524 THE EXCRETION OF URINE 



excretion is not demonstrably augmented by an increase in the volume 

 of the urine, an increase in the rate of urea excretion induced by the 

 ingestion of urea is accompanied by a larger volume of urine. That these 

 two factors may not stand in a causal relationship to each other is sug- 

 gested by recent work of Addis and Watanabe, 3 who find no quantitative 

 relationship between the rate of increase in urea excretion and the 

 increase in urine volume, and who believe that the apparent relationship 

 is due to a common cause, such as alteration in the rate of circulation or 

 change in the activity of the kidney cells. Nevertheless, there appears to 

 be a limit set to the power of the kidney to take the urinary salts or water 

 from the plasma and to place them in the urine in quite different propor- 

 tions. The definite amount of water required to hold the urinary salts 

 has been termed the "volume obligative" (Ambard r> ). These limits of 

 concentration may be fixed by the energy which the kidney can bring to 

 act against the osmotic resistance. 



The inconstancy in the behavior of the kidney toward ingested salts is 

 probably due to the fact that the salts reach the kidney in the concen- 

 tration in which they are held by the blood plasma, and not as they were 

 ingested. If salt is absorbed rapidly enough to disturb the salt equilib- 

 rium of the tissues and plasma, then water will be abstracted from the 

 tissues, and the plasma on reaching the kidney will eliminate the salt 

 and water together. The difference in the reaction arises from the 

 varied activity in the tissues in general rather than in the kidney itself. 



The Reaction of Urine 



In man and the carnivora this reaction is generally acid to litmus or 

 phenolphthaleih. The cause is found in the fact that the end products 

 of protein metabolism give rise to sulphuric and phosphoric acids the 

 acidity of which gives the urine an acid reaction. In the herbivorous 

 animals the alkaline reaction is due to the fact that vegetables and 

 fruits contain salts of dibasic or polybasic acids, such as acid potassiuiv 

 malate, citrate, acetate, and tartrate. Oxidation of these in the bod" 

 gives rise to carbonates. Some of the carbonic acid is excreted through 

 the lungs, and hence the associated base, generally sodium or potassium, 

 is combined so as to form a weak basic salt. 



The measurement of the acidity of the urine in terms of gram anions 

 or cations, like the same measurement in blood, requires the use of the 

 rather difficult electrical or indicator method, the principle of which has 

 been described in Chapter V. Expressed in terms of C H , the acidity 

 varies between 4.7 x 10- 7 and 100 xlO" 7 . The total potential acidity 

 that is, the number of. II ions which will be formed in the face of a con- 



