AMOUNT AND COMPOSITION OF THE URINE 



557 



(Compiled from Mosenthal's figures.) 



The proportion of water to total solids is often very similar in plasma 

 and urine, but when water is taken in large quantities the urine shows 

 much greater changes than does the blood, and the solids may sink to a 

 very low concentration. On the other hand, when little fluid is taken or 

 when the skin and bowel eliminate a large amount of fluid, the urine 

 may become very concentrated without any change in the blood plasma. 

 The total solids in urine can be determined with approximate accuracy 

 by multiplying the last two figures of the specific gravity by the con- 

 stant coefficient 0.233 (Haeser). 



The Depression of Freezing Point. While the solids of the blood con- 

 sist, for the most part, of proteins and colloids, those of the urine are 

 made up of inorganic salts and small organic molecules. The mole- 

 cular concentration that is, the total number of molecules in a given 

 quantity of fluid is under ordinary conditions much greater in the 

 urine than in the blood. The molecular concentration may be deter- 

 mined by the depression of the freezing point of a fluid below that of dis- 

 tilled water (see page 10). Blood freezes almost constantly at -0.56 C., 

 while urine may freeze at temperatures which vary between -1 C. and 

 -2.5 C.; if very concentrated it may freeze at a temperature as low as 

 -5 C., or if dilute the freezing point may be as high as -0.075 C. 



The variability of the freezing point and the specific gravity of the 

 urine lead us to a consideration of the relationship of the urinary volume 

 to its concentration. In the first place, the volume of water ingested is 

 more frequently than otherwise in excess of the minimum absolutely re- 

 quired by the body, and is subject to greater variation than the sub- 

 stances excreted in the urine. The kidney is able to eliminate one con- 

 stituent of the plasma which may be present in excess without involving 

 any changes in others. For example, when salt is added to the food and 

 excreted in the urine, the total chlorides are increased, but the amount 

 of urine and the other constituents may remain unchanged ; or, again, as 

 may happen, excess of salt leads to an increase in the volume of the 

 urine, but the salt concentration remains constant while that of the 

 other urinary bodies is decreased. Similarly, although the rate of urea 

 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 



