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diaphragm

di·a·phragm

/ˈdīəˌfram/

Noun

The diaphragm is the muscle that divides the abdominal cavity from the thoracic cavity. When stimulated by the brain, the diaphragm contracts, increasing the size of the thoracic cavity. This results in a small but significant negative pressure inside the chest, which causes air to rush into the lungs. At the end of this inhalation phase, the brain signals the diaphragm to stop contracting. As the muscle relaxes, the now positive pressure in the chest pushes air out of the lungs.