Lungs and heart, coloured chest X-ray. The lungs and heart are enclosed and protected by the ribs (horizontal bars) which meet at the breastbone, or sternum (down centre). The heart (red) is partly seen protruding from behind the sternum at lower right. The lung airways (green) branch out into the rest of the chest cavity. The lungs bring air close to blood capillaries, allowing oxygen to be taken up by the body, and carbon dioxide to be released. The heart pumps blood around the body.