n 1 a  :  the placing of air or dissolved gases in intimate contact with the circulating medium (as blood) of a multicellular organism (as by breathing)  b  :  a single complete act of breathing <30 s per minute>   2  :  the physical and chemical processes by which an organism supplies its cells and tissues with the oxygen needed for metabolism and relieves them of the carbon dioxide formed in energy-producing reactions  3  :  any of various energy-yielding oxidative reactions in living matter that typically involve transfer of oxygen and production of carbon dioxide and water as end products <cellular >  .
 
 
 
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