390 THE RESPIRATION 



tension of the alveolar air rises very rapidly during the firsJb part of 

 the apneic period, and then more gradually, the explanation being that 

 during the forced breathing the C0 2 has been washed out from the blood 

 but not from the body as a whole. 



As the C0 2 tension becomes lowered in the blood, C0 2 will diffuse 

 out of the tissues to maintain equilibrium between blood and tissues. 

 There must be some considerable lag in this process however, so that 

 when the forced breathing ceases a much lower tension exists in the 

 blood than in the tissues. The rapid rise in alveolar C0 2 early in the 

 apnea is therefore due to diffusion from the tissues up to the equi- 

 librium point. 



Periodic breathing is produced by forced respiration more readily in 

 rarefied air than at sea level. It was found by Douglas, 26 after breath- 

 ing forcibly for one minute at sea level, that the breathing when it 

 returned showed 8 to 10 different periods of apnea and hyperpnea. On 

 repetition of the experiment at an altitude giving a barometric pres- 

 sure of 600 mm., 25 such periods followed the apnea; at a height cor- 

 responding to 520 mm., 40 periods. Indeed, at high altitudes periodic 

 breathing may be brought about by the slightest alteration in normal 

 respiration ; even taking a deep breath may be sufficient to cause distinct 

 periodicity in the succeeding respirations, and in many persons living 

 at high altitudes periodic breathing is very apt to occur during sleep. 

 As in pathological cases exhibiting Cheyne-Stokes respiration, the peri- 

 odic breathing at high altitudes can be immediately removed by inspir- 

 ing oxygen. 



