432 THE PRESERVATION OF NEUTRALITY 



pressure. If too much CO2 is present it will in the first instance 

 combine with any available base to form a bicarbonate ; if no 

 base is available, the excess COg wdll be eliminated by the lungs. 

 If any acid stronger than carbonic acid finds its way into the 

 blood it replaces some of the bound CO2, thereby increasing the 

 free CO2 temporarily. This causes increased ventilation of the 

 lungs and elimination of the excess COo, bringing the ratio 



free CO 

 -_ \ — back to its original level, and hence restoring the 



combined CO2 



original pYi. 



The bicarbonate of the plasma represents the excess of base "which 

 is left over after all the non-volatile acids have been neutralised. It 

 is the alkali reserve of the body which can be drawn upon to 

 neutralise any free acid stronger that CO^ which may find its 

 way into the blood stream (Demonstration, Part II.). Not until 

 practically all the alkali reserve has been used up will the blood 

 show any change in hydrogen ion concentration. Long before 

 that point can be reached other mechanisms will be brought into 

 action to preserve neutrality. The bicarbonate is a nest egg of 

 potential base which may be drazvn upon when required, but the 

 inroad must be made good at the first opportunity. It is really 

 an emergency measure useful to tide one over the difficulties that 

 occur suddenly and frequently. It is not a widoiv's cruse of oil — 

 always magically replete. If the ratio of H0CO3 to NaHCOg is 

 kept within normal limits even though the reserve is permanently 

 lowered, the acidosis necessitating the draft on the reserve is 

 called compensated. If, on the other hand, the amount of H2CO3 

 increases to a value greater than 1/20 of the alkali reserve in 

 arterial blood, the acidosis is said to be uncompensated. 



II. The lungs eliminate COg. The amount eliminated per unit 

 of time is a function of the capacity of the lungs and the rhythm 

 of respiration. The rate and depth of respiration are controlled 

 by the amount of CO2 in the blood perfusing the respiratory 

 centre in the medulla oblongata. Any increase in the COg of the 

 blood causes an increase in the rate of respiration. Similarly, the 

 process of respiration may be slowed down, till it stops, by de- 

 creasing the amount of COo in the blood. It has been stated that 

 this regulatory action of the medulla is caused not by COg, but by 

 the hydrogen ion concentration of the blood, i.e. any acid perfusate 

 will quicken respiration. But, as is ob\i()us from the context, 

 )U) free acid but CO2 can occur in the blood of a living animal. 

 Further, careful research has shown that the pH of blood does 

 not alter in health, so nice is the regulation. 



