Break it down to a chemical equation that demonstrates the difference. C + O2 ----> (1)CO2. Hence we are adding a carbon atom to an O2 molecule, such that the body is emitting carbon bound to O2. In reality when CO2 remains disolved in blood for too long it converts to H2CO3 - carbonic acid. This is why EMS used to give sodium bicarb IV to hypercarbic patients particularly in the setting of cardiac arrest.