RESPIRATION. THE MECHANISM OF BREATHING 405 



tory gases in the blood and the alveoli as would occur with slower 

 and deeper breathing. 



Forced Expiration. Although in ordinary quiet breathing, as 

 we have seen, expiration is a passive collapse of the chest, active 

 expiratory effort is of frequent occurrence. In talking, singing, 

 whistling, as well as in coughing, sneezing, and straining, the ex- 

 piratory muscles are functioning. The ease with which these are 

 brought into play suggests that the part of the center which 

 controls them, although not normally in action, is hung on a 

 "hair trigger" so to speak, requiring very slight additional in- 

 fluence to arouse it to action. 



Sensitiveness of the Respiratory Center. The respiratory 

 center is responsive to very slight changes in the amount of carbon 

 dioxid in the blood. A small increase quickens the breathing 

 notably. The effect of the quickened breathing is to ventilate the 

 lungs more thoroughly, and thus to bring about a more rapid 

 movement of carbon dioxid from the blood to the alveolar air (see 

 next chapter). In this manner the increased amount of carbon 

 dioxid is gotten rid of. The respiratory center may be thought of 

 as a delicate governor which serves to keep the carbon dioxid 

 content of the blood at a uniform level. The chief source of carbon 

 dioxid is in the active muscles. Muscular exercise is, therefore, 

 the most common cause of quickened breathing. If so careful a 

 regulation of the amount of carbon dioxid in the blood seems at 

 first not very important we can better appreciate its significance 

 by recalling that carbon dioxid is the end product of oxidation, 

 so that any change in the amount of carbon dioxid in the blood 

 means a corresponding change in the consumption of oxygen by 

 the Body; and, furthermore, that the changes in lung ventilation 

 which serve to keep the carbon dioxid level steady have the effect 

 at the same time of increasing or diminishing the supply of avail- 

 able oxygen. Thus an increase in carbon dioxid brings about a 

 more pronounced ventilation of the lungs, and thus, in turn pro- 

 vides more oxygen than is brought to the lungs during ordinary 

 breathing. There will be no need for a change in the amount of 

 oxygen unless there is a change in the bodily oxidations, so when- 

 ever more oxygen is needed the need is signalized by an increase in 

 the carbon dioxid, and this acts to bring into play the mechanism 

 by which the required oxygen is supplied. 



