CHAPTER XVIII. 

 EXCRETION. 



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

 process by which the end-products of tissue metabolism are removed from 

 the body, the nature of the process, however, differing in no essential particu- 

 lars from that underlying the process of secretion. 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 ingredi- 

 ents 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, 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. 



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 urobilin, 

 urochrome, and uroerythrin, all of which are derivatives of the bile pig- 

 ments absorbed from the liver or the alimentary canal. 



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

 phates 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, especially 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 gastric glands and the consequent liberation of 

 bases which are excreted in the urine. The phosphoric acid which enters 

 into combination with sodium and potassium bases is a product of tissue 

 metabolism. 



The specific gravity is about 1.020, though it varies from 1.015 to i-Q 2 5- 

 It will diminish, other things being equal, with increased consumption of 

 water and diminished activity of the skin; it will be increased of course by 

 the opposite conditions. 



The quantity of urine excreted in twenty-four hours varies from 1200 to 



470 



