RESPIRA TION 237 



lessens but slowly as the pressure is reduced, till at about 

 25 to 30 mm. of mercury an abrupt diminution takes place. 

 It is found that a solution of pure haemoglobin crystals 

 behaves towards oxygen just like blood ; and there is no 

 doubt that the body in blood with which the oxygen is 

 loosely united is haemoglobin. 



We may suppose that at the ordinary temperature and pressure* 

 some oxygen is continually escaping from the bonds by which it is 

 tied to the haemoglobin : but, on the whole, an equal number of 

 free molecules of oxygen, coming within the range of the haemoglobin 

 molecules, are entangled by them, and thus equilibrium is kept up. 

 If now the atmospheric pressure, and therefore the partial pressure of 

 oxygen, is reduced, the tendency of the oxygen molecules to break off 

 from the haemoglobin will be unchanged, and as many molecules on 

 the whole will escape as before ; but even after a considerable 

 reduction of pressure the haemoglobin, such is its avidity for oxygen, 

 will still be able to seize as many atoms as it loses. The more, how- 

 ever, the partial pressure of the oxygen is diminished that is to say, 

 the fewer oxygen molecules there are in a given space above the 

 haemoglobin the smaller will be the chance of the loss being made 

 up by accidental captures. At a certain pressure the escapes will 

 become conspicuously more numerous than the captures ; and the 

 gas-pump will give evidence of this, although it could give us no 

 information as to mere molecular interchange, so long as equilibrium 

 was maintained. 



The higher the temperature of the haemoglobin is, the greater will 

 be the average velocity of the molecules, and the greater the chance 

 of escape of molecules of oxygen. The ' dissociation tension ' of oxy- 

 haemoglobin, or the partial pressure of oxygen at which the oxyhaemo- 

 globin begins to lose more oxygen than it gains, is increased by raising 

 the temperature. The curve of dissociation of oxyhaemoglobin at a 

 temperature of 35 C. is shown in Fig. 84. 



The Carbon Dioxide of the Blood. Blood freed from gas 

 absorbs carbon dioxide partly in proportion to the pressure, 

 and in part independently of it. Some of the carbon dioxide 

 must therefore be simply dissolved ; some, and this the 

 greater portion, is chemically combined. The serum con- 

 tains a larger percentage of carbon dioxide than the clot, but 

 this percentage is not great enough to allow us to assume 

 that the whole of the carbon dioxide is confined to the 

 serum. Some of it must belong to the corpuscles. 



* The partial pressure of oxygen in air at 760 mm. atmospheric pressure 

 is x 760, or 159*6 mm. 



