Final answer:
During quiet breathing expiration, the process is passive. The lung tissue recoils and the thoracic cavity and lungs decrease in volume, causing an increase in intrapulmonary pressure.
Step-by-step explanation:
The process of normal expiration is passive, meaning that energy is not required to push air out of the lungs. Instead, the elasticity of the lung tissue causes the lung to recoil, as the diaphragm and intercostal muscles relax following inspiration.
In turn, the thoracic cavity and lungs decrease in volume, causing an increase in intrapulmonary pressure. The intrapulmonary pressure rises above atmospheric pressure, creating a pressure gradient that causes air to leave the lungs.