14 



Tlie By-Prociucts 



or MetaDolisni— Excretion 



Excretion keeps a balanced content in the internal environment of the body. 

 This content is continually tipped between income, expenditure, and re- 

 mainder, between too little and too much. Food and water furnish the income; 

 activity with respiration is the expenditure; excretion removes the remainder. 

 Altogether this is metabolism, the continual buildup and breakdown that 

 liberates energy and leaves an ordinarily useless remainder. 



Residues must be thrown out of the body because they are in the way and 

 even poisonous. The excretory organs carry on these processes; they are the 

 regulators of body content, keeping water, gases, salts, and other substances 

 from increasing beyond an essential standard. Excretion maintains a chemical 

 balance in the internal environment of the body; it includes separation, collec- 

 tion, and elimination of undesirable substances. The excretory organs of verte- 

 brates are the gills, lungs, liver, and the kidneys, also called renal organs 

 (L., ren, kidney), and in lower animals nephridia (Gr., nephros, kidney). The 

 responsibility for maintaining the delicate, complex adjustments of the blood 

 rests mainly with the liver and kidneys, the latter being the chief excretory 

 organs. 



All living cells give off by-products of the chemical reactions that take place 

 within them. Since every cell surface is capable of excretion, this occurs 

 whether the animal has kidneys and other excretory organs, or no excretory 

 organs, as in hydra (Fig. 14.1). Except for the contractile vacuoles the struc- 

 tural arrangements of excretory organs are basically similar and the chemicals 

 excreted are the same. The oxidation of carbon frees energy and creates an end 

 product of carbon dioxide; most, but not all, of this is excreted in the gills or 

 lungs. Almost all excreted hydrogen is in the form of water. Nitrogen from the 



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