CHAPTER V 



RESPIRATION AND EXCRETION 



Food taken into the body represents merely stored or potential energy. 

 To release this energy and make it available for work, the food must 

 be burned — i.e., combined with oxygen; and this must occur within 

 the cells that do the work. Large amounts of oxygen must therefore be 

 taken into the body and distributed to all the cells. The supply must be 

 continuous, for even the energy necessary for merely keeping alive comes 

 from the burning of food, and the cells soon die if deprived of oxygen. 



The oxidation of food materials not only releases energy in the form 

 of heat and work but also gives rise to new chemical substances. Carbon 

 dioxide and water are always produced ; they are the only products of the 

 oxidation of carbohydrates, almost the only ones from the burning of fats, 

 and a part of the results of protein oxidation. Water is useful in the 

 metabolism of the body; but carbon dioxide is a substance that, although 

 necessary in minute amounts, becomes harmful if allowed to accumulate. 

 In addition to water and carbon dioxide, the burning of proteins results 

 in the production of nitrogenous compounds and inorganic salts that are 

 toxic in any except low concentrations. Certain substances, notably 

 salts, may be ingested with the food in amounts greater than can be 

 utilized. There is also to be considered the residue of undigestible mate- 

 rial left in the intestine after absorption of the usable parts of the food. 

 All these useless or harmful substances must in some manner be removed 

 from the body. 



The intake of oxygen and the removal of carbon dioxide — both gases- 

 are provided for by a single mechanism, the respiratory system. The food 

 residues are eliminated by the action of the digestive canal itself. All 

 the remaining metabolic wastes — nitrogenous compounds, excess salts, 

 and other waste substances — are soluble solids and are disposed of in 

 aqueous solution by the operation of the excretory system. 



THE RESPIRATORY SYSTEM 



Oxygen and Carbon Dioxide Exchange 



The taking in of the oxygen needed by the body and the expulsion of 

 the waste carbon dioxide are accomplished by a special respiratory sys- 



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