READJUSTMENTS OF REGULATION 51 
just compensated sensibly for diminished alkalinity 
of the blood. This we confirmed on Pike’s Peak at a 
higher altitude. 
As a result of the whole of the Pike’s Peak and 
previous experiments we came to the conclusion that 
the point of alkalinity to which the kidneys, etc., regu- 
late the blood is altered in the direction of slightly 
diminished alkalinity, so that, assuming the reaction 
of the respiratory centre to alkalinity to be steady, the 
alveolar CO, pressure has to be kept lower in order 
to preserve the balance. The very slight diminution 
of alkalinity required to account for the increased 
breathing is so small as to be at present beyond the 
range of measurement, as already explained. Hassel- 
balch and Lindhard have more recently published the 
results of electrometric measurements of the arterial 
blood alkalinity which show a sensibly unaltered 
reaction after acclimatisation to lowered barometric 
pressure in a steel chamber, with the alveolar CO, 
pressure much reduced. 
It thus appears that the regulation of the alkalinity 
of the blood by the kidneys and liver is dependent on 
the oxygen pressure of the air. The change in envi- 
ronment has altered the setting of the regulator. This 
is a very striking example of the intimate connection 
between internal physiological regulation and external 
environment; but we have now to consider other 
instances. 
It has long been known that the percentage of 
haemoglobin and relative number of red corpuscles 
increases at high altitudes. Figure 3 represents the 
