CHAPTER XXXI 

 EXCRETION AND THE EXCRETORY ORGANS 



Exogenous and Endogenous Excreta. It is usual to include un- 

 der the general head of excreta all waste materials of any kind that 

 are given out from the Body. We shall see, however, that under 

 this general definition come two very distinct classes of materials. 

 Many substances are taken into the Body with the food which 

 have of themselves no food value, and escape absorption during 

 the passage of the food through the alimentary tract; these appear, 

 of course, among the excreta. Other substances have an accessory 

 food value, in arousing appetite, or in stimulating some of the 

 bodily processes; these may be absorbed from the alimentary 

 tract into the blood, but they do not enter in any intimate fashion 

 into the metabolic activities of the living tissues, and after a longer 

 or shorter sojourn in the blood they appear among the excreta. 

 The third substances to be grouped with those just described are 

 the ammonia compounds which are split off from the fuel-proteins 

 during absorption. These, from the moment of their separation, 

 are waste products, to be conveyed as rapidly as possible to the 

 excretory organs and gotten rid of. All these excretory materials 

 are grouped together as exogenous excreta, the term suggesting 

 that they are derived from sources outside the actual life processes 

 of the tissues. 



The second group of excreta, the endogenous excreta, includes 

 those substances that are produced by the living cells of the Body 

 in the course of their metabolic activities. Most of our knowledge 

 of cell metabolism has been gained through studies of the en- 

 dogenous excreta. 



The Channels of Excretion. Four channels are recognized 

 through which the body discharges waste materials ; these are : the 

 lungs, the skin, the urinary system, the rectum. The lungs are the 

 channel for the discharge of gaseous wastes, carbon dioxid, and 

 water vapor; the skin and urinary system together dispose of the 

 major part of the endogenous excreta other than gaseous, and 



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