What gas does the body need for the heart and lungs?

Heart has two sides (4chambers) separated by a septum.One side it collects the impure blood ie from all the parts of our body and pumps it to lungs. At lungs through alveoli the co2 diffuses out and oxygen enters the blood. this pure blood (oxygenated blood) enters the other side of heart from where it is pumped to all body parts AND cycle continues.