AMOUNT AND COMPOSITION OF THE URINE 561 



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, and with the nature of the protein 

 (page 605). With the ordinary mixed diet it is usually between 12 and 

 20 grams a day, corresponding to from 75 to 125 grams of protein. Ordi- 

 narily, nitrogen is not retained and an increase in the protein ingested is 

 followed by an increase of nitrogen in the urine. In periods of growth and 

 after undernutrition, however, protein is stored in the body. 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. 



Urea. The chief of the nitrogenous bodies of the urine is urea, the 

 origin of which has been fully described in the chapters on metabolism. 

 No constituent of the urine is subject to greater variation both in abso- 

 lute and in relative amounts. On an average diet containing 120 grams 

 of protein per day, the absolute urea excretion may amount to about 30 

 grams ; on a low protein diet it may be only a few grams. When the pro- 

 tein intake is high, the nitrogen eliminated as urea may be 90 per cent 

 of the total nitrogen; but when the protein intake is low, this proportion 

 may fall to 60 per cent. The difference is because on a low protein diet 

 the greater percentage of nitrogen eliminated is endogenous in origin, 

 and urea, which is the chief constituent of the exogenous nitrogen moiety 

 of the urine, is accordingly decreased on low diets. 



In recent years the importance of the relationship between the con- 

 centration of the urinary constituents in the blood and the urine has 

 been much insisted upon, and since the estimation of the amount of 

 urea in the blood and the urine is relatively simple, most of the work 

 has been done by using these values. Ambard and Weil 5 believe that a 

 quantitative relationship exists between the rate of urine excretion and 

 the concentration of urea in the blood and the urine, since the urea in 

 the blood acts as a stimulus to the renal cells. By comparing the rate 

 of urea excretion and the concentration of urea in the blood and urine 



