CHAPTER XI 

 RESPIRATION AND THE DISPOSAL OF WASTES 



Respiration. — For the same reason that an engine 

 requires fuel whose oxidation or burning supplies the 

 driving power or energy, so the vital machine depends 

 upon foods and a steady supply of oxygen to oxidize them. 

 The higher animals when deprived of oxygen die in a 

 few minutes; lower forms, like the amoeba, may live for 

 twenty-four hours, but sooner or later protoplasm for- 

 ever ends its activities when deprived of a supply of this 

 substance. Oxygen, therefore, is as essential to the life 

 of an animal or plant as water, salts of various kinds, 

 sugar, and higher compounds usually classed as foods. 



One of the results of the oxidation of cell substances 

 is the formation of carbon dioxid and water that as waste 

 products escape to the exterior. This process, embracing 

 the introduction of oxygen into the cell and the giving 

 off of carbon dioxid gas, is known as respiration. In 

 higher forms, however, where vast numbers of cells are 

 packed together, accessory respiratory structures to carry 

 oxygen to the cells and to remove the carbon dioxid from 

 them are usually present. They comprise such organs 

 as gills or lungs, fitted to absorb the oxygen from the 

 surrounding medium and deliver it to the blood, that in 

 turn transfers it to the cells. Hence, in addition to the 

 cell or internal respiration, there is this accessory, or 

 external, respiration. 



DiFFT'sioN OF Gases. — In every liter of air there are 

 210 cubic contimotors of oxygen, 790 cubic centimeters 

 of nitrogen, and 0.03 cubic centimeters of carbon dioxid 

 gas. In fresh water, every liter contains dissolved in it 



