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