Ill 



REGULATION OF THE ENVIRONMENT, 

 INTERNAL AND EXTERNAL 



We must now attempt to analyse the meaning of 

 the fact that the pressure of oxygen may be, and at 

 high altitudes always is, higher in the arterial blood 

 than in the alveolar air. The layer separating the 

 blood from the alveolar air in the lungs appears under 

 the microscope as an extremely thin layer of moist 

 albuminous material made up of flattened cells. In 

 such a layer gases are soluble, just as they are in 

 water ; and it seems natural that the membrane should 

 take up a gas in contact with it till it is saturated, and 

 give it off on the other side if the gas pressure is lower 

 there. During rest at sea level this is in fact what 

 happens with oxygen, as well as with every other gas 

 which has been tested. The gas passes so readily that 

 complete equilibrium between the gas pressure in the 

 alveolar air and that in the blood has occurred before 

 the blood leaves the lungs ; and the gas pressure in the 

 arterial blood is thus equal to that in the alveolar air. 

 For CO 2 and nitrogen this has been shown by the 

 aerotonometer and other methods: for oxygen it has 

 been shown by the carbon monoxide method, the aero- 

 tonometer method being unreliable for oxygen. 



But at high altitudes the moist albuminous material 



