AMOUNT, COMPOSITION, AND CHARACTER OF URINE 527 



solution containing an average of 1.00, or concentrates it at least 30 

 times. 



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 iri the blood and urine 

 in a mathematical formula, they have obtained a value which they be- 

 lieve is more or less fixed for the normal kidney. This expression is 

 known as Ambard' s coefficient and formula* and has been used as a 

 means of evaluating the functional capacity of the kidney. 



Whatever the value of the formula may be in expressing the relationship 

 existing between the rate of urea excretion and the concentration of this 

 salt in the blood, it is certain that, in diseased conditions where impair- 

 ment of the kidney is certain, the concentration of urea in the blood re- 

 mains permanently at an abnormally high average level, although the 



*Ambard and Weil's formula is: 



Ur 



K = , in which: 



70 vT" 



D x x 



P V2S 



K = coefficient of urea excretion (Constant of Ambard). 



Ur = grams of urea per liter of blood. 



D output of urea in grams per 24 hours. 



P == weight of the patient. 



C = grams of- urea per liter of urine. 



70 = standard weight. 



25 = standard concentration of the urine. 



The average value for this constant in normal individuals is said to lie between .06 and .09. - 

 Critical reviews of the work have been published recently by Maclean 6 and by Addis and 

 Watanabe.* 



