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 2 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 2 

 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 



