Where does impure blood from heart go?

The premise of your question is not accurate. The heart does not pump impure blood. Oxygen-poor blood returns to the heart from the body's tissues via the superior and inferior vena cava. This blood is not "impure" but needs to have its oxygen replenished by flowing to the lungs for gas exchange. From the lungs, oxygenated blood flows back to the heart through the pulmonary veins and is then redistributed by the left side of the heart to circulate through the systemic arteries and carry oxygen to all the body's tissues.