406 THE HUMAN BODY 



Eupnea, Hyperpnea, Dyspnea, Apnea. Ordinary quiet breath^ 

 ing is known as eupnea. Rapid breathing, such as follows 

 moderate exercise, is designated as hyperpnea. When the breath- 

 ing is forced, and especially when forced expiration enters, we 

 have the condition called dyspnea. This results from abnormal 

 excitement of the respirator}*- center either reflexly, as from stimula- 

 tion of pain nerves, or by a greater increase in the carbon dioxid 

 content of the blood than that which causes simple hyperpnea. 

 The dyspnea of the early stages of suffocation arises from this 

 latter cause. Apnea, or absence of breathing, may result from 

 one of two conditions or from both acting together. The first 

 of these is a deficiency of carbon dioxid in the blood, so that the 

 respiratory center is not stimulated. The second is inhibition of 

 the center through vigorous and repeated inflation of the lungs. 

 Since inflation of the lungs with ordinary air brings about both 

 conditions the apnea which results from this treatment is partly 

 chemical and partly inhibitory. That inhibition enters in the 

 production of apnea in this way is shown by the greater difficulty 

 of producing the condition in animals with both vagi cut. 



Holding the Breath. When one holds his breath he is sending 

 impulses to the respiratory center which inhibit its discharge. 

 Meanwhile the bodily oxidations go right on, so the longer this 

 inhibition continues the greater becomes the amount of carbon 

 dioxid in the blood, and the more powerfully does the normal 

 excitation of the center act. In a very short time the carbon 

 dioxid stimulation becomes more p6tent than the volitional in- 

 hibition, and when that time comes a breath must be taken in 

 spite of the effort to hold it. Evidently any procedure that will 

 diminish the amount of carbon dioxid in the blood to begin with 

 will prolong the time the breath can be held. This can be done by 

 forced breathing for several minutes. The over-ventilation of the 

 lungs thus carried on sweeps out so much carbon- dioxid from the 

 blood that a much longer time elapses than ordinarily before the 

 accumulation overcomes the volitional inhibition. Since, as we 

 shall learn (p. 421), over-ventilation of the lungs does not ma- 

 terially increase the supply of available oxygen, this procedure 

 may bring about severe oxygen deficiency, which shows itself by 

 blueness of the skin and mucous membranes, a blueness caused by 

 the venous condition of the blood in the arteries. 



