CHAPTER IX 

 THE SKIN AND THE KIDNEYS 



-J 276. The Formation of Waste Products. We have learned 

 that the food materials, prepared by the digestive processes, 

 are taken up by the branches of the portal vein, or by the 

 lymphatics, and carried to the tissues to provide them with 

 nourishment. 



We have learned also that oxygen, introduced into the 

 body by the lungs, is being continually carried to the tissues, 

 and that the blood is constantly being purified by being 

 deprived in the lungs of its excess of carbon dioxide. 



From this tissue activity, which is mainly a process of 

 oxidation, are formed certain waste products which, as we 

 have seen, are carbon dioxide, water, and urea, or some 

 closely allied body. 



Remember, then, that carbon dioxide, urea, salts, and water 

 are the chief waste products of the body, and that they are cast 

 out of the body by three main channels, the lungs, the 

 skin, and the kidneys. 



A 277. How the Waste Products are eliminated. The elimi- 

 nation of these waste products is brought about by a special 

 apparatus called the organs of excretion. The worn-out sub- 

 stances themselves are called excretions, as opposed to secretions, 

 which are elaborated for use in the body (see note, p. 99). 



As already shown, the lungs are the main channels for 

 the elimination of carbon dioxide. They also excrete a con- 

 siderable quantity of water as vapor. 



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