonic acid in alveolar air during 

 exercise. According to the recent observations of Krogh, however, the 

 tension remains practically unaltered, and the response of the respiratory 

 centre to the stimulus of carbonic acid is as delicately adjusted during 

 exercise as during rest. As the result of several observations on the 

 same person, Krogh obtained the following figures for the alveolar 

 tension of carbonic acid : 



Tension of Alveolar C0 2 . 

 Rest . .... . 5-06 



Exercise 5'10 



We may, therefore, regard the regulation of the respiratory move- 

 ments and of the amount of air passing into and out of the lungs as 

 dependent, under normal conditions, upon the tension of carbonic acid 

 in the blood which supplies the respiratory centre ; and since the 

 tension of carbonic acid in the respiratory centre itself must vary with 

 that in the blood, the ultimate stimulus to the respiratory movements 

 is evidently the tension of carbonic acid in the respiratory centre. 



Since the slightest rise in the tension of carbonic acid in the blood 

 increases the respiratory movements, a diminution might be expected 

 to lessen the respiratory movements by diminishing or abolishing the 

 stimulus to the centre ; and such is found to be the case. 



When an individual takes a number of deep breaths (forced 

 respiration), more carbonic acid is removed from the lungs than is 

 entering them from the blood. Hence the tension in alveolar air, and 

 in blood, falls to such a level that it no longer stimulates the respiratory 

 centre ; and respiration ceases for a short time (apncea). During the 

 period of apnoea, carbonic acid continues to reach the blood from the 

 tissues, and the tension in the blood and in the alveolar air gradually 

 rises, until it again reaches a level sufficient to stimulate the respiratory 

 centre ; when this occurs respiration recommences (fig. 111). 



Apno3a can also be produced in animals by repeated inflation of the 

 lungs, whereby carbonic acid is, so to speak, washed out of the lungs 

 and its tension in the alveolar air falls. 



A marked fall in the tension of carbonic acid in the alveoli gives rise 



