530 THE EXCRETION OF URINE 



earliest signs of renal insufficiency is now held to be a failure of the 

 kidney to prevent the uric acid of the blood from increasing. Perhaps 

 the reason for the inability of the kidney to excrete uric acid readily 

 lies in the fact that its salts are among the least soluble of those in the 

 urine. It is on this account that when the urine cools, a red sediment of 

 urates containing certain pigments often separates out. 



The uric acid of the urine is possibly derived entirely from the purine 

 metabolism of the body, in which the nucleins either of the body cells or 

 of the exogenous food take part. It is decreased during starvation and 

 increased by eating food rich in nucleins, such as liver and sweet- 

 breads. 



Under ordinary conditions the excretion of uric acid amounts to from 

 0.3 to 1.2 gm. per day (0.02 to 0.10 per cent), the variation being de- 

 pendent upon the state of health, diet, or personal idiosyncrasy. The 

 blood of a normal individual contains on the average 1.8 mg. of uric 

 acid per 100 c.c. The kidneys are therefore able to concentrate the 

 uric acid in the urine from 30 to 60 times over its concentration in the 

 blood plasma. 



The purines found in coffee and tea (caffeine, etc.) are excreted in 

 the urine as salts not of uric acid but of methylated xanthines. 



Hippuric Acid. This is a constant constituent of the urine of her- 

 bivorous animals, and is usually present in small amounts in human 

 urine. The amount rarely exceeds 0.7 gm. a day, but on a diet rich in 

 fruits and vegetables it may exceed 2 gm. It is interesting, since it is 

 the only urinary constituent that is synthesized by the renal cells. 



Ammo acids are always present in small amounts in the urine, con- 

 stituting, according to D. D. Van Slyke, about 1.5 per cent of the total 

 nitrogen. The estimation of the ammo-acid nitrogen of the urine has 

 not been found to be of any clinical significance. 8 



The aromatic oxyacids are normally present in the urine in varying 

 amounts. These include phenol, indoxyl, skatoxyl, and phenylacetic, 

 paraoxyphenyl, propionic, oxymandelic and homogentisic acids. These 

 bodies are derived from phenylamino acids, such as tyrosine, tryptophane, 

 and phenylalanine. It is believed that the putrefactive decomposition 

 of proteins in the large intestine results in the production of these toxic 

 bodies. The body protects itself by oxidizing them and uniting them 

 to sulphuric acid to form the ethereal or conjugated sulphates, w r hich 

 are found in the urine in the form of sodium or potassium salts. The 

 determination of the amounts of these bodies in the urine has therefore 

 been taken as an index of the putrefaction going on within the bowel. 



The chief of these bodies is urinary indican, which is found usually as 

 a potassium salt. The test for indican in the urine consists in oxidiz- 



