CHAPTER XLI 

 EXCRETION 



Vocabulary 



Urine, the liquid excreted by the kidneys. 



Urea, a nitrogenous substance in the urine, waste. 



Duct, tube which carries excreted or secreted matter. 



Excretion, throwing off of waste. 



Secretion, production of useful substance by glands. 



All the activities of the body require energy, whether in the mus- 

 cles, nerves, or glands. Energy implies oxidation, and oxidation 

 produces waste products which must be removed. The main 

 wastes of the body are carbon dioxide and water and nitrogenous 

 compounds (mainly urea) together with some mineral salts, chiefly 

 sodium chloride (common salt). 



Organs of Excretion. The most important organs of excretion 

 are the kidneys and lungs; then come the intestine, liver, and last, 

 the skin which has other more important functions. 



Kidneys. The kidneys are bean-shaped glands located near the 

 spine at the " small of the back." They are about two by four 

 inches in size and are usually imbedded in fat. Their internal 

 structure is too complicated for description here, but is perfectly 

 fitted for removing from the blood, urea, uric acid, other nitrogen 

 compounds, mineral salts, and water. Their blood supply is very 

 large and under high pressure, which is important in removal of 

 these wastes. As it leaves the kidneys in the renal veins, the blood 

 is actually purer than anywhere else in the body though it may still 

 be dark in color, due to lack of oxygen. 



The ducts from the kidneys lead to the bladder where the urine 

 (which is constantly being excreted) is stored. The amount of 

 urine is usually about three pounds per day and the nitrogenous 



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