THE COMPOSITION OF THE URINE 1085 



differently into urea. Schroder^ has shown that one of these simple 

 compounds, although not the principal one, is ammonium carbonate. 

 Thus, it may be concluded that urea is a synthetic product of the liver 

 cells. 



In accordance with our previous discussion, urea may be regarded 

 as partly exogenous, and partly endogenous, because it is derived, on the 

 one hand, from nitrogenous bodies which have been absorbed but have 

 not become intimate constituents of the tissue cells, and, on the other, 

 from bodies which have been discharged by the cells after they have 

 previously formed a part of them. In other words, urea finds its 

 origin in the circulating proteins, as well as in the tissue proteins. It 

 may then be reasoned that a person in nitrogen-equilibrium discharges 

 only a small and rather constant amount of tissue proteins and that, 

 therefore, the endogenous urea must possess a small and constant value. 

 Contrariwise, it may be assumed that the amount of the exogenous 

 urea is much larger and variable, because it is taken from the variable 

 and excess quantities of proteins ingested. It is true, however, that 

 the endogenous variety may also undergo marked alterations, for 

 example, in fevers and other pathological conditions causing a rapid 

 destruction of the tissue proteins. 



Although subject to variations for reasons just stated, the amount 

 of urea excreted in the course of a day is usually given as 33 to 35 

 grm., provided about 100 to 120 grm. of protein are ingested. ^ 

 Its amount becomes greatest three hours after a mixed meal and may 

 constitute as much as 90 per cent, of the total nitrogen if large quanti- 

 ties of protein are ingested. Upon a low protein diet, such as has been 

 advocated by Chittenden, the urine shows a nitrogen-content consid- 

 erably below that ordinarily regarded as normal. The proportion 

 of urea may then be diminished to 60 per cent., because its chief source, 

 the exogenous nitrogen, has been eliminated in part. Muscular exer- 

 cise does not affect the urea output, showing that the energy is derived 

 in this case from the combustion of non-nitrogenous substances, 

 chiefly the carbohydrates. Some authors also state that a direct 

 relationship exists between the rate of urine secretion and the amount 

 of urea in the blood and urine, and claim to be able to evaluate the 

 functional power of the kidney by a comparison of these factors.^ 



Urea possesses the formula CO(NIl2)2 and is isomeric with ammo- 

 nium cyanate (NH4CNO) . This implies that it has the same empirical 

 but not the same structural formula. This substance was employed 

 by Wohler in 1828 in the synthetic preparation of urea. Crystals of 

 this substance may be obtained by warming potassium cyanate to- 

 gether with ammonium chlorid. In this form, urea is readily soluble 

 in water and alcohol and possesses a salty taste and a neutral reaction 

 to litmus On treatment with nitric acid, the nitrate of urea is formed 



1 Archiv fur exp. Path, und Pharm., xv, 1882, 364. 



2 Addison and Watanabe, Jour. Biol. Chem., xxvii, 1917, 381. 



3 Ambard and Weil, Physiol, norm, et path, des reins, Paris, 1914. 



