The urine removes the soluble products of metabolism from the organism and the respiration eliminates the gaseous products. The two mechanisms combined maintain .the normal reaction of the blood.

In general, the organic constituents of the urine comprise compounds which contain nitrogen. Dakin,2 however, finds that formic acid, H.COOH, is a constant constituent of urine during fasting, and that the quantity is considerably increased after carbohydrate and after fat ingestion, to a lesser extent also after protein ingestion. He interprets this as signifying that all three foodstuffs yield formic acid as an end-product of their metabolisms. Although the production of formic acid may be considerable, it is so readily oxidizable that it is eliminated in only small amounts in the urine.

The study of creatinin and creatin as excretory products was stimulated by the discovery by Folin of a quick and accurate method of determination.

He gave a diet of milk, cream, and carbohydrate which is free from creatinin and the purin bases, and noted the effect of high and low protein content of the diet on the composition of human urine.

One of Folin's3 experiments may thus be tabulated, percentages being rendered in black type (see Appendix, p. 573):

1 Further details, Lusk: "Archives of Internal Medicine," 1915, xv, 939. 2 Dakin, Janney, and Wakeman: "Journal of Biological Chemistry," 1913, xiv, 341.

3 Folin: "American Journal of Physiology," 1905, xiii, 117.

Influence Of High And Low Protein Diets On The Relative Amounts Of The Nitrogenous Constituents Of The Urine

Food.

Composition of the Urine in Grams.

In Grams.

In Calories.

Total

N.

Urea

N.

Ammonia

N.

Uric Acid

N.

Creatinin

N.

Undetermined

N.

Protein, 118 = 19

2786

16.8

14.70

0.49

0.18

0.58

0.85

N.............

Fat, 148..........

87.5%

3.0%

1.1%

3.6%

4.9%

Carb., 225........

Protein, 6 = 1 N .

2153

3.6

2.20

0.42

O.09

0.60

0.27

Fat, 52...........

Carb., 400........

61.7%

".3%

2.5%

17.2%

7.3%

A study of this table will reveal the fact that if a man ingest a diet containing a medium amount of protein, and again one that is nearly free from protein, the difference in the character of the urine in the two cases is almost exclusively due to a difference in the output of urea. The quantity of creatinin eliminated remains independent of the quantity of protein metabolized, and the same thing holds true, as a rule, for uric acid (see p. 540). This led Folin to distinguish between an endogenous protein metabolism which resulted in the constant and even production of creatinin and was a manifestation of cell metabolism, and an exogenous protein metabolism as represented by the urea elimination which is in greater part derived from ingested protein.