46 PHYSICOCH^MICAL BASIS OF PHYSIOLOGICAL PROCESSES 



The bottle is then connected with the manometer with the precautions 

 described elsewhere in this volume. When temperature conditions have 

 been allowed for, saturated tartaric acid is mixed with the plasma solu- 

 tion and the gas evolved measured by the displacement of the fluid in the 

 manometer. The apparatus may also be used with blood in place of 

 plasma. In this case, however, it is necessary that the oxygen be removed 

 before adding the tartaric acid. This precaution is necessary, since acid 

 can dislodge some of the 2 from hemoglobin. The blood is therefore first 

 of all laked with ammonia containing some saponin, then shaken with 

 0.25 c.c. saturated potassium ferricyanide solution, and finally with the 

 saturated acid solution. If blood is used, the estimations must be made 

 on strictly fresh blood, since on standing the C0 2 -combining power 

 greatly deteriorates. 



3. Indirect Methods 



There are several other methods by which the alkaline reserve may be 

 measured. These include: 



1. Determination of the Tension of C0 2 in Alveolar Air (page 344). 

 Since this method is employed more particularly in investigating the 

 hormone control of the respiratory center, we shall defer a description 

 of it until later. The alveolar C0 2 tension corresponds to the C0 2 ten- 

 sion in arterial blood and this is proportional to the alkaline reserve as 

 determined by Van Slyke's method as is proved by the fact that the ratio, 



plasma C0 2 



-, is satisfactorily constant. 



alveolar C0 2 tension' 



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 616), 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 



