Body & HealthHow it works
How air gets into you without you sucking it in.
Your diaphragm is a muscular dome beneath the lungs. When it contracts it flattens, the chest cavity grows and the pressure inside drops below the air outside. Air flows in, about 500 millilitres per breath, 12 to 20 times a minute. Exhaling at rest is passive: the diaphragm relaxes and the lungs spring back.
Useful: If your belly rises as you inhale, the diaphragm is working; if only your chest rises, accessory muscles are.
Source: en.wikipedia.org