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 faeces. 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 

 constituents 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 some inter- 

 ference 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 

 constituents of the urine and to summarise their properties, preparation, 

 and normal significance. 



The urine of man is a clear yellow fluid which froths when shaken. On 

 standing, a cloud of mucus is deposited, consisting of a very small amount 



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