Double Circulation of the Blood refers to the passage of the blood firstly through the lungs (the pulmonary circulation – where it picks up oxygen and releases carbon dioxide) and then through the body (the systemic circulation) where it delivers its cargo of oxygen and picks up carbon dioxide.
These two circuits are powered by different sides of the heart. The right side of the heart pushes the blood at relatively low pressure through the lungs. The left side of the heart pushes the blood at relatively high pressure through the whole body.