ACIDOSIS 47 



that the taking of food invariably raises the alveolar C0 2 -combining power. 

 This would seem to indicate that it must be the excitability of the respi- 

 ratory center rather than the acid-base equilibrium that becomes altered 

 so as to cause variations in alveolar C0 2 . 



Technical difficulties have also to be overcome in the collecting of the 

 alveolar air, for it is now well established that the original method of Hal- 

 dane and Priestley is approximately accurate only when it is carried out 

 under strictly controlled conditions so strict that they can not be prac- 

 tised in the clinic and even then, as R. G. Pearce, Carter, Krogh, Sie- 

 beck and others have shown, we can not be certain of the results. At best, 

 therefore, the alveolar C0 2 can serve as an accurate index of the acid-base 

 equilibrium of the blood only under strictly controlled conditions. 



2. The Measurement of the Acid Excretion by the Kidney. As might 

 be expected, the acid-base equilibrium of the body may also be gauged by 

 measurement of the acid excretion of the urine, in which the acids are 

 contained partly in combination with ammonia or a fixed base, and partly 

 in a free state. We shall first of all consider the methods of acid 

 excretion and then examine the evidence showing that the total acid 

 excretion is proportional to the alkaline reserve as measured by the 

 above described methods. 



EXCRETION OF ACID IN COMBINATION WITH AMMONIA. The production 

 of ammonia is essentially an endogenous process, and when excessive 

 quantities of acid make their appearance in the organism, the fixed alkali 

 may not be sufficient to neutralize it all, so that ammonia, derived from 

 the breakdown of amino acids (page 650), instead of being converted 

 into urea is employed to neutralize the excess of acid. Most workers 

 have in this way explained the very large ammonia excretion that has 

 long been known to occur in such conditions as diabetic acidosis. Some 

 recent workers are, however, inclined to question the significance of 

 ammonia in this connection, believing that the increased ammonia ex- 

 cretion is, like the acetone bodies themselves, a product of perverted 

 metabolism. Be this as it may, it is no doubt true that ammonia is used 

 for neutralizing acid in disease, although it may not be an important 

 factor in the maintenance of neutrality under normal conditions. It is 

 a factor of safety, in that it helps to care for an increase in acid when 

 the normal mechanism of the body is overtaxed. 



EXCRETION OF PHOSPHATES. The more permanent control of neutrality 

 depends on the excretion of phosphates by the kidney. The principle 

 governing this process is exactly the same as that already discussed in 

 connection with carbonic acid. In the one case it is the volatile acid 

 C0 2 , and in the other, the fixed phosphoric acid that is concerned in the 

 reaction. The ratio between the acid salts of phosphoric acid, MH 2 P0 4 , 



