Iil 
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, 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 
