170 HUMAN PHYSIOLOGY. 



Organic Constituents of Urine. Urea is one of the most im- 

 portant of the organic constituents of the urine, and is present to 

 the extent of from 2.5 to 3.2 per cent. Urea is a colorless, neutral 

 substance, crystallizing in four-sided prisms terminated by oblique 

 surfaces. When crystallization is caused to take place rapidly, the 

 crystals take the form of long, silky needles. Urea is soluble in 

 water and alcohol; when subjected to prolonged boiling, it is decom- 

 posed, giving rise to carbonate of ammonia. In the alkaline fer- 

 mentation of urine, urea takes up two molecules of water with the 

 production of carbonate of ammonia. 



The average amount of urea excreted daily has been estimated 

 at about 500 grains. As urea is one of the principal products of the 

 breaking up of the albuminous compounds within the body, it is 

 quite evident that the quantity produced and eliminated in twenty- 

 four hours will be increased by any increase in the amount of al- 

 buminous food consumed, or by a rapid destruction of albuminous 

 tissues, as is observed in various pathologic states, inanition, febrile 

 conditions, fevers, etc. A farinaceous or vegetable diet will diminish 

 the urea production nearly one half. 



Muscular exercise when the nutrition of the body is in a state of 

 equilibrium does not seem to increase the quantity of urea. 



Seat of Urea Formation. As to the seat of urea formation, little 

 is positively known. It is quite certain that it preexists in the blood 

 and is merely excreted by the kidneys. It is not produced in 

 muscles, as even after prolonged exercise hardly a trace of urea is 

 to be found in them. Experimental and pathologic facts point to 

 the liver as the probable organ engaged in urea formation. Acute 

 yellow atrophy of the liver, suppurative diseases of the liver, diminish 

 almost entirely the production 01 urea. 



Uric acid is also a constant ingredient of the urine and is closely 

 allied to urea. It is a nitrogen-holding compound, carrying out of the 

 body a portion of the nitrogen. The amount eliminated daily varies 

 from five to ten grains. Uric acid is a colorless crystal belonging 

 to the rhombic system. It is insoluble in water, and if eliminated 

 in excessive amounts, it is deposited as a " brick-red " sediment in 

 the urine. It is doubtful if uric acid exists in a free state, being 

 combined for the most part with sodium and potassium bases form- 

 ing urates. It is to be regarded as one of the terminal products 

 of the decomposition of nucleic acid which in turn is derived from 

 nuclein, a constituent of cell nuclei. 



