190 



RESPIRATION 



It has already been shown in Chapters VI and VII that until 

 the oxygen pressure of the inspired air is lowered by about a third, 

 or that of the alveolar air to about half (i.e., from about 100 mm. 

 to 50 mm.) there is no marked immediate increase in the breath- 

 ing. The effect on the respiratory center of the very distinct degree 



Cos pressure 

 jnm.cf 800 



Altitude 



7SO 70O 



BOO 75O 7OO 65O 6OO SSO 5OO 4SO 4OO 3SO 3OO 2SO 2OO 

 Atmospheric pressure in mm. of mercury. 



Figure 57. 

 Alveolar gas pressures in relation to barometric pressure or altitude. 



of anoxaemia which is undoubtedly produced, in the manner ex- 

 plained in Chapter VII, is almost entirely masked by the con- 

 trary effect due to extra washing out of CO 2 and consequent 

 lowering of the PH in the arterial blood. But if exposure to the 

 lowered oxygen pressure is continued, not merely for perhaps an 

 hour, but for days or weeks, there is a quite marked increase in 



