The diaphragm is a sheet of muscle that stretches like a dome over the organs of the upper abdomen and serves as a boundary between the abdominal cavity and the thoracic. It separates the lungs from the main body cavity and, together with the ribs, forms a seal that allows the lungs to inflate as part of the process of respiration.
The diaphragm is firmly anchored to the lumbar vertebrae and the inferior ribs of the thorax. As it contracts and relaxes, the diaphragm creates a slight pressure differential between the thoracic cavity and the external air. This partial vacuum draws fresh air into the lungs to participate in gas exchange with the blood. At the end of the cycle of inspiration, the diaphragm rises again to restore pressure and expel the used air from the lungs and back into the environment. At an average respiratory rate of 16 breaths per minute, the diaphragm delivers nearly 1,000 breaths in a single hour and well over half a billion breaths in an average 80-year lifespan. The diaphragm is controlled by two areas of the brain, the pons and the medulla oblongata, which keep it in operation continuously without the need for conscious control.