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 2 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 2 in the blood. 

 In the absence of CO 2 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 2 normally 

 present in circulating blood, the pressure of oxygen 

 would require to be lowered to about half the pressure 



