KIDNEY AND SKIN AS EXCRETORY ORGANS. 851 



secretory activity of the kidney serves to maintain a normal bal- 

 ance between the acid and basic equivalents in the blood, and the 

 fact that on an ordinary mixed diet the urine has an acid reaction 

 indicates that the acids formed in the body during metabolism 

 must exceed the bases. It is evident from this consideration that 

 the kidneys take an important part in maintaining the substantial 

 neutrality of the blood. The kidneys and the lungs co-operate in 

 this function, the former by the secretion of the excess of acid salts 

 and organic acids or the excess of bases, as the case may be, the 

 latter by the elimination of carbon dioxid. 



The composition of the urine is very complex. In addition to 

 the water and inorganic salts the following elements are important, 

 namely, urea, the purin bodies (uric acid, xanthin, hypoxanthin), 

 creatinin, hippuric acid, oxalic acid (calcium oxalate), several 

 conjugated sulphates and conjugated glycuronates, several aromatic 

 oxyacids and nitrogenous acids, fatty acids, dissolved gases (N 

 and CO 2 ), and the urinary pigments urochrome and urobilin. This 

 list is not complete; a number of additional substances have been 

 described as occurring constantly or occasionally in traces within 

 the limits of health, and some substances are secreted whose compo- 

 sition is unknown. Under pathological conditions the composition 

 may be still further modified. The complexity of the composition 

 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 have been observed. Attention may 

 be directed to those that quantitatively or otherwise are of chief 

 physiological interest. 



The Nitrogen Elimination in the Urine. In the metabolism of 

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

 products 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 C02 is eliminated in the expired 

 air, and in smaller part in dissolved form in the secretions (sweat, 

 urine) . The nitrogenous excretion, representing the breaking down 

 of protein material, is found in minute part in the sweat, to a larger 

 extent in the feces, but in by far the main amount in the urine. 

 In all problems concerning protein metabolism in the body, both 

 as regards its character and extent, the quantitative study of this 

 excretion is of paramount importance. In order to determine the 

 total amount of protein metabolism it is customary to determine 



