382 EXCRETION 



It has been proven that the kidney does not form urea; in fact the kid- 

 neys may be removed from the body, and urea will continue to accumulate 

 in the blood. Urea is formed chiefly in the liver, but may in part be con- 

 structed in other organs, as described more fully on page 411. It follows 

 that the kidney is only the channel for the elimination of this nitrogenous 

 compound. 



Decomposition of the urea with development of ammonium carbonate 

 takes place from the action of bacteria (micrococcus ureafi) when urine is 

 kept for some days after being voided, which explains the ammoniacal odor 

 then evolved. The urea is sometimes decomposed before it leaves the bladder, 

 when the mucous membrane is diseased and the mucus secreted by it is 

 abundant; but decomposition does not occur unless atmospheric germs have 

 had access to the urine. 



Quantity Excreted. The quantity of urea excreted is, like that of the urine 

 Itself, subject to considerable variation. For a healthy adult about 30 grams 

 \ er day may be taken as rather a high average. Its percentage in healthy 

 urine is from 2 to 2.5. Its amount is materially influenced by diet, being 

 greater on a diet of high proteid content. The quantity of urea excreted 

 by children, relatively to their body-weight, is much greater than by adults; 

 thus the quantity of urea execreted per kilogram of weight was found to be, 

 in a child, 0.8 gram; in an adult only 0.4 gram. Regarded in this way, too, 

 the excretion of carbonic acid gives similar results, the proportions in the 

 child and adult being as 82 to 34. 



Uric Acid. Uric acid, C 5 H 4 N 4 O 3 , is rarely absent from the urine 

 of man or animals, though in the feline tribe it seems to be sometimes entirely 

 replaced by urea. In birds and reptiles uric acid or its salts is the chief 

 form in which nitrogen is eliminated from the body. 



Properties. Uric acid is a colorless, crystalline compound of the purin 

 group, figure 295. It is odorless and tasteless. It is very slightly soluble in 

 water, quite insoluble in alcohol and ether, and freely soluble in solutions 

 of the alkaline carbonates and other salts. 



A study of the elimination of nitrogen in birds, i.e., geese, has shown that 

 uric acid, like urea in mammals, is formed largely in the liver from antecedent 

 proteid nitrogen. In man the elimination of uric acid increases or decreases 

 with the proteid content of the daily diet. It does not, however, follow the 

 variations of the food nitrogen so closely as in the case of urea. Any food 

 with a rich nuclein content increases the excretion of uric acid. This ob- 

 servation has led to the inference that uric-acid nitrogen is derived from 

 nuclear metabolism, page 413. 



Other representatives of the purin group are adenin, guanin, xanthin, 

 hypoxanthin, etc. Chemically, caffeine from coffee is a trimethyl xanthin. 



The most common form in which uric acid is deposited in urine is that 

 of a brownish or yellowish powdery substance, consisting of granules of 



