THE GASES OF THE BLOOD 



253 



385 



7 



pressure only produced an additional absorption of I c.c., and above 

 this increasing the pressure had very little effect. 



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 mole- 

 cules 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 n e-ce"tage of OxvQC'> >s>. 



off from the haemoglobin 

 will be unchanged, and as 

 many molecules on the 

 whole will escape as before ; 

 but even after a consider- 

 able reduction of pressure 

 the haemoglobin, such is its 

 avidity for oxygen, will still 

 be able to seize as much 

 oxygen as it loses. The 

 more, however, 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 cap- 

 tures. At a certain pressure 

 the escapes will become 

 conspicuously more numer- 

 ous than the captures ; and 

 the gas-pump will give evi- 

 dence of this. The higher 

 the temperature of the 

 haemoglobin is, the greater 

 will be the average velocity 

 of the molecules, and the 



Fig. 119. Curves of Dissociation of Oxyhaemo- 

 globin freed from salts by dialysis (after Bar- 

 croft). Along the horizontal axis are plotted 

 the partial pressures (numbers below the curve) 

 of oxygen in air, to which a solution of haemo- 

 globin was exposed. The corresponding per- 

 centages of oxygen are given above the curve. 

 Along the vertical axis is plotted the percentage 

 saturation of the haemoglobin with oxygen. 



greater the chance of escape of molecules of oxygen. 



It is easily proved that the substance in the corpuscles which 

 unites with oxygen is the blood-pigment. Although a solution of 

 oxyhaemoglobin crystals behaves towards oxygen somewhat differ- 

 ently from blood containing the same proportion of the native pig- 

 ment, the maximum amount of oxygen taken up is the same for 

 each. Much labour has been spent in determining curves which 

 express the relation between the partial pressure of oxygen to which 

 blood or a haemoglobin solution is exposed, and the proportion to, 

 which the blood pigment becomes saturated with oxygen under 

 each pressure. The differences in the results of the various in- 

 vestigators who have worked out the curves of dissociation for 

 haemoglobin and for blood (Figs. 119, 120, 121) have been largely 

 cleared up by the researches of Barcroft and his co-workers. One 



