CHAPTER 12 

 DISPOSAL OF WASTES 



Substances which cannot be built up into protoplasm, or do not 

 yield energy when decomposed, or do not act as vehicles for important 

 substances, or do not stimulate cells to activity can be of little use to 

 animals. Such substances must be eliminated if they are incidentally 

 acquired, as are the indigestible parts of various foods, or if they are 

 produced as a consequence of physiological processes. Indigestible 

 portions of objects taken in as food are removed as feces by the digestive 

 tract itself. Those which result from the life processes are thrown off 

 by the general process of excretion. It is only the latter group, the wastes 

 which originate within the organism, that are dealt with in this chapter. 



Origin of Wastes. — Since oxidation (page 37) is the main source 

 of energy in living things, some of the principal wastes result from that 

 process. Carbon is abundant in all protoplasm and in all the classes of 

 organic foods (proteins, carbohydrates, lipids). Oxidation of these 

 things results therefore in quantities of carbon dioxide (CO2). This sub- 

 stance, as previously explained, is very stable and contains very little 

 potential energy, besides being toxic in large quantities; hence it is waste 

 matter. Water must be taken in as a vehicle for other substances, but 

 in larger quantities than can be retained; the excess is waste. Destruc- 

 tion of proteins, whether those of protoplasm or unutilized food, must 

 yield some nitrogenous wastes, the principal one being urea. There are 

 minor substances of many kinds, but these three — carbon dioxide, water, 

 and urea — form the bulk of the material that has to be removed. 



Gaseous Wastes. — The removal of carbon dioxide has already been 

 mentioned (page 119) as part of the process of respiration. Cells accumu- 

 late quantities of this substance as a result of their own oxidations and in 

 man usually contain it at a pressure equivalent to about one-fifteenth of 

 an atmosphere, or more. Since this pressure is double the pressure of 

 the same substance in the blood of the capillaries, carbon dioxide diffuses 

 from the cells into the blood. In the lungs, the pressure of the carbon 

 dioxide in the blood is distinctly greater than in the air of the lungs; 

 hence diffusion is outward. Gills operate in the same way as lungs, but 

 the differences in pressure are smaller; hence the rate of elimination of 

 carbon dioxide is slower. 



Small quantities of other gases, especially those arising from bacterial 

 action in the intestine, or from defective digestion, are also removed by 



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