Final answer:
Air is expelled from the lungs when intrapulmonary pressure is greater than atmospheric pressure, due to the natural decrease in lung volume during expiration.
Step-by-step explanation:
Air moves out of the lungs when the intrapulmonary pressure, or pressure inside the lungs, is greater than atmospheric pressure. This occurs because air flows from a region of higher pressure to one of lower pressure. During expiration, the muscles of respiration and the elasticity of the lung tissue cause the lungs to decrease in volume, which increases the intrapulmonary pressure above the atmospheric pressure, leading to air being pushed out of the lungs.