56 DONALD D. VAN SLYKE 



The Mechanisms Whereby the Normal Acid-Base 

 Balance Is Maintained 



The mechanisms by which the acid-base balance of the body in general, 

 and of the blood plasma in particular, is maintained may be divided into 

 two groups, those which retard change in the blood pH as a result of 

 the normal gain and loss of CO 2 during the cycle of the gas exchange, and 

 those which retard pH change when non-volatile acids enter the blood, 

 or when abnormally great CO 2 retention occurs. The mechanism that cares 

 for the normal changes of the gas cycle has been recently discussed in detail 

 elsewhere (Van Slyke(i), 1921). It is the mechanism directed against 

 non-volatile acids, and abnormally retained CO 2 , which is of interest in 

 acidosis, and which we shall consequently discuss here. 



The order in which the components of this mechanism respond to acid 

 invasion may be outlined as follows: 



1. The buffers of the blood, seconded more or less promptly by those 

 of other parts of the body, partially neutralize the acid the instant it enters 

 the blood. The neutralization is described as partial, because the buffers 

 unaided do not completely prevent reaction change, although they may 

 reduce it to insignificance. This first defense is purely chemical, as dis- 

 tinguished from the three following, which are at least partly functional. 



2. The lungs accelerate the rate of C0 2 excretion, and thereby reduce 

 the H 2 CO 3 of the blood until a normal H 2 CO 3 : BHCO 3 ratio is restored, 

 and therefore a normal pH. When this has been accomplished the BHCO, { 

 (total bicarbonate, B representing Na + K +. other bases) is still low, if 

 the invading acid is other than carbonic, but all the other buffers have 

 regained their normal supplies of alkali. The bicarbonate alone registers 

 by its fall the amount of buffer alkali neutralized by the retained acid. The 

 condition is now one of "compensated alkali deficit" ; the buffer alkali is 

 reduced, but compensating reduction of free H 2 CO 3 has restored a nor- 

 mal pH. 



3. Finally the kidneys excrete acid products, and 



4. The organism forms ammonia,, which is also excreted. 



By (3) and (4) the invading acid is removed, and replaced by bicar- 

 bonate until the BHCO 3 has been restored to its normal concentration. 

 The acid-base balance of the blood is now again normal, both. in pH and 

 in buffer alkali. 



If the rate of elimination of acid by excretion and ammonia formation 

 together fails to equal the rate of acid formation, however, then, instead 

 of returning from compensated alkali deficit to normality, the condition 

 passes to one of terminal uncompensated acidosis. The bicarbonate is 

 reduced to a small fraction of normal, and in addition the available alkali 

 of the hemoglobin and other buffers is drawn upon. When by these 



