Which process is generally considered a passive process?

Expiration is generally considered a passive process during normal, quiet breathing. This is because the primary mechanism of expiration relies on the natural elastic recoil of the lungs and thoracic cavity after inhalation. When the diaphragm and intercostal muscles relax, the lung volume decreases, leading to an increase in intrathoracic pressure which allows air to flow out of the lungs without the need for muscular effort.

In contrast, inspiration is an active process that requires muscular contraction, specifically of the diaphragm and intercostal muscles, to expand the thoracic cavity and decrease pressure within the lungs, causing air to flow in. Forced expiration, which involves the contraction of abdominal muscles and internal intercostal muscles, is also an active process since it requires energy expenditure to push air out forcefully.

Thus, while both expiration and inspiration are components of the respiratory cycle, expiration can occur passively during normal breathing.