In terms of breathing mechanics, expiration is generally considered to be?

Expiration is generally considered a passive process under normal circumstances. During quiet breathing, expiration occurs primarily due to the relaxation of the diaphragm and intercostal muscles. When these muscles relax, the thoracic volume decreases, which causes the pressure in the thoracic cavity to increase relative to the atmospheric pressure. As a result, air is expelled from the lungs without the need for additional muscular effort.

This passive nature is in contrast to active expiration, which can occur during vigorous activities such as exercise or in situations requiring forceful exhalation. During these times, accessory muscles of expiration, like the abdominal muscles and internal intercostals, may engage to actively push air out of the lungs. However, in the context of normal, relaxed breathing, expiration is predominantly a passive process driven by elastic recoil of lung tissues and the relaxation of respiratory muscles.