154 HUMAN PHYSIOLOGY. 



believed that urea is derived from ammonium compounds and from amino- 

 acids absorbed from the intestine. 



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 in- 

 soluble 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 forming 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. 



Hippuric acid is found very generally in urine, though it is present only 

 in small amounts. It is increased by a diet as asparagus, cranberries, plums, 

 and by the administration of benzoic and cinnamic acids. It is probably 

 formed in the kidney. 



Kreatinin resembles the kreatin derived from muscles. It is a colorless 

 crystal, belonging to the rhombic system. Its origin is unknown, though it 

 is largely increased in amount by albuminous food. About fifteen grains 

 are excreted daily. 



Xanthin, hypo-xanthin, and guanin are also constituents of urine. 

 They are nitrogenized compounds and are also terminal products of nucleic 

 acid. 



Urobilin, the coloring-matter of the urine, is a derivative of the bile 

 pigments. It is particularly abundant in febrile conditions, giving to the 

 urine its reddish-yellow color. 



Inorganic Constituents of Urine. Earthy Phosphate. Phosphoric 

 acid in combination with magnesium and calcium is excreted daily to the 

 extent of from fifteen to thirty grains. The phosphates are insoluble in 

 water, but are held in solution in the urine by its acid ingredients, alkalinity 

 of the urine being attended with a copious precipitation of the phosphates. 

 Mental work increases the amount of phosphoric acid excreted, a condition 

 caused by increased metabolism of the nervous tissue. 



Sulphuric acid in combination with sodium and potassium constitutes 

 the sulphates, of which about thirty grains are excreted daily. Sulphuric 

 acid results largely from the decomposition of albuminous food and from 

 increased destruction of animal tissues. 



The gases of urine are carbonic acjd and nitrogen. 



