ACIDOSIS 37 



are equivalent quantities of free H 9 C0 3 and bicarbonate i. e., | 



the H-ion concentration will be exactly the same as the dissociation con- 

 stant of carbonic acid (i.e., 5 x 1O 7 ) ; therefore 0.000,000,5 N (P H = 6.3)*, 

 or about five times the value of neutrality, 0.000,000,1 N (P H = 7). If ten 

 times as much free carbonic acid as bicarbonate is present, then the H-ion 



concentration will be fifty times that of neutrality, i. e., 



x 0.000,000,5 = 0.000,005 (P H = 5.31) ; if there is ten times less carbonic 

 acid than bicarbonate, the H-ion concentration will be one-half that of 



neutrality, i. e., 4wTT = T7i x 0.000,000,5 = 0.000,000,05) (P H = 7.31) ; or 



if twenty times less, one fourth (P H = 7.6). Since a large amount of 

 bicarbonate is actually present in blood (enough to yield from 50 to 65 c.c. 

 C0 2 per 100 c.c. of blood), and the free carbonic acid undergoes fluctua- 

 tions which are only trivial when compared with those which have been 

 chosen in the above examples, it is clear that there must be very little 

 change in the H-ion concentration of the blood in comparison with the 

 variations which would occur were no bicarbonate present. 



Another weak acid which acts like carbonic in maintaining neutral- 

 ity is acid phosphate (MH 2 P0 4 ), and for the same reason namely, that 

 its dissociation constant is of similar magnitude to the H-ion concen- 

 tration. Although the blood plasma itself contains much less phosphate 

 than bicarbonate, the tissues contain a considerable amount, which en- 

 ables them to maintain their neutrality. This action of bicarbonates and 

 phosphates is styled the buffer action, meaning that it serves to damp 

 down the effect on the H-ion concentration which additions of acids or 

 alkalies would otherwise have. As pointed out by Bayliss, however, a 

 better word to use would be "tampon action,'* since the substances 

 actually soak up much of the added H- or OH' ions. It is not confined 

 to the fluids of the higher animals, but is very widely distributed 

 throughout nature ; for example, in the ocean and in the fluids of marine 

 organisms and animalcules (see L. J. Henderson). 11 



Although the actual reaction by which neutrality is maintained is 

 purely of a physicochemical nature, some provision must obviously be 

 made so that the acid and basic substances that take part in it may be 

 supplied and those produced by the reactions removed as occasion re- 

 quires. The source of supply is partly exogenous and partly endogenous. 

 The exogenous source is the basic and acid substances present in the 

 food; and although we do not ordinarily attempt to control the amounts 

 of these substances ingested, we may do so, as, for example, by the 

 persistent administration of soda in cases of pathological acidosis. The 

 endogenous source depends on the constant production in metabolism 



*For calculating this value see p. 27 



