38 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



of acids such as carbonic, phosphoric, lactic, and sulphuric, and of 

 alkalies such as ammonia and fixed alkali, a considerable reserve of which 

 is undoubtedly available in the animal organism. 



The removal is affected by three pathways: (1) through the lungs 

 gaseous carbonic acid is eliminated; (2) through the kidneys, the fixed 

 acids; and (3) through the intestines, some of the phosphoric acid. 



Carbonic acid is produced in large amounts in the normal process of 

 metabolism, and is excreted in a gaseous condition by the lungs. Varia- 

 tion in its excretion is the most important mechanism for controlling 

 fl 'temporary changes in C H . In order to make this clear, it may be well to 

 revert for a moment to the physicochemical equation by which carbonic 



acid is enabled to maintain neutrality. This may be written: C H = 

 TT r*o 



molecular ratio ^-^r^ Tne rat i mav De increased either by adding 

 JMaxlbUg 



free carbonic acid to the blood (as by causing an animal to respire some 

 of the gas), or by the addition of some other acid (e. g., oxybutyric, as in 

 diabetes) which will decompose some of the NaHC0 3 and produce 

 H 2 C0 3 . The increase which these changes would cause in C H of the 

 blood is prevented by the remarkable sensitivity of the respiratory cen- 

 ter to changes in C H . An increase which is much less than can be 

 measured by physicochemical means stimulates the center, causing in- 

 creased pulmonary ventilation, so that the carbonic acid is immediately 

 eliminated through the lungs. This elimination does not stop when the 



old level of carbonic-acid concentration is reached, but proceeds until 

 TT p<r\ 



the original ratio TrA * s a gai n attained in the blood, and C H is 



restored exactly to its original value. If it stopped at the old CO, con- 

 centration, the ratio would be too high because there is less NaHC0 3 . 



THE THEORY OF ACIDOSIS 



Although these considerations indicate that variations may occur in 

 the bicarbonate content of the blood without any significant change in 

 C H , they also show that the bicarbonate content must be a criterion of 

 the acid-base balance of the blood, and probably of the body fluids in 

 general. As pointed out by Van Slyke, 12 bicarbonate represents the ex- 

 cess of base which is left over after all the fixed acids have been neu- 

 tralized. It represents the base that is available for the neutralization of 

 any excess of such acids that may appear a measure of the reserve of 

 "buffer substance" or, more specifically, the alkaline reserve of the body. 

 Under normal conditions the amount of NaHCO 3 in blood plasma is very 

 constant (amounting to 50-65 vols. per cent C0 2 ), and when it is reduced, 

 it indicates that an excess of fixed acid must be present. This is taken 



