CHAPTER XLIII 



THE CONTROL OF RESPIRATION (Cont'd) 

 APNEA PERIODIC BREATHING 



Apnea 



If a man breathes forcibly and quickly for about two minutes, he 

 will experience no desire to breathe for a further period of about the 

 same duration lie becomes apneic. When the desire to breathe returns, 

 the breathing is at first very shallow, and frequently periodic in type, 

 but gradually it becomes more marked, until at last normal respiration 

 is reestablished. How may the results be explained? The cause for the 

 absence of breathing is lowering of the C0 2 -tension in blood. This re- 

 moves the natural stimulus to the respiratory center because a temporary 

 condition of alkalosis with low C0 2 -tension is induced. Evidence of the 

 establishment of alkalosis by forced breathing has been obtained by ex- 

 amination of the acid excretion in the urine, (see page 381). The C0 2 - 

 tension is lowered because this gas has been "washed out" or "blown 

 off" from the blood into the overventilated alveoli. Although this over- 

 ventilation also raises the pressure (tension) of oxygen in the alveoli, 

 little, if any, more of this gas can be absorbed into the blood because 

 even at the normal alveolar tension, the blood takes up at least 95 per 

 cent of its possible load, in consequence of the dissociation curve. These 

 differences in the rate of C0 2 -loss and 2 -gain cause the respiratory quo- 

 tient to become very high during the forced breathing ; it may indeed rise 

 nearly to 2. During the apneic period the person by an effort can expire 

 some alveolar air, and if this be analyzed it will be found that very little 

 C0 2 is being expelled from the blood though the 2 is being absorbed as 

 usual. Consequently R.Q. becomes very low (0.2 has been observed). 

 Gradually, however, the C0 2 rises again in the blood and the alkalosis 

 disappears so that the respiratory center becomes excited, although at 

 first only feebly and irregularly. If alveolar air be analyzed when the 

 first indication of breathing returns it is said by Haldane that the C0 2 

 tension is not yet back to the normal level (see Fig. 131). This indi- 

 cates that some other influence is helping to excite the respiratory center 

 and this no doubt is anoxemia for the percentage of 2 in the alveolar 

 air has now fallen to a very low level. 



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