526 THE EXCRETION OF UEINE 



The Normal Organic Salts of the Urine 



Nitrogenous Constituents. The greater number of the organic salts of 

 the urine are made up of bodies which contain nitrogen, and which are 

 derived from the protein element of nutrition. The proteins, which form 

 the chief building material of the body, are broken up into their con- 

 stituent amino acids in the intestinal tract and absorbed as such by the 

 blood. Portions of these acids are taken up by the tissues to repair and 

 to replace those proteins which have been discarded, and the remaining 

 protein, in excess of the body need for amino acids, is deamidized, the 

 major portion of the carbon, oxygen and hydrogen being oxidized to 

 form C0 2 and water, and the lesser portion of these elements being com- 

 bined with the nitrogen to form urea, ammonia, uric acid, etc. A similar 

 fate later awaits the nitrogen moiety which found a place in the tissues, 

 and which is replaced in turn by new nitrogenous bodies.* 



Since all the ingested nitrogen, except a small and rather constant 

 amount which is lost by the feces and the sweat, is excreted in the urine, 

 the total nitrogen of the urine has been taken as a measure of the nitro- 

 gen or protein metabolism of the body. In normal conditions the protein 

 metabolism is adjusted in such a manner that the nitrogen intake is 

 equal to the nitrogen output, a condition known as nitrogenous equilib- 

 rium. If the nitrogen intake is reduced below the actual body needs, 

 the excretion of nitrogen is greater than the intake which indicates that 

 the body protein is replacing the protein usually furnished by the food. 

 The minimum amount of protein that the body must have to maintain 

 equilibrium varies in individuals, but is on the average between 5 and 6 

 grams of nitrogen a day, which corresponds to about 40 grams of pro- 

 tein. With the ordinary diet it is usually between 12 and 20 grams a 

 day, or represents from 75 to 125 grams of protein. Since protein is not 

 stored by the body except in periods of growth or after periods of undsr- 

 nutrition, an increase in the protein food is accompanied by an increase 

 in the nitrogen excreted in the urine. For this reason, unless the amount 

 of nitrogen ingested is known, the study of the total nitrogen of the urine 

 gives no information concerning the nature of the nitrogen metabolism 

 of the body. The total output of nitrogen per day usually amounts to 

 10 to 15 grams from 1 to 2 per cent of the urine by weight. 



All the nitrogenous bodies of the urine are normally nonprotein, and 

 arise from similar bodies in the blood, where they exist in concentra- 

 tions of from 20 to 30 mg. per 100 c.c. In excreting the nitrogen of the 

 urine the kidney therefore takes it from a solution in which it is found 

 in a concentration of 0.03 per cent on the average and delivers it to a 



*For further details see page 610. 



