Diaphragm

The diaphragm is a domed, internal muscular sheet that separates the abdomen from the thoracic cavity, which houses the heart, lungs, and the ribs. The diaphragm has a very important function in respiration. When the diaphragm contracts, the thoracic cavity increases in volume, lowering internal pressure and causing air to move into the lungs. This is called inspiration and is aided by the contraction of the intercostal muscles and expansion of the rib cage.  At rest, the diaphragm lies at level with the lowest part of the breastbone, also known as the sternum. Its natural dome shape increases the internal pressure and, with aid from ribcage and intercostal muscles as they relax, the air is expelled from the lungs in expiration.