Why does air enter the lungs when the chest cavity expands?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

Air enters the lungs during inspiration because of a pressure difference created by the expansion of the chest cavity.12

When the muscles of the chest and abdomen—primarily the diaphragm and external intercostal muscles—contract, they increase the volume of the chest cavity. This expansion increases the space enclosing the air within the lungs, which causes the internal air pressure to fall below atmospheric pressure.12

In accordance with the principles of gas dynamics, air naturally flows from an area of higher pressure to an area of lower pressure. Because the pressure inside the lungs is now lower than the pressure of the outside atmosphere, air is drawn into the lungs until the pressures reach equilibrium.12