A broad sheet of skeletal muscle separates the chest from the abdomen and rests in a dome beneath the lungs. When it contracts, the dome flattens and drops. The chest gets larger, pressure falls, and air rushes in.
Relax, and the dome rises again. Air leaves. That cycle is most of quiet breathing. The rib muscles help, but the diaphragm does the main work.
You can also aim it on purpose, which is why a singer or a sigh can change the breath. Most of the time it simply keeps the rhythm without asking.
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