Healthy heart. Coloured X-ray of a chest showing a healthy heart (red, lower centre). The heart lies between the lungs (black spaces), which are themselves enclosed by the ribs (blue bands). At the front of the ribcage is the sternum, a hard bony plate that further protects these organs. Below the heart and lungs is the diaphragm, a muscular sheet whose contraction and relaxation allows air to be drawn into and expelled from the lungs respectively. The heart is a muscular organ that is responsible for pumping blood around the body. The blood picks up oxygen from the lungs before returning to the heart to be sent to the rest of the body.