I think that it's when our body uses energy and so cellular respiration occurs. Cellular respiration is when our cells use the food we ate to create energy. Basically carbon dioxide is made when cells use nutrients to make energy.
Carbon dioxide is a waste product of cell metabolism.
I think it's more that it exhausts it. Waste gas from lungs. In with the good air out with the bad.transfer of trash from the blood to the lungs and out from the mouth/nose.