NEUTRALITY OF THE TISSUES AND TISSUE-FLUIDS 277 



dred cubic centimeters of ^ acid. In the body, it must be remembered, 

 this remarkable neutralizing-power of the blood is assisted by the added 

 ventilation of carbon dioxide from the lungs, which occurs in conse- 

 quence of the Dyspnea or rapid breathing which results from a slight 

 decrease of the alkalinity of the blood, and by the excretion of acid 

 salts by the kidneys and by the production of ammonia from the tissues. 

 The origin of the neutralizing-power of the blood is threefold : in the 

 first place the Bicarbonates of the blood are capable of neutralizing 

 large quantities of acid without any great change in the hydrogen ion 

 concentration by undergoing the reactions : 



NaHCOs + HA = NaA + H 2 CO 3 

 H 2 CO 3 = H 2 O + CO 2 



Thus if the acid HA is strongly dissociated, the effect of these trans- 

 formations is to replace it by the exceedingly weakly dissociated car- 

 bonic acid or by the neutral gas carbon dioxide. In a similar manner 

 the Phosphates of the blood contribute to maintain neutrality by under- 

 going the reaction : 



Na 2 HPO 4 + HA = NaH 2 PO 4 + NaA 



whereby the strongly dissociated acid HA is replaced by the faintly 

 acid salt, monosodium phosphate. Finally the Protein Salts in the 

 blood also assist in the preservation of neutrality by entering into 

 reactions of the type: 



Na Protein + HA = H Protein + NaA 



the strong acid being in this instance replaced by practically neutral 

 uncombined protein. 



Of these three agencies the bicarbonates are quantitatively much 

 the most important. This arises from their abundance in plasma and 

 also from the fact that the dissociation-constant of carbonic acid, or 

 proportion of hydrogen ions to undissociated acid in the reaction of 

 dissociation : 



H 2 CO 3 ^ H+ + HCO-3 



is very nearly equal to the hydrogen ion concentration in distilled water 

 at absolute neutrality (0.8 x 10~ 7 normal). Now L. J. Henderson has 

 shown that the rate of change in the alkalinity or acidity of a solution 

 of an acid when alkalies or acids are added to it is a minimum when the 

 dissociation-constant of the acid is of this magnitude. He illustrates 

 this principle by the following table, showing the amount of tenth 

 normal alkali required to secure a definite but arbitrarily chosen change 

 in alkalinity when added to equal amounts of the undermentioned acids : 



Dissociation-constant. Cubic centimeters of 



Acid. X10~ 7 alkali required. 



Phenol 0.0013 0.01 



Boric acid 0.017 0.08 



Hydrogen sulphide 0.57 1.10 



Monosodium phosphate 2.0 1 . 00 



Carbonic acid 3.0 0.72 



Picolinic acid 18.0 0.10 



Acetic acid .180.0 0.03 



