RESPIRATION 163 



pressures, but at higher pressures the haemoglobin is nearly 

 saturated, so that a further rise of pressure causes only the 

 small increase due to the increase of solubility associated with 

 higher pressure. The relation instead of being linear is 

 parabolic. 



From the diagram it can be seen that at 70 mm. pressure 

 of oxygen the blood contains almost as much (90 per cent.) 

 oxygen as at 100 mm. pressure (pressure of oxygen in lung 

 alveoli). A fall to 10 mm. pressure liberates 76 per cent, 

 of the combined oxygen, and would therefore allow the escape 

 of 66 (76 10) per cent., or 13-2 volumes of oxygen from 

 every hundred cubic centimetres of blood containing 15 per 

 cent, of haemoglobin. 



This process accounts for the transport of oxygen from 

 lungs or gills to the tissues. In the respiratory organs the 

 haemoglobin is exposed to comparatively high pressures of 

 oxygen and the haemoglobin takes up oxygen until almost 

 saturated. In the tissues there is a low oxygen pressure : 

 the oxygen is given off from the haemoglobin and the haemo- 

 globin passes back to the respiratory organs to obtain a fresh 

 supply of oxygen. 



In the blood the haemoglobin is not dissolved in the plasma 

 but it is contained in the red blood corpuscles. It is due to 

 the haemoglobin in these corpuscles that the blood has a 

 red colour. If the haemoglobin were not contained in the 

 corpuscles it would escape by the kidneys and be lost to the 

 body. 



The curve relating oxygen pressure to the amount of oxygen 

 united with haemoglobin is not always the same, as it varies 

 from animal to animal. Bohr believed that this difference 

 indicated that the haemoglobin from different animals was of 

 different composition. The presence of different proteins 

 united to the iron containing group (haematin) was held to be 

 responsible for the different oxyhaemoglobin curves and the 

 difference in shape of the oxyhaemoglobin crystals. The 

 experiments of Barcroft show that the differences in the 

 oxyhaemoglobin curves can be reproduced with the same 

 haemoglobin. 



Barcroft found that the haemoglobin curve varies according 

 to whether the haemoglobin is dissolved in distilled water or 

 solutions of different salts. If the same haemoglobin is 

 dissolved in solutions containing salts in the proportions in 

 which they occur in the corpuscles of different animals, the 

 oxyhaemoglobin curves agree with those obtained from the 



