CHAPTER VI 



THE REGULATION OF NEUTRALITY IN THE ANIMAL BODY 



AND ACIDOSIS 



Nothing is more constant in the animal economy than the H-ion con- 

 centration (C H ) of the fluids which bathe the tissues. This regulation 

 is fundamentally of a physicochemical nature, depending on the inter- 

 action of alkalies with acids, of which carbonic and phosphoric acids 

 are the most important.* When different amounts of acids or al- 

 kalies are added to water, the range of variation in H ion is 

 very extensive, whereas in blood the range is very limited indeed, not 

 extending beyond P H 7 and P H 7.52 (i. e., C H never goes above that of a 

 0.000,000,1 N solution or below that of a 0.000,000,03 N solution). In 

 other words blood can withstand considerable additions of acid or al- 

 kali without much change. 



Buffer Substances. The chemical reactions upon which this remark- 

 able constancy in reaction depends have been explained by Lawrence 

 J. Henderson. 10 The fundamental equations are as follows: 



M ,HPO 4 + HA = MII 2 PO 4 -f MA, and 

 MHCCX + HA z= H 2 C0 3 + MA, 



when M a basic radicle, and A, an acid radicle. 



Now it has been discovered that weak acids, like carbonic and phos- 

 phoric, possess the remarkable property of maintaining the reaction 

 constant when they are present in a solution which also contains an 

 excess of their salts. Under these circumstances the concentration of 

 ionized hydrogen is almost exactly equal to the product of the dissocia- 

 tion constant! of the acid (see page 1.9) multiplied by the ratio be- 

 tween free acid and salt; in other words, 



If carbonic acid is present in a solution of bicarbonates so that there 



*Under certain circumstances, proteins may also act either as acids or as alkalies. They are 

 therefore called amphoteric. The neutralizing properties of proteins are, however, of little conse- 

 quence in the neutrality regulation in the animal body (Bayliss 20 ). 



tThe dissociation constant has already been referred to as a figure which expresses the tendency 

 of a weak acid or base to dissociate in an aqueous solution. "It expresses the proportion in which 

 the nondissociated part is capable of existing in the presence of its ions," and therefore is a gauge 

 of the strength. The dissociation constant amounts to about 0.000,000,5 for carbonic acid ; that is, 

 the dissociation of HoCOs into H' + ITCO?/ at room temperature will be such that the concentra- 

 tion of H-ion equals a 0.000,000,5 N solution. 



36 



