ALKALI RESERVE 333 



of the dissociation of NaHCO 3 shows that it is a reversible 

 reaction : 



e.g. NaHCO 



H 2 



Na + 

 OH 



HCO 

 H + 



Strong. Weak. 



This means that the direction of the reaction will, in the main, 

 depend on the relative amount of CO 2 (H 2 O) present compared 

 with the amount of NaHCO 3 . This ratio has been determined 

 experimentally. To give the normal pH. of 7-4, plasma must have 

 3-75 grams of CO 2 bound in NaHCO 3 present for every gram of 

 uncombined CO 2 : or, by volume, one volume of CO 2 remains 

 constant when associated with 20 volumes combined as carbonate. 

 In tabular form this reads : 



FreeC0 2 1 1 



- by weight, by volume. 

 Bound CO 2 3-75 20 



If excess of NaHCO 3 over this ratio is present, some will dissociate 

 to balance the dissociation pressure. If too much CO 2 is present 

 it will in the first instance combine with any free base to form a 

 bicarbonate ; if no free base offers, the excess CO 2 will be elimi- 

 nated by the lungs. 



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 than CO 2 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 drawn 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 widow's cruse of oil 

 always magically replete. If the ratio of H 2 CO 3 to NaHCO 3 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 H 2 CO 3 

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

 arterial blood, the acidosis is said to be uncompensated. 



