The diaphragm is a sheet of muscle that pulls air into the lungs.

The longer version

Breathing depends on a dome-shaped sheet of muscle separating the chest from the abdomen. When it contracts, the dome flattens and moves downward, increasing the volume of the chest cavity. Because the space around the lungs is sealed, that expansion lowers the pressure inside, and air flows in from outside to equalise it.

When the muscle relaxes, the elastic tissues of the lungs and chest wall recoil, pressure rises again, and air is pushed out. The muscle is controlled automatically by nerves from the neck region, though it can also be driven deliberately for speaking or holding breath. Rib muscles assist during exertion, lifting the rib cage to increase the volume change.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.