Final answer:
During expiration, the intra-alveolar pressure in the lungs increases above atmospheric pressure due to muscles like the diaphragm relaxing. This creates a pressure gradient that forces air out of the lungs, equalizing the pressure throughout the conducting division with atmospheric pressure.
Step-by-step explanation:
When air flows out of the lungs and into the atmosphere during expiration, the pressure within the lungs, also known as intra-alveolar pressure, increases to a point where it is higher than the atmospheric pressure. This occurs because muscles such as the diaphragm and the intercostals relax, reducing the volume of the thoracic cavity. As the volume decreases, the pressure inside the lungs rises.
This difference in pressure between the inside of the lungs and the outside environment forces air to flow out of the lungs. The process is driven by a pressure gradient, and under normal conditions, a small difference, as little as 1 to 2 torr, is sufficient to facilitate breathing. Therefore, during expiration the entire conducting division of the air passages becomes equalized with atmospheric pressure allowing air to exit the lungs.