diaphragm

The diaphragm is the broad, dome-shaped muscle that forms the floor of the chest, separating the lungs and heart above from the abdomen below. It is the main engine of breathing: every quiet breath you take is driven mostly by this single sheet of muscle.

When it contracts, the dome flattens and descends, enlarging the chest cavity and lowering the pressure inside, so air is drawn into the lungs — this is inspiration. When it relaxes, it springs back upward and air is pushed out. It is controlled by the phrenic nerves, which arise from the neck (segments C3–C5).

Because it does most of the work of breathing, problems with the diaphragm matter: paralysis from phrenic-nerve injury, flattening from the over-inflated lungs of severe COPD, or splinting from pain or a swollen abdomen all make breathing harder. Hiccups are involuntary diaphragmatic spasms.