ACTION OF ERYTHROCYTES 433 



III, Excess of base, and acids in condiination with bases are 

 eliminated by the kidney. The cells of this organ have a low 

 threshold for such salts. 



IV. The part phit/cd hi/ ihc red blood corpuscles- in preserving 

 neutraliUj. In conmion with other tissue elements, the erythrocytes 

 have a phosphate buffer system. Potassium dihydrogen phos- 

 phate, KH0PO4, is an acid salt and reacts with bicarbonate^ for 

 instance, to form the basic salt KoHPO^, 



viz. : 2KH2PO4 + 'iNaHCOgirNaaHPO^ + K^HPO^+HaO-l-COa 



acid phosphate. basic i^hosphate. 



A mixture of these two phosphates such as is found in all tissues 

 obviously will not increase in acidity till nearly all the disodium 

 phosphate has been converted into dihydrogen phosphate, nor 

 will the [H]. markedly decrease till all the dihydrogen phosphate 

 has been converted into the basic salt. 



In addition to this, the pigment haemoglobin plays a very 

 important part in the preservation of neutrality, as might be 

 deduced from its function in the transport of respiratory carbon- 

 dioxide. As explained in the chapter on respiration, haemoglobin, 

 either in the reduced or oxygenated form, appears to be present 

 in the cells either as a weak acid, or in combination with cell base 

 as a salt. Increase in the concentration of acid in the cell due to 

 increase in the acid in the plasma results in reactions of the type 

 BHb + HA — BA + H • Hb, as long as the acid HA is a stronger 

 acid than haemoglobin. Acids weaker than hi^moglobin would 

 have a negligible effect on the hydrogen ion concentration because 

 of their slight dissociation. Thus in the presence of a stronger acid, 

 base is liberated from combination with haemoglobin to neutralise 

 the stronger acid, while the resulting acid haemoglobin has a 

 minimal effect on the hydrogen ion concentration because of its low 

 dissociation. 



In addition to this " buffer " action of the haemoglobin, in 

 which it acts simply as a weak acid and salt buffer system like the 

 free and bound carbon-dioxide of the plasma, the pigment has a 

 still more important influence on the preservation of neutrality by 

 virtue of its change in acidity on oxygenation as described in 

 Chap. XXIII,, whereby, without change in hydrogen ion con- 

 centration, the mere passage of haemoglobin from oxygenated to 

 reduced form renders available about 25 per cent, of base combined 

 with it for cond^ination with other acids. It is these physical and 

 chemical properties of luemogl()l)in, together with the re-distribu- 

 tion of electrolytes between cells and serum which accompanies the 

 reactions between haemoglobin and acids, which confer on blood 

 a " buffering " power su})erior to that of plasma. 



B. 28 



