The unloading of oxygen from the blood 159 



the tissue and therefore at twenty -five times its former velocity. Let 

 us take the influence of salts next. The effect of salts is to aggregate 

 the molecules of the haemoglobin ; this aggregation introduces the 

 double contour into the dissociation curves, causing the blood to give 

 out its oxygen much more readily at low oxygen pressures whilst it 

 takes it up more readily at high ones. Compare, for instance, the 

 thick lines in Fig. 86 A and B. The former is the dissociation curve 

 of dialysed haemoglobin at 37 C., the latter, that of human blood in 

 the absence of C0 2 at the same temperature, is very close to that of 

 haemoglobin in a solution of potassium chloride isotonic with blood. 



The salts not only produce a clumping of the haemoglobin 

 molecules so that each clump has on the average 2'5 molecules, 

 but they have a very important effect in maintaining the state of 

 aggregation at this level in spite of physiological changes in the 

 reaction of the blood. The effect of C0 2 in moderate quantities in 

 the presence of the salts is therefore to alter the value of K in the 



equation 



y Kx n 



100 ~ 1 + Kx n ' 



without more than a negligible alteration in n ; the effect of 40 mm. 

 CO 2 pressure on the dissociation curve of blood is to replace the 

 thick line in Fig. 86 B by the dotted line, thereby raising the " final 

 capillary pressure-head" to 25 mm.; indeed in the blood of many 

 persons the oxygen pressure corresponding to 50 / saturation is 

 as high as 30 mm. of mercury. This marvellous thing has there- 

 fore happened as the result of the combined effect of temperature, 

 salts and carbonic acid; the "final capillary pressure-head" has 

 become elevated about 100 times, making provision for oxygen to 

 diffuse into the tissues at 100 times the speed and this without 

 sensibly reducing the percentage saturation of the blood in the 

 pulmonary alveoli. For the sake of simplicity we have only dis- 

 cussed the capillary pressure of oxygen at one point in the capillary, 

 namely the point at which the blood leaves it. The reader must 

 recollect that diffusion of oxygen is taking place all along the capillary 

 and he may work out for himself the effects of temperature, salts 

 and carbonic acid at any particular percentage saturation of oxygen 

 which he selects. 



The effect of clumping produced by electrolytes is best grasped by 

 a comparison of the effect of a decrease in the equilibrium constant 

 (K} in clumped and unclumped solutions. A decrease in K alone 



