CHAPTER XVII 



WASTE PRODUCTS. EXCRETORY ORGANS; DESCRIPTION OF THE 

 ORGANS CONSTITUTING THE URINARY SYSTEM; GENERAL 

 CHARACTERS OF URINE; COMPOSITION OF URINE 



In the previous chapters we have seen that tlie blood is con- 

 stantly supplied by means of the respiratory and digestive mechan- 

 isms, with all the chemical substances it requires to maintain the 

 life, growth, and activity of the body. These substances, entering 

 the current of the blood, are carried to all the tissues, and are in- 

 cessantly combining with the chemical substances of which these 

 tissues are composed. One of the results of these chemical com- 

 binations is the formation of waste products, which must be re- 

 moved from the body, as many of them are toxic. 



WASTE PRODUCTS 



The principal waste products formed in the body are urea, uric 

 acid, creatinin, hippuric acid, carbon dioxide, and other organic 

 substances ; inorganic salts and water. Waste products are classed 

 as excreta and the process by which they are removed from the 

 body as excretion or elimination. 



EXCRETORY ORGANS 



The organs whose sole function is the elimination of waste 

 products are the organs of the urinary system. These are called 

 the excretory organs and consist of the following : — 



2 Kidneys, which form the urine from materials taken from the 



blood. 

 2 Ureters, ducts which convey the urine away from the kidneys. 

 1 Bladder, a reservoir for the reception of urine. (See Fig. 120.) 

 1 Urethra, a tube through which the urine passes from the 



bladder and is finally voided. 

 Other organs that assist in the process of elimination are : (1) the 

 lungs, (2) the skin, (3) the liver, and (4) the intestines. 



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