When we breathe in ，why pressure inside the chest become lo

breathe in action:

1. intercostal muscle contract. ribs move upward and outward
2. diaphragm muscle contract. diaghragm flatten
3. the volume of thoraxic cavity incresase.
4. the pressure inside the thoraxic cavity decrease
5. lung inside the cavity expand
6. the pressure inside the lungs decrease
7. the pressure inside the lungs is lower than that in the atmosphere
8. air enters the lungs

It is because the air will not get into our lung when the pressure inside the chest is even higher than atmosphere.