The blood that carries oxygen will be pumped from lungs to the left part of the heart (specifically:left atrium, then to the left ventricle) and will go out through the aorta, the blood now is transported into the different cell of the body wherein the gas exchange will happen, the oxygen from the blood will exchange with the waste materials and carbon dioxide of the cell. Oxygen is needed by the cell to produce energy. Then, the blood will come back to the heart carrying the waste materials, the blood will enter the right part of the heart (right atrium then right ventricle) and will be deliver into the lungs back and the carbon dioxide will be breathe out.