24 A MANUAL OF PHYSIOLOGY 



acids or with bases, so that, when excess of base is added to 

 blood, the proteins act as acids, and neutralize the base ; when 

 excess of acid is added, the proteins act as bases, and neutralize 

 the acid. (2) The equilibrium of certain of the inorganic con- 

 stituents of the blood (carbon dioxide, the carbonates, and the 

 phosphates) is such that even great variations in the concentration 

 of any of these, such as may normally occur, produce scarcely any 

 effect upon the concentrations of the hydrogen and hydroxyl ions. 



The so-called ' titratable ' alkalinity of blood or serum, measured 

 by the amount of standard acid which must be added before the 

 colour of the indicator used changes from alkaline to acid, bears no 

 necessary or fixed proportion to the actual alkalinity. When blood, 

 for instance, is titrated with hydrochloric acid, with methyl orange 

 as indicator, at the point where the red colour appears all the disodium 

 phosphate and sodium bicarbonate will have been changed into 

 monosodium phosphate and carbon dioxide, all the alkali removed 

 from combination with proteins, a certain amount of acid-protein 

 compounds formed, and other minor reactions produced (Henderson) . 

 It is difficult to correlate the quantity deduced from such a titration 

 with any physiological condition, although undoubtedly it bears 

 some relation to the acid-neutralizing power of the blood, and some 

 relation to its real reaction. 



What is estimated here is the quantity of acid required to satisfy 

 the proteins and to react with the carbonates and phosphates 

 before that concentration of hydrogen and hydroxyl ions just 

 necessary to cause the change of colour is established. This is not 

 the same for different indicators, since there is a certain minimum 

 ratio in the concentration of these ions at which each indicator 

 turns in one or the other direction, none turning precisely at the 

 neutral point. Thus serum appears to be acid when tested with 

 phenolphthalein, and alkali must be added to the serum before the 

 pink colour indicating alkalinity is produced. On the other hand, 

 with litmus or methyl orange it gives the alkaline reaction, and a 

 considerable amount of acid must be added before the colour of the 

 indicator which denotes acidity appears. The true reaction of the 

 serum is not, of course, at one and the same time both alkaline and 

 acid ; but it is so near neutrality that it falls just below the degree 

 of alkalinity necessary to give the pink colour with phenolphthalein, 

 and just below the degree of acidity which gives the pink colour 

 corresponding to an acid reaction with methyl orange. Certain 

 indicators for example, rosolic acid turn so as to give sharp 

 colour reactions at about the concentration of hydrogen and hydroxyl 

 ions in the blood, and these may possibly be of use in determining 

 the changes in the true reaction for clinical purposes (Adler). 



Much more closely related to the true alkalinity of the blood 

 than the titratable alkalinity is the carbon dioxide content. 

 This follows from the facts that much the greatest part of the 

 carbon dioxide is united with bases, chiefly with sodium, and 

 that the quantity in simple solution is approximately constant. 

 The estimation of the total carbon dioxide in a sample of blood 

 throws light upon the capacity of the blood to perform one of 

 its most important functions the transportation of carbon 



