II 
THE READJUSTMENTS OF REGULATION IN 
ACCLIMATISATION AND DISEASE 
We have seen that under ordinary conditions the 
regulation of breathing is dependent on very small 
variations in the degree to which the arterial blood 
leaving the lungs is saturated with CO,, and that a 
normal CO, pressure of about 40 mm. is maintained 
in the alveolar air of the lungs during rest. Never- 
theless this normal pressure may become altered. Thus 
if the oxygen percentage or pressure in the lung air 
becomes very low in consequence of great deficiency 
in the oxygen percentage of the air breathed, or from 
the barometric pressure being very low, as at great 
altitudes, the breathing is increased and the alveolar 
CO, pressure falls. A similar fall occurs after mineral 
acids have been taken, or in diseases in which abnor- 
mal quantities of acid are discharged into the blood, 
or after severe muscular exertion. To understand 
how the breathing is affected under these various con- 
ditions, and on what the normal conditions of breath- 
ing ultimately depend, it is necessary to consider the 
blood, and particularly the gases contained in it. 
When a liquid is brought into intimate contact with 
a gas the liquid takes up the gas in solution until a 
point is reached at which equilibrium or saturation 
occurs. At this point as many molecules of gas are 
being given off from the liquid as enter it, and the 
