48 ORGANISM AND ENVIRONMENT 



be found that the alveolar CO 2 pressure has fallen, 

 which shows that the breathing is deeper. This fall 

 reaches a certain amount, depending on the altitude, 

 and then ceases. On the subject's return to sea level 

 the alveolar CO 2 pressure does not at once return to 

 normal again, but may take many days, or even some 

 weeks, to do so. Figure 3 shows graphically the aver- 



Mttflar 



60 5io 00 * 50 * * so 3CO 

 I | | I I I I I I I 



2000 4000 6000 8000 MOOD MOM (6000 20000 26000 



FIG. 3. Alveolar pressures of oxygen and CO2 and per- 

 centages of haemoglobin in the blood of persons ac- 

 climatised to altitudes from sea level to 14,000 feet, 

 barometric pressures from 760 to 45 mm. of mercury. 



