528 THE EXCRETION OF URINE 



amount of urea excreted during twenty-four hours may be exactly the 

 same as under normal conditions. Probably the increased concentration 

 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 urea of the blood which is indicated by the 

 term urea retention in nephritis. 



It must not be lost sight of, however, that the approximate constancy 

 of the combined formula is due in large part to the mathematical con- 

 struction, and also to the fact that any increase in the concentration of 

 urea in the blood is usually accompanied by an increased rate of urea 

 excretion. The factors which are most variable occur as the square or 

 the square roots of their values, and thus the disturbing effect they pro- 

 duce on the constancy of the resultant of the formula is greatly re- 

 duced, while the most constant factor, the concentration of urea in the 

 blood, is used with modification. In such a complex mechanism as the 

 renal function it is very probable that other factors are of great im- 

 portance in controlling the rate of urinary excretion. Many of these 

 factors can not admit of mathematical expression. The writer seriously 

 doubts the advisability of adopting an empirical formula as a means 

 of expressing unknown physiological laws. Such measures are apt to 

 give a sense of knowledge altogether false, and thus hinder research 

 progress. 



The upper limit of blood urea-nitrogen is about 20 jjfrg. 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 from the ni- 

 trogenous portion of the deamidized amino 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 ingested or formed 

 within the body. The amount of ammonia in the urine, therefore, is an 

 indirect measure of the extent of urea formation and of the acid bodies 



