30 ORGANISM AND ENVIRONMENT 
sociation of human oxyhaemoglobin under the condi- 
tions so far known to exist in circulating human blood, 
including the rise of CO, pressure as the blood passes 
the capillaries. It will be seen that the curve has a 
very peculiar shape, with a double bend, which is of 
great physiological significance. At the steep part of 
the curve oxygen will evidently come off freely with a 
comparatively slight fall in oxygen pressure. The 
haemoglobin is thus admirably adapted for maintain- 
ing the oxygen pressure approximately constant within 
the pressures corresponding to the steep part as the 
blood passes through the capillary vessels of the body. 
So far as we know the circulation is never, under 
normal conditions, so slow that the oxygen pressure 
in the body capillaries falls below the steep part of the 
curve, and is seldom so rapid as to bring the oxygen 
pressure above the steep part. The oxygen pressure 
in the alveolar air is normally about 100 mm., or 13 
per cent of an atmosphere, which corresponds to the 
flat upper part of the curve. 
The general form of the dissociation curve of the 
oxyhaemoglobin in blood was discovered a few years 
ago by Bohr of Copenhagen. He and his pupils also 
found that the curve is much affected, not only by 
temperature, but by the pressure of CO, in the blood. 
In the absence of CO, the curve (as represented in the 
figure) shifts to the left, so that oxygen is given off 
much less readily. For a specified amount of oxygen 
to be given off in the absence of the CO, normally 
present in circulating blood, the pressure of oxygen 
would require to be lowered to about half the pressure 
