SALTS OF HAEMOGLOBIN 335 



carbon-dioxide in the erythrocyte is, therefore, subject to consider- 

 able variation, and will tend to decrease in the lung, since the 

 alveolar carbon-dioxide tension is low. The " strength " of the 

 carbon-dioxide in solution as carbonic acid is unchanged, but, as 

 has already been indicated, oxygenation in the lung causes a marked 

 increase in the acid strength of haemoglo})in. The reaction 

 BHCO3 + H.Hb z= B.Hb -f HgO + CO2 is facilitated in the 

 direction from left to right, both by virtue of the increased acidity 

 of the oxyhaemoglobin, which, therefore, needs more alkali to 

 neutralise, and also by the escape of the carbon-dioxide, which 

 lowers the concentration of carbonic acid in the cells relative to the 

 concentration of haemoglobin. 



The quantitative importance of this redistribution of base may 



be appreciated from the following figures taken from Van Slyke. 



At the ordinary hydrogen ion concentration of blood (pH 7-4), 



1 mol. reduced haemoglobin (= 1 mol. O2) combines with about 



1-5 equivalents of alkali. 



This same haemoglobin, in oxygenated form, combines with a 



little over 2 equivalents of alkali. 

 .'. the reduction of 1 molecule of oxygenated haemoglobin in 

 the tissues sets free about 0-6 equivalents of alkali to combine 

 with the CO2 produced in the tissues. 



If the respiratory quotient is 0-8, and 0-8 molecules of COg are 

 produced for every molecule of oxygen absorbed, then 0-6 molecules 

 of CO2 {i.e. 75 per cent, of the total) will be carried by alkali 

 liberated from combination with oxy-haemoglobin by the reduction 

 of the latter. The remaining 25 per cent, of the carbon-dioxide is 

 carried by reaction with certain salts of weak acids, including 

 H2HbOo, present in the blood, the reactions being of the type 

 B2HPO4 + H2CO3 = BH2PO4 + BHCO3. 

 These latter reactions are accompanied by a very slight increase 

 in hydrogen ion concentration. The weak acids involved in these 

 reactions are mainly phosphates and the proteins of plasma and 

 cells. 



It would seem, then, that cells carry at least 80 per cent, of the 

 respiratory carbon-dioxide of the blood {i.e. the carbon-dioxide 

 picked up by the blood from the tissues, and excreted in the lungs, 

 as distinct from that concentration of carbon-dioxide which is 

 present alike in venous and arterial blood). Examination of 

 arterial and venous blood from any individual under given condi- 

 tions reveals, however, that the excess of carl)()n-dioxide carried 

 by the venous blood over that carried by the arterial blood is 

 distributed in approximately the ratio of 3 : 3 between serum and 

 cells. (Table XLVIII.) 



