562 THE EXCRETION OF 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 may be the value of the formula in expressing the relation- 

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

 this substance in the blood, it is certain that, in diseased conditions where 

 there is impairment of the kidney the concentration of urea in the 

 blood remains permanently at an abnormally high average level, al- 

 though the amount of urea excreted during twenty-four hours may bo 

 exactly the same as under normal conditions. Probably the increased con- 

 centration of urea in the blood under these conditions is a compensatory 

 measure to provide sufficient pressure to cause its excretion through a 

 damaged outlet. It is this increase in iirea of the blood which is indicated 

 by the term urea retention in nephritis. 



The upper limit of blood urea-nitrogen is about 20 mg. per 100 c.c., 

 which would correspond to about 0.45 gm. of urea per liter of blood. 

 The average figure is half of this amount. The maximum concentration 

 of urea in the urine is seldom over 8 per cent, On this basis the kidney 

 can raise the concentration of the urea in the urine, at a conservative 

 estimate, from 100 to 200 times. Normally the daily output of urea 

 nitrogen may range from 8 to 12 gm., and the nitrogen which it contains 

 is roughly 80 per cent of the total excretion for the day. 



Ammonia. The chief source of ammonia in the body is the ni- 

 trogenous portion of the deamidized ammo acids. The ammonia found 

 in excess in the portal blood is derived from ingested ammonium salts 

 and from ammonia resulting from bacterial action on proteins in the 

 intestinal tract. The ammonia of the body is present chiefly in the form 

 of ammonium carbonate, and it is this salt that is the precursor of urea. 

 Because ammonium carbonate is so readily converted into urea by the 

 tissues of the body, little ammonia is normally present in the systemic 

 blood. The greater portion of the ammonia that finds its way into the 

 urine serves as a base to transfer acid radicles either invested or formed 



*Ambard and Weil's formula is: 



Ui 



70 

 D x x 



P V25 



K r= 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 r= weight of the patient. 

 C = grams of urea per liter of urine. 

 70 r= 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. 3 



