CHAPTER XXIV. 



THE FLUID EXCRETIONS. 



The Excretion of Urine. 



The Composition of the Urine. The waste substance result- 

 ing from the processes of metabolism in the tissues are eliminated 

 from the body in a gaseous, fluid, or solid state. With the excep- 

 tion of the carbon dioxide and water of the expired air, and cer- 

 tain substances which are excreted into the intestines or appear 

 in the secretions of the skin glands, the metabolic products are 

 eliminated in the urine. 



The composition of the urine is therefore rather complex and 

 varies greatly with the nature of the food and the amount of 

 water taken. By careful analysis of the urine from a number of 

 individuals on ordinary diet, the average amount of the various 

 constituents in what may be considered a normal urine can be 

 estimated. Fresh human urine is a clear yellow fluid, a little 

 heavier than water, having a specific gravity of 1.016 to 1.02. If 

 tested with litmus paper it usually shows an acid reaction, which 

 is mainly due to the presence of acid salts, such as sodium dihy- 

 drogen phosphates, but partly also to acid substances derived 

 from proteins. Herbivorous animals secrete an alkaline urine, 

 which is no doubt caused by the presence of the large amount of 

 alkaline earths and the relatively small amount of protein mat- 

 ter in their diet. The human urine becomes alkaline in reaction 

 when vegetables are the main ingredients of the diet. 



The character of most of the urinary constituents and the man- 

 ner by which they are derived from the foodstuffs have been de- 

 scribed in the chapter on metabolism, and in the following ac- 

 count only a brief review of their physical and chemical nature 

 is necessary. 



THE ORGANIC SUBSTANCES OP THE URINE. These comprise a 

 number of nitrogenous compounds. The following figures, ob- 



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