CHAPTER XVII 

 EXCRETION 



Excretion may be defined as the process by which the end-products of 

 tissue metabolism are removed from the body, the nature of the process, how- 

 ever, differing in no essential particulars from that underlying the process 

 of secretion as stated in preceding chapters. The histologic structures 

 involved and the forces at work being of the same general character, it is 

 impossible to draw any sharp line of distinction between them. As a general 

 fact it may be'stated, that by virtue of their composition all the characteristic 

 ingredients of the excretions are incapable either of entering into the forma 

 tion of tissue or of undergoing oxidation for the purpose of heat-production. 

 As the retention of these end-products in the body would exert a deleterious 

 influence on normal metabolism, their prompt removal becomes essential to 

 the maintenance of physiologic activity. The principal excretions of the 

 body urine, perspiration, and bile are, with the exception of those given 

 off in the lungs, complex fluids in which are to be found in varying propor- 

 tions the chief end-products of metabolism. 



THE URINE 



The urine is a fluid formed by the activities of the kidneys and is character- 

 ized by well-marked physical properties and a complex chemical composi- 

 tion. After its formation in the kidneys it passes through the ureters into 

 the bladder, where it is temporarily retained before being discharged from 

 the body. 



Physical Properties. Normal urine has a pale yellow or amber color, 

 an aromatic odor, an acid reaction, and a specific gravity of 1.020. As a 

 rule, it is perfectly transparent, though its transparency may be diminished 

 from the presence of mucus, calcium and magnesium phosphates, and mixed 

 urates. 



The color, which varies within physiologic limits from a pale yellow to a 

 reddish-brown, is due to the presence of the coloring-matters, urochrome, 

 urobilin, and uroerythrin, all of which are derivatives of the bile pigments ab- 

 sorbed from the liver or the alimentary canal. 



The odor of the urine is characteristic and due to the presence of volatile 

 organic compounds. 



The reaction of the urine is generally acid to litmus. The reaction has 

 been for a long time attributed to the presence of sodium dihydrogen phos- 

 phate, though it is more probably due to the presence of various acid radicals. 

 The intensity of the acidity will depend on the extent to which any particular 

 acid is dissociated with the liberation of the hydrogen ions as it is the latter 

 factor that imparts acidity to the fluid. The degree of acidity, however, 

 varies at different periods of the day. Urine passed in the morning is strongly 

 acid, while that passed during and after digestion, especially if the food be 



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