AC1DOS1S 47 



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 , 

 and the alkaline salts, M 2 HP0 4 , in blood is approximately 1 to 5, but in 

 the urine this ratio varies according to the amount of H ion that must 

 be eliminated from the blood. In other words, a definite amount of phos- 

 phoric acid is enabled to carry variable amounts of H ion out of the body 

 by causing the amount of alkali excreted in combination with it to be- 

 come altered. For example, in the form of MH 2 P0 4 a given amount of 

 P0 4 obviously carries out more H ion than when it is excreted as 

 M 2 HP0 4 . The adjustment between these two salts is a function of the 

 kidney. We may accordingly measure the amount of alkali retained by 

 the organism by finding how much standardized alkali must be added 

 to a given quantity of urine until the reaction of the blood is obtained. 

 Since the latter value is constant, the titration can be done simply by 

 titrating the urine with an indicator such as sulphonephenolphthalein, 

 which changes tint at about P H of blood. 



A more serviceable indicator to use, however, is phenolphthalein, be- 

 cause its end point is such that when human urine just reacts neutral 

 to it that is, when the titrable acid approaches zero the C0 2 -absorb- 

 ing power of the plasma is at its maximum of 80 vols. per cent and the 

 ammonia excretion by the urine is zero (Van Slyke). It is advantageous, 

 therefore, to use this indicator, because it happens to have its turning 

 point situated for a reaction which is well to the alkaline side of neu- 

 trality, and which is reached in urine when the blood is at its maximal 

 acid-combining power and no ammonia is being used for neutralization 

 purposes. As the C0 2 -combining power of the blood decreases, there 

 should, therefore, be a proportionate increase in ammonia and in the 

 titrable acidity of the urine. 



Although a general parallelism exists between these values in cases of 

 diabetes, etc., there is no strict proportionality. The expedient has 

 therefore been tried of comparing the alkaline reserve of the blood with 



