What primarily causes expiration in the respiratory system?

Expiration in the respiratory system is primarily caused by the elastic recoil of the lungs and thoracic wall. During inhalation, the diaphragm and intercostal muscles contract, expanding the thoracic cavity and drawing air into the lungs. Once inhalation occurs and the muscles relax, the elastic properties of the lung tissue, coupled with the elasticity of the thoracic wall, allow the lungs to return to their resting state. This recoil is a passive process in which the lungs push air out of the respiratory tract.

While the relaxation of the diaphragm is a component of the expiration process, it is not the primary mechanism driving the actual expulsion of air. Instead, it is the inherent elasticity of the lung tissue that is essential for efficient expiration. Air pressure differences certainly play a role in ensuring air moves in and out of the lungs, but they are not the primary cause of expiration itself. Muscle contraction, during inhalation, facilitates the process of breathing, but it does not directly account for the process of expiration once the air has been inhaled. Thus, the elastic recoil of the lungs and thoracic wall is the central factor driving expiration.