Oxygen is the gas required by our body cells to break down glucose to produce energy by the mitochondria which r transported by the RBC's from the lungs to the tissues and co2 is form due to the oxidation of the  glucose and is removed by the rbc's by transporting it from the tissues to the lungs to be exhaled
 meghachaddha05 are u satisfied wid dis answer ???
Actually I am not sure of the answer
But we breathe in various gases which includes  O_2 and we breathe out all the unnecessary gases which are not needed
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