Coloured 3D computed tomography (CT) scan of a healthy heart (upper left) and lungs (yellow and green). The heart (red) is a hollow sac of muscular tissue that pumps oxygenated blood around the body and deoxygenated blood to the lungs. The lungs (yellow) are either side of the heart. They are the site of gaseous exchange, where carbon dioxide is removed from the blood and oxygen added.