CHAPTER XVII 

 RENAL EXCRETION 



SECTION I 



THE COMPOSITION AND CHARACTERS OF 



THE URINE 



THE main product of the oxidation of carbon, namely, carbon dioxide, 

 is discharged by the lungs and to a slight extent by the skin. Water, 

 taken as such with the food, but also derived to a slight extent from 

 the oxidation of hydrogen, is got rid of by the lungs, skin, and kidneys. 

 The salts of heavy metals when administered are excreted for the 

 most part by the alimentary canal, e.g. iron, bismuth, mercury. A 

 certain proportion of the pigmentary waste products of the body, 

 derived from the breakdown of the blood-pigment, is also got rid of 

 with the fseces. With these exceptions practically all the waste 

 products resulting from metabolism are excreted in the urine by the 

 kidneys. We have thus to seek in the composition of this fluid the 

 last chapter in the metabolic history of a large number of the con- 

 stituents of the body. Since, moreover, the kidneys may excrete almost 

 any substance which circulates through their blood-vessels, many 

 of the intermediate metabolites may be found in minute quantities 

 in the urine and may be isolated by working up large quantities of 

 this fluid. Under pathological conditions these metabolites may appear 

 in the urine in larger amounts and serve then as an index to a faulty 

 metabolism in which there is some interference with the later stages 

 in the metabolism of fats, carbohydrates, or proteins. 



The composition of the urine must therefore be a variable one, 

 according to the activity of the body, the quantity and nature of 

 the food taken, and the relative amount of water escaping by the 

 kidneys, lungs, and skin respectively. But just as we can describe a 

 normal diet for an adult man of average weight, so we can describe 

 an average composition for the urine. The history of the urinary 

 constituents has been given for the most part in the chapter dealing 

 with the metabolism of the proximate constituents of the food. It 

 will be useful, however, to enumerate in this chapter the various 



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