24 



IV. 



ORGANIC CONSTITUENTS. 



Urea is, in amount, the principal constituent of the solids, 

 of the urine. It is the most important product of the decomposi- 

 tion of proteid in the food. In round numbers it forms from 

 2% to 3.5% of the urine, averaging about 2.5%. About one-half 

 of the total solids in the urine consists of urea. Urea has no effect 

 on litmus, it is odorless, has a weakly cool and bitter taste like 

 saltpeter. It is verj^ soluble in water and alcohol, but it is almost 

 insoluble in ether and benzine. About 90% of the total nitrogen 

 of the urine is excreted in the form of urea. 



Physiologically, urea is increased by a proteid diet ; -exercise 

 and muscular vigor; by drinking much water. It is decreased by 

 fasting; noq-nitrogenous food; reduction of water in the d(et ; 

 alcoholic beverages, tea or coffee; indolence of mind and boidy. 

 Pathologically, it is increased in all acute fevers; dyspnoea; 

 diabetes; and phosphorus poisoning. It is decreased in, uremia; 

 acute yellow atrophy of the liver and in chronic diseases. In 

 general, any disease that interferes with the activity of the liver 

 .decreases the urea. Any disease affecting the uriniferous tubules 

 " may modify the appropriation of the urea from the blood and 

 affect its passag:e into the urine. An increase of the urea inde- 

 pendently of the physiologic variations and aniount of nitro- 

 genous food eaten is an approximate index of the amount of 

 tissue waste in the system; on the other hand, when the urea is 

 decreased it is an evidence of a diseased condition of the liver 

 (the producer of urea) or the kidney (the eliminator of urea). 



A simple method of detecting urea is to concentrate 

 a small amount of dog * or human urine in an evaporating 

 dish to about half of its .original volume. Place a drop 

 or two of this concentrated urine upon a glass slide and 

 ■ after adding a drop of nitric acid, gently warm over the 

 flame. If urea be present, upon evaporation, the micro- 

 scope will show the characteristic crystals of nitrate of 

 urea, of rhombic or hexagonal form. (Fig. 4). 



* For this experiment, dog urine is more satisfactory than humart 

 because of the larger percentage of urea present. 



