The lungs are the essential organs of mammalian respiration. Human lungs are paired in the chest with the heart situated in between. The external surface of a lung is smooth and conforms to the shape and size of the thoracic cavity or chest, bounded by the rib cage and below by the movable diaphragm. Inspiration or breathing air rich in oxygen is achieved by enlarging the chest by contracting the diaphragm, and thereby, reducing the external pressure on the lungs, letting them expand. This creates suction, taking in the fresh air. Expelling depleted air, rich in carbon dioxide, is achieved as the diaphragm returns to its resting place and the thoracic cavity is shortened, adding external pressure to the paired lungs and squeezing air out of them.