128 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



amount of gas dissolved is doubled. The oxygen in the blood does 

 not vary directly with oxygen pressure, for the amount of that gas 

 in simple solution forms only a small fraction of the toi)al present. 

 This small amount is of course doubled by doubling the pressure, but 

 such an increase is insignificant, the bulk of the gas being in chemical 

 union with haemoglobin. The oxygen of oxyhaemoglobin can be 

 replaced by equivalent quantities of other gases, such as carbonic 

 oxide. The tension or partial pressure of oxygen in the air of the 

 alveoli is less than that in the atmosphere, but greater than that in 

 venous blood; hence oxygen passes from the alveolar air into the 

 blood ; the oxygen immediately combines with the haemoglobin, and 

 thus leaves the plasma free to absorb more oxygen ; and this goes on 

 until the haemoglobin is entirely, or almost entirely, saturated with 

 oxygen. The reverse change occurs in the tissues where the partial 

 pressure of oxygen is lower than in the plasma, or the lymph that 

 bathes the tissue elements ; the plasma parts with its oxygen to the 

 lymph, the lymph to the tissues ; the oxyhaemoglobin then undergoes 

 dissociation to supply more oxygen to the plasma and lymph, and this 

 in turn to the tissues once more. This goes on until the oxyhaemo- 

 globin loses a great portion of its store of oxygen, but even in asphyxia 

 it does not lose all. 



The following values are given for the tension of oxygen in per- 

 centages of an atmosphere : 



External air 20'96 



Alveolar air 13-16 & 



Arterial blood ..... 14 j 



Tissues 



The arrow shows the direction in which the gas passes. 



The methods of obtaining the gases of the blood and analysing 

 them are described in the Appendix. When the gases are being 

 pumped off from the blood, very little oxygen comes off until the 

 pressure is greatly reduced, and then, at a certain point, it is suddenly 

 disengaged. This shows it is not in simple solution, but is united 

 chemically to the haemoglobin as oxyhaemoglobin, which is dissociated 

 when the pressure is extremely low. 



The ability of the tissues to form reduction products is shown by 

 Ehrlich's experiments with methylerie blue and similar pigments. 

 Methylene blue is more stable than oxyhaemoglobin ; but if it is injected 

 into the circulation of a living animal, and the animal killed a few 

 minutes later, the blood is found dark blue, but the organs colourless. 

 On exposure to oxygen the organs become blue. In other words 

 the tissues have formed a colourless reduction product from the 



