THE EXCRETORY SYSTEM 189 



the urine is more concentrated. In the disease known as 

 diabetes mellitus the specific gravity of the urine is con- 

 siderably heightened, owing to the presence of sugar. 

 The specific gravity is of greatest value when it is obtained 

 from a twenty-four-hour collection. 



The Composition of Urine. The chief constituents of 

 normal urine are water, urea, uric acid, coloring matter, 

 and salts. Of these constituents, urea is by far the most 

 important, for it is the chief waste product of the body. 

 To eliminate urea is the special work of the kidneys, and if, 

 for any reason, they fail to do their work, the accumula- 

 tion of the urea in the system eventually leads to the ter- 

 mination of life. Urea is the final product of all proteid 

 substances, and, consequently, a diet rich in proteids will 

 increase the amount of urea in the system. 



Of the salts, sodium chlorid occurs in the largest quan- 

 tity. In certain inflammatory diseases, in which it is 

 needed by the blood, it sometimes disappears temporarily 

 from the urine. 



When the kidneys are diseased, it is customary for 

 physicians to lighten their work as far as possible by 

 regulating the diet. 



The chief abnormal constituents that are liable to appear 

 in the urine are albumin and sugar; the former gives rise 

 to a condition known as albuminuria; the latter, to the 

 disease called diabetes. 



The normal quantity of urine passed in twenty-four 

 hours is from 40 to 50 ounces, or about 3 pints. This 

 amount will vary in health according to the condition of 

 the skin and the amount of fluid taken into the body. 

 The excretion of water by the kidneys is closely related to 

 a similar process conducted by the skin. When the body 

 is exposed to cold the blood-vessels in the skin are con- 

 stricted, and the discharge of water in the form of sweat is 

 checked ; at the same time the blood-vessels of the kidneys 



