1 96 PRINCIPLES OF GENERAL PHYSIOLOGY 



centration serves to get rid of the two products of muscular activity carbon 

 dioxide by the increase of respiratory ventilation, and lactic acid by increased 

 supply of oxygen. At the same time, unless there were an efficient mechanism 

 for moderating the changes in hydrogen ion concentration, there would be serious 

 disturbance of the delicate action of protoplasmic processes. 



This mechanism does in fact exist, and has been elucidated chiefly by the work 

 of Lawrence J. Henderson, whose article on the subject (1909) should be consulted 

 for a more detailed account than can be given here. 



The possibilities of a means of soaking up, as it were, excess of hydrogen 

 or hydroxyl ions would naturally be looked for in the more complex forms of 

 electrolytic dissociation of the salts of the bi- or tri valent acids, in combination 

 with the hydrolytic dissociation of salts of weak acids with strong bases. This 

 latter process has not been as yet discussed in these pages, and will require some 

 consideration presently. 



There are two systems of this kind to which early investigators turned their 

 attention. They are both found widely spread throughout the animal organism. 

 The first is that of the bicarbonates and carbon dioxide, which is to be met with 

 chiefly in the blood, but also in the cells of the tissues generally. The second 

 is that of the acid and alkaline phosphates, of greater importance in the cells. 

 There are also, of course, interactions between the two systems, thus : 



Na. 2 HPO 4 + H 2 CO 3 = NaH 2 PO 4 + NaHCO 3 , 



so that there is always present a complex state of equilibrium between the two 

 phosphates in addition to that between the bicarbonates and carbon dioxide. 

 The proteins, as amphoteric electrolytes, and therefore capable of combination 

 with both acids and bases, although, in all probability, only with strong acids 

 and strong bases, except in rare instances, must also be taken into account. 

 As we shall see, however, the part played by proteins appears to be compara- 

 tively unimportant. Adsorption, possibly, may also play a subordinate part. 



In the further treatment of the question, I follow closely that of Lawrence J. 

 Henderson. 



We must remenyber that, contrary to what happens in simple homogeneous systems, such 

 as true solutions in water, we have to deal in the blood and tissues with the complication due 

 to phases and the phenomena, such .as adsorption, which take place at their contact surfaces. 

 It is well, however, to understand the less complex case to begin with. The results can 

 afterwards be modified, if necessary, by the introduction of further factors. 



It has long been known that the blood is able to withstand the addition of 

 considerable amounts of free acid or alkali without much change in its reaction. 

 This has been correctly described as being chiefly due to the carbonates and 

 phosphates present, although the mechanism could not receive a satisfactory 

 explanation until the electrolytic dissociation theory was propounded. 



Let us consider first the phosphate system. The mono-sodium phosphate 

 (NaH 2 PO 4 ) behaves as a very weak acid owing to the way in which it dissociates, 

 while the di-sodium phosphate (Na 2 HPO 4 ) is a very weak base. The dissociation 

 of these salts may be represented as taking place in stages, thus (marking the 

 equations for convenience of future reference) : 



(1) Na 2 HPO 4 ^Na- + NaHPO 4 '. 



(2) NaH 2 P0 4 :|:Na- + H 2 PO 4 '. 



(3) NaHP0 4 '^Na- + HPO 4 ". 



(4) H 2 P0 4 '^ 



(5) H 2 O^IH- + OH'. 



(6) HP0 4 * + H 2 0^: 



Hydrolytic Dissociation. With respect to the two last equations, we note that 

 the source of the OH' ions giving alkalinity to solutions of Na.,HPO 4 is the 

 reaction in which the ion HPO 4 " combines with the H- ion of water, leaving OH' 

 in excess. The electrolytic dissociation of water itself has not yet been discussed, 



