The mechanics of breathing involve changing the volume and pressure of the thoracic cavity. By using the principles of Boyle's Law, one can see that the pressure in the thoracic cavity is inversely proportional to its volume. When the intercostal muscles contract the ribs are elevated. At the same time the diaphragm contracts. These events expand the thoracic cavity, decreasing its internal pressure. The lungs expand, filling the thoracic cavity. The resulting pressure in the lungs is lower than that outside the body. Air enters the lungs until equilibrium is reached. When the diaphragm and the intercostal muscles relax, the thoracic cavity recoils. The resulting increase in pressure causes the air within the lungs to be expelled.