774 PHYSIOLOGY OF DIGESTION AND SECRETION. 



olism (3.6 per cent, of total nitrogen). (4) The purin body nitrogen 

 (uric acid, xanthin, hypoxanthin), also indicative of a special metab- 

 olism. 



Origin and Significance of Urea. Urea has the formula, CO- 

 N 2 H 4 . It may be considered as an amid of carbonic acid, and 



has, therefore, the structural formula of CO<^ NH 2 . It occurs in the 

 urine in relatively large quantities (2 per cent.). As the total quan- 

 tity of urine secreted in twenty-four hours by an adult male may 

 be placed at from 1500 to 1700 c.c., it follows that from 30 to 34 

 gms. of urea are eliminated from the body during this period. It 

 is the most important of the nitrogenous excreta of the body, the 

 chief end-product, so far as the nitrogen is concerned, of the phys- 

 iological metabolism of the proteins and the albuminoids of the 

 foods arid the tissues. If we know how much urea is secreted in a 

 given period, we know approximately how much protein has been 

 broken down in the body in the same time. In round numbers, 

 1 gm. of protein will yield J gm. of urea, as may be calculated easily 

 from the amount of nitrogen contained in each. Since, however, 

 some of the nitrogen of protein is eliminated in other forms uric 

 acid, creatinin, etc. even an exact determination of all the urea 

 is not sufficient to determine with accuracy the total amount of 

 protein of all kinds that has been metabolized. This fact is arrived 

 at more perfectly, as stated above, by a determination of the total 

 nitrogen of the urine and other excretions. In addition to the urine, 

 urea is found in slight quantities in other secretions in milk (in 

 traces) and in sweat. In the latter liquid the quantity of urea in 

 twenty-four hours may be quite appreciable as much, for instance, 

 as 0.8 gm. although such a large amount is found only after active 

 exercise. It has been ascertained definitely that urea is not formed 

 by the kidneys; it is brought to the kidneys by the blood for elimi- 

 nation. That urea is not made in the kidneys is demonstrated 

 by such facts as these : If blood, on the one hand, is irrigated through 

 an isolated kidney, no urea is formed, even though substances (such 

 as ammomium carbonate) from which urea is readily produced are 

 added to the blood; on the other hand, urea is constantly present 

 in the blood (0.0348 to 0.1529 per cent.), and if the two kidneys 

 are removed, it continues to accumulate steadily in the blood as 

 long as the animal survives. It has been ascertained that the urea 

 is produced in part in the liver. The most important questions 

 to be decided are: Through what steps is the protein molecule 

 metabolized to the form of urea? and What is the antecedent 

 substance brought to the liver, from which it makes urea? It is 

 impossible to. answer these questions perfectly, but recent investi- 

 gations have thrown a great deal of light on the whole process, 

 and they give hope that before long the entire history of the deriva- 



