RESPIRATION 253 



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 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 no in- 

 formation 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 ' dissocia- 

 tion tension ' of oxyhaemoglobin, or the partial pressure of oxygen 

 at which the oxyhaemoglobin begins to lose more oxygen than it gains, 

 is increased by raising the temperature. Curves of dissociation of 

 oxyhaemoglobin and blood are shown in Figs. 108 and 109. According 

 to Bohr, Fig. 109 represents the curves for blood and a haemoglobin 

 solution of equal strength. It will be observed that the two curves 

 are not identical, the blood-pigment in the corpuscles not behaving 

 just as the artificially-produced oxyhaemoglobin. 



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 contains 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. About a third of it 

 belongs to the corpuscles. 



In the serum the combined carbon dioxide exists chiefly as 

 carbonate and bicarbonate of sodium, the relative amount of 

 each depending on the quantity of carbon dioxide and of other 

 acids, such as phosphoric acid, which dispute with it the posses- 

 sion of the bases. That its relations are peculiar, however, is 

 shown by the fact that from defibrinated blood the whole of the 

 carbon dioxide can in time be pumped out without the addition 

 of an acid to displace it from the bases with which it is combined. 

 It is hardly necessary to say that this could not be done with a 

 solution of sodium carbonate. Yet when sodium carbonate is 



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

 is ^-x 760, or i59'6 mm. 



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