KIDNEY AND SKIN AS EXCRETORY ORGANS. 853 



products give an index of the amount of nucleic acid metabolized 

 in the body, and the nitrogen thus eliminated may be given an 

 average value of 1 to 2 per cent, of the total nitrogen. 



5. The Amino-acid Nitrogen. * Some of the nitrogen is excreted 

 in the form of amino-acids, either free or combined. An example 

 of the combined form is the compound hippuric acid, which 

 consists of benzoic acid in combination with amino-acetic acid 

 (glycin). 



6. Unknown Nitrogen. Certain nitrogenous substances are 

 excreted in the urine whose structure and physiological significance 

 are as yet undetermined. Among these substances are the so- 

 called oxyproteic acids which contain both nitrogen and sulphur. 

 They are derived presumably from the metabolism of proteins, but 

 the special significance to be attributed to them cannot be stated. 

 The unknown nitrogen may constitute as much as 3 to 5 per cent, 

 of the total nitrogen. 



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 of the foods and the tissues. 

 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 appreci- 

 able as much, for instance, as 0.8 gm. although such a large 

 amount is found only after active exercise. Urea is present 

 normally in the blood in an amount equal to 27 to 28 mgms to 100 

 c.c. of blood (0.028 per cent.), and it has been shown that the tissues 

 generally contain the urea in about the same concentration, f 

 In fact, the urea diffuses into the tissues with great ease, and when 

 for any reason the concentration in the blood varies the amount in 

 the tissues rises or falls in a corresponding way. The kidneys 

 constantly remove the urea as it is formed, and by their secretory 

 activity the amount of urea in the blood is kept normally at the 

 low level given above. When the kidneys are removed the amount 

 of urea in the blood rises, showing that this substance is formed 

 elsewhere in the body. The history of the formation of urea in the 



* Henriques and Sorenson, "Zeitschrift fur physiol. Chemie," 64, 128, 1910. 

 t Marshall and Davis, "Journal of Biological Chemistry," 18, 53, 1914. 



