READJUSTMENTS OF REGULATION 57 
acclimatisation, be practically as fully saturated as 
usual ; and considering the increase in the haemoglobin 
percentage the amount of oxygen in the arterial blood 
must be greater than normal. The oxygen consump- 
tion during rest was the same on Pike’s Peak as at 
sea level, and the circulation rate, so far as our tests 
could determine it, was about the same.t Hence the 
oxygen pressure in the capillaries of the body would 
be somewhat higher than usual, and our unusually 
rosy color seemed to confirm this. 
The most probable explanation as to how oxygen 
want produces these effects is that there is some sub- 
stance which normally undergoes almost complete oxi- 
dation in the lungs at each round of the circulation. 
At high altitudes it escapes past the lungs in abnormal 
quantity in consequence of the lowered oxygen pres- 
sure, and probably also of the longer time required by 
the blood in the lungs to reach its full oxygen pres- 
sure. There are many facts pointing to the assump- 
tion that such a substance exists and that its presence 
in the blood is the source of various phenomena accom- 
panying oxygen want. 
The increase in the capacity of the lung epithelium 
to secrete oxygen is comparable to the increased 
efficiency produced in almost any organ by increased 
use. This increased capacity suggests the probable 
explanation of why in the original human experiments 
of Lorrain Smith and myself we obtained much more 
1By more accurate tests Krogh and Lindhard have re- 
cently shown definitely that there is no alteration in the 
circulation rate after acclimatisation in a steel chamber. 
