364 HERMAN O. MOSENTHAL 



partly by direct experiment and partly by plotting curves, established 

 the normal threshold value for sodium chlorid as 5.62 gm. per liter of 

 plasma. Therefore the sodium chlorid above 5.62 gm. per liter determines 

 the rate of excretion, and the law may be expressed as for urea. 



Excess NaCl over 5.62 gm. per liter of plasma 



NaCl in 24 hrs. 



= Constant. 



TTT= : V^aCl per liter of urine 



Wt. in kilos 



For practical use it appears best to calculate the plasma sodium chlorid 

 from the rate of excretion, and to compare the calculated concentration 

 with that actually found. The formula, as derived with the use of values 

 actually found for the constant in the above formula, reads 



Blasma NaCl = 5.62 + 



(The symbols have the same meaning as in the urea formulas. See sec- 

 tion on the coefficient of urea excretion). 

 This, in its simplest form, reads 



Plasma NaCl = 5.62 + t / Gm - NaC1 P er 24 hra - V Gm " NaC1 ? er liter of urine ' 



4.23 X Wt. in kilos 



The constancy of this formula depends on two factors: (1) the con- 

 stancy of the threshold, and (2) the constancy of the rate of excretion of 

 sodium chlorid above the threshold. In their original contribution Am- 

 bard and Weill believed that the threshold was quite constant in normal 

 individuals. Later they recognized some variation in the threshold in 

 normal individuals, though this variation seems to be relatively slight. 

 Assuming that the laws for rate of excretion of sodium chlorid over the 

 threshold remain constant in normal individuals, one may calculate the 

 threshold by subtracting the calculated excess from the sodium chlorid 

 actually found in the plasma, and our figures for the variations in the 

 threshold are based on the following formula : 



Threshold - = Plasm i NaCl M ' V ( 



t.23 Wt. 



This formula is subject to error if the rate of excretion over the threshold 

 varies. We have not yet found that the urea excretion gives any basis 

 for estimating the rate of sodium chlorid excretion, and accordingly have 

 not used Ambard's combined formula for calculating the threshold of 

 sodium chlorid excretion. Our figures on the threshold are only to be re- 

 garded as approximate, since we have so far no means of recognizing 

 variations in rate of excretion over the threshold. 



The principles of the laws of urea and chlorid excretion may be illus- 



