QUANTITY AND COMPOSITION OF THE BLOOD IN MAN 905 



15 to 38 C., the alkalinity increases fourfold; moreover, since the reaction 

 depends upon the relation of the free carbonic acid to the bases which are 

 present in the fluid, any escape of C0 2 from the blood will diminish the 

 hydrogen ions present and increase the alkalinity. An easier method than 

 the electrical is to employ indicators which vary in their sensitiveness to 

 changes of reaction. By previous experiment it has been determined what 

 concentration of hydrogen ions is sufficient to cause a change of colour in the 

 different indicators. In the following Table, taken from a paper by Roaf, 

 are given the colours of a number of different indicators and the degree 

 of acidity i. e. the hydrogen ion concentration which just suffices to change 

 their colours. 



It should be remembered that in distilled water of the highest state of purity the 

 concentration of H and OH ions respectively is about 1 X 10 ~ 7 . 



By these methods the hydrogen ion concentration of the blood at 38 

 is found to be 04 x 10 ~ 7 so that it is just on the alkaline side of neutrality. 

 It might be thought that with such a feeble alkalinity the merest trace of 

 acid added to the blood would suffice to make it acid. It is found however 

 that a relatively large proportion of an acid must be added to the blood in 

 order to produce an appreciable change in its reaction. This is due to the 

 fact that the sodium bicarbonate acts as a. ' buffer ' i. e. a substance which 

 can take up acid or alkali with a minimal change of reaction/ Thus, if some 

 acid be added to the plasma, it combines with the sodium and the equivalent 

 amount of C0 2 escapes, so that if the concentration of the latter gas be 

 retained constant in the atmosphere to which the plasma is exposed, the 

 reaction remains almost the same as before. In the same way, if some alkali 

 be added to blood in contact with an atmosphere containing 5 per cent. 

 C0 2 , it combines with the C0 2 to form sodium bicarbonate, and the reaction 

 is again practically unaltered. This property of the blood of retaining a 



