358 HERMAN O. MOSENTHAL 



the kidney to eliminate urea. This method of estimating renal function has 

 given rise to many investigations ; some of these have devoted themselves 

 to the clinical application of Ambard's coefficient, others to the revision 

 of Ambard's formula. 



Ambard's Coefficient. (Ambard, Ambard and Weill} McLean(a) 

 summarizes "Ambard's laws" regarding urea excretion as follows: 



Ambard's first experiments were with dogs. He found that when the 

 concentration of urea in the urine is constant the rate of excretion varies 

 directly as the square of the concentration of urea in the blood. His first 

 law thus formulated is : 



(Urea in blood) 2 Urea in blood 



^5 * - = Constant, or ======== = Constant 



Kate of excretion y Excretion per unit of time 



(when concentration in urine is constant). 



Secondly, he found, by comparing different experiments in which the 

 concentration of urea in the blood was the same, that the rate of excretion 

 then varied inversely as the square root of the concentration in the urine ; 



Rate of excretion I V Concentration II 

 Rate of excretion II V Concentration I 



This may also be expressed 



Rate I V Concentration I Rate II V Concentration II 



or, in other words, when the blood urea remains constant, the rate times the 

 square root of the concentration in the urine remains constant. Intro- 

 ducing this factor into the first law we have : 



Urea in blood 



i== Constant. 



|/ Rate of excretion V Concentration in urine 



This was shown to hold true for the same animal or for the same individual, 

 or for animals or individuals of the same or nearly the same weight. For 

 individuals of different weight it became necessary to introduce a further 

 modification, in order to make all individuals comparable. This modifica- 

 tion is based on the assumption, supported by the results of experiment, 

 that, other factors remaining constant, the rate of excretion varies di- 

 rectly with the weight of the individual. One assumes that the amount 

 of active kidney tissue and the circulation of blood through the kidneys 

 vary directly with the weight. This may be expressed as 



Rate 



WT . , = Constant (other factors remaining constant). 



