KIDNEY AND SKIN AS EXCRETORY ORGANS. 773 



may be understood when it is recalled that through this organ are 

 eliminated some of all the end-products formed in the various tis- 

 sues, together with products arising from bacterial fermentation 

 in the gastro-intestinal canal and various more or less foreign sub- 

 stances taken with the food. It is not possible to describe all the 

 numerous constituents that f have been observed. Attention may 

 be directed to those that quantitatively or otherwise are of chief 

 physiological interest. 



The Nitrogen Elimination in the Urine. Nearly all of the ex- 

 cretion of nitrogen occurs in the urine. In the metabolism of the 

 usual foodstuffs carbohydrates, fats, and proteins the end-prod- 

 ucts of their destruction or physiological oxidation in the body are 

 water, carbon dioxid, and nitrogenous waste products (and sulphates 

 and phosphates from the sulphur and phosphorus in the proteins). 

 The water is eliminated in the urine, the sweat, saliva, etc., and the 

 expired air. The CO 2 is eliminated in the expired air, and in smaller 

 part in dissolved form in the secretions (sweat, urine). The nitrog- 

 enous excretion, representing the breaking down of protein material, 

 is found in minute part in the sweat, to a larger extent in the feces r 

 but in by far the main amount in the urine. In all problems con- 

 cerning protein metabolism in the body, both as regards its char- 

 acter and extent, the quantitative study of this excretion is of par- 

 amount importance. In order to determine the total amount of 

 protein metabolism it is customary to determine the total nitrogen 

 eliminated in the urine, without regard to its specific form. This 

 determination is made usually by the method of Kjeldahl. The 

 total weight of nitrogen multiplied by 6.25 gives the amount of pro- 

 tein broken down, since nitrogen forms, on the average, 16 per 

 cent, of the weight of the protein molecule. In an average-sized 

 man the total nitrogen eliminated in a day varies, let us say, between 

 14 and 18 gms., which would correspond to 88 and 117 gms. of pro- 

 tein. It being often necessary to distinguish between the forms in 

 which this nitrogen is eliminated, the following distinctions are made: 

 (1) The urea nitrogen, that is, the nitrogen eliminated as urea. 

 According to some recent analyses by Folin,* the urea nitrogen in 

 man averages 87.5 per cent, of the total nitrogen. (2) The am- 

 monia nitrogen, that is, the nitrogen found in the form of am- 

 monia salts which liberate free ammonia on the addition of a fixed 

 alkali. The proportion of this ammonia nitrogen often varies, 

 especially under pathological conditions affecting the liver. Its 

 quantitative determination is a matter of importance. The aver- 

 age amount in health may be stated (Folin) as 4.3 per cent, of the 

 total nitrogen. (3) The creatinin nitrogen, that is, the amount 

 excreted as creatinin and indicative of a special (muscular) metab- 

 * " American Physiological Journal, " 13, 45, 1905. 



