CHAPTER XVI. 

 EXCRETION. 



As stated in the preceding chapter, the term excretion is limited to 

 the process by which the end-products of tissue metabolism are re- 

 moved from the body, the nature of the process, however, differing 

 in no essential particulars from that underlying the process of secre- 

 tion. 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 in their composition all the characteristic ingredients 

 of the excretions are incapable either of entering into the formation 

 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 

 complex fluids in which, with the exception of those given off in the 

 lungs, are to bejound in varying proportions the chief end-products 

 of metabolism. 



THE URINE. 



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 = n5i^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 urobilin, urochrome, and uroerythrin, all of which are de- 

 rivatives from the bile pigments absorbed from the liver or the 

 alimentary canal. 



The reaction of the urine is acid, owing to the presence of the acid 

 phosphates of sodium and calcium. 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, espe- 

 cially if the food be largely vegetable in character and rich in alkaline 

 salts, is either neutral or alkaline in reaction. The diminished acidity 

 after meals is attributed to the formation of hydrochloric acid by the 



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