METABOLISM IN NEPHRITIS 359 



Introducing this correction into the previous formula, we have 

 Concentration of urea in blood 



Weight 

 Eate 



= Constant. 



V Concentration in urine 



This formula expresses all of Ambard's laws in the simplest form. In 

 order to standardize the formula for use in human subjects, it was ex- 

 pressed by Ambard as follows: 



Ur 



^z K (constant). 

 O 



25 



Ur = gm. of urea per liter of blood. 

 D = gm. of urea excreted per 24 hours. 

 Wt = weight of individual in kilos. 

 C = gm. of urea per liter of urine. 



This formula is known as Ambard's coefficient, and the value obtained for 

 K by Ambard and Weill in normal human subjects ranged beween 0.060 

 and 0.070. 



The introduction of a standard weight of 70 kilos, a standard con- 

 centration of 25 gm. per liter, and the expression of rate of excretion as 

 the rate per 24 hours are purely arbitrary, and can not affect the general 

 relationship between the variable factors. When these arbitrary factors 

 are kept constant, they, in conjunction with the constant relationship 

 between the four variable factors, tend tq keep K constant, and the actual 

 numerical value of K is determined by these arbitrary additions to the 

 formula. All the observations of the French school are based on the for- 

 mula as thus stated. 



McLean found the normal variations to be somewhat greater than 

 Ambard and Weill, the usual normal being about 0.080. Lewis agrees 

 with McLean in that he believes the coefficient to vary between 0.060 

 and 0.090 although his average is somewhat lower, being 0.074. 



McLean's Coefficient. In order to measure the rate of urea excretion 

 in percentage of the' normal standard, McLean (a) has derived a formula, 

 from Ambard's figures, whose normal value is 100, the maximal range 

 being between 80 and 250. "McLeans index" is as follows: 



Substituting for K (Ambard's coefficient) and simplifying, 



Gm. urea per 24 hrs. VGm. urea per liter of urine X 8.96. 

 Wt. in kilos X (Gm. urea per liter of blood) 2 



When K = 0.080, the standard normal, I (index of urea excretion) = 100. 



