KIDNEY AND SKIN AS EXCRETORY ORGANS. 753 



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 of the physiological oxidation of the proteids of 

 the body, and also of the albuminoids when they appear in the food. 

 If we know how much urea is secreted in a given period, we know 

 approximately how much proteid has been broken down in the 

 body in the same time. In round numbers, 1 gm. of proteid 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 proteid is eliminated in other forms uric acid, creatinin, etc. 

 even an exact determination of all the urea is not sufficient to de- 

 termine with accuracy the total amount of proteid broken down. 

 This fact is arrived at more perfectly, as stated above, by a deter- 

 mination 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 kid- 

 neys by the blood for elimination. 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 ammonium 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 

 proteid 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 investigations have thrown a great deal of light on the whole 

 process, and they give hope that before long the entire history of 

 48 



