558 THE EXCRETION OF URINE 



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 urinarj^ salts or water from the plasma 

 and to place them in the urine in quite different proportions. The definite 

 amount of water required to hold the urinary salts has been termed the 

 "volume obligative" (Ambard 5 ). 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 THE URINE 



In man and the carnivora the reaction of the urine is generally acid 

 to litmus or phenolphthalein, but alkaline to methyl orange. The acid 

 responsible for the reaction is phosphoric, not in a free state, but as a 

 mixture of the salts Na 2 , HP0 4 and NaH 2 P0 4 , in which there is an excess 

 of the latter. If alkali be added to a solution of H 4 PO 3 , containing a 

 little methyl orange, the tint changes from red to yellowish when the 

 H,P0 4 has all been changed into NaH 2 P0 4 ; if more alkali be added, and 

 the indicator be now changed to phenolphthalein the tint turns red, when 

 the H 3 P0 4 has been changed to Na 2 HP0 4 . The reaction of urine lies be- 

 tween these two points.^ In the herbivorous animals the alkaline re- 

 action is due to the fact that vegetables and fruits contain salts 

 of dibasic or polybasic acids, such as acid potassium malate, citrate, 

 acetate, and tartrate. Oxidation of these in the body gives rise to 

 carbonates.^ Some of the carbonic acid is excreted through the lungs, 

 and hence the associated base, generally sodium or potassium, is com- 

 bined so as to form a weak basic salt. 



The measurement of the acidity of the urine in terms of the H-ion con- 

 centration like the same measurement in blood, requires the use of the 



