CHAPTER XXXI 

 EXCRETION AND THE EXCRETORY ORGANS 



Exogenous and Endogenous Excreta. It is usual to include 

 under 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 ma- 

 terials. 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; thes^ 

 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 nitrogen containing compounds which are split off from the 

 fuel-proteins in the process of deaminization. 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 eliminates a part of the water and traces of 

 the nitrogenous excreta; the urinary system disposes of the major 

 part of the endogenous excreta other than gaseous, and also of 



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