How does passive expiration occur?

Passive expiration occurs primarily through the relaxation of the diaphragm. During the process of inhalation, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and allowing air to flow into the lungs. When the diaphragm relaxes, it returns to its original dome shape, which decreases the volume of the thoracic cavity. This change in volume increases the pressure within the lungs relative to the atmospheric pressure, causing air to flow out of the lungs naturally, without the need for additional muscular effort.

While other options involve muscle actions that relate to the mechanism of breathing, they do not primarily facilitate passive expiration. The contraction of the intercostal muscles is more relevant during active expiration, when forceful exhalation may be needed. The utilization of abdominal muscles also plays a role in forced expiration but is not needed for passive expiration. External pressure in the lungs is a misleading concept since passive expiration relies on the pressure gradient created by the relaxation of the diaphragm and not external pressure acting on the lungs directly.