176 ELEMENTS OF BIOLOGY 



a complex iron-containing compound. This substance unites readily 

 with oxygen, forming an unstable molecule, an oxide of haemo- 

 globin known as oxYHifiMOGLOBiN. The oxyhaemoglobin, being un- 

 stable, readily yields its oxygen in the presence of substances with a 

 greater affinity for oxygen. Thus it is converted back again into 

 haemoglobin. The usefulness of this substance in the conveyance of 

 oxygen by the blood of Man is briefly this: 



The liquid plasma of the blood is composed chiefly of water. Now 

 oxygen dissolves in water and in watery liquids, but in the presence 

 of air at ordinary temperature and pressure pure water is saturated 

 when it contains approximately 0.6 per cent of dissolved oxygen. 

 The temperature of the bodies of mammals is much higher than 

 that of the room and since the solubility of gases such as oxygen in 

 liquids decreases as the temperature increases, the solubility of 

 oxygen in the plasma of the human blood is materially less than 0.6 

 per cent. The solubility is further reduced by reason of the colloidal 

 nature of the blood plasma, for the solubility of gases is less in solu- 

 tions of salts and in colloids than in pure water. Consequently, if 

 the cells of a complex animal body such as Man were dependent 

 for their oxygen supply solely on the oxygen that dissolves in the 

 liquid portion of the blood, their energy-transforming metabolism 

 would be carried on at a very low rate, due to inadequate oxygen 

 supply. But the haemoglobin in the blood cells very much increases 

 the oxygen-carrying power of the blood by reason of its reaction 

 with oxygen. Thus the supply of oxygen for consumption in the 

 metabolism of the tissue cells is raised to a quantity adequate for 

 the maximum activity of which the tissue is at the moment capable. 

 So during its course through the exceedingly fine blood vessels in 

 the tissues the oxyhaemoglobin is deprived of its oxygen, reduced to 

 haemoglobin and during its circulatory course in the respiratory 

 membranes it comes in contact with free oxygen and is again 

 oxidized. 



The carbon dioxide that is produced as a waste product of oxida- 



