During quiet expiration, which occurs?

During quiet expiration, which occurs?

• A. Diaphragm contracts
• B. Air flows into lungs
• C. Diaphragm relaxes
• D. Rib cage muscles contract

Answer: (C)

Quiet expiration is a passive process driven by elastic recoil of the lungs and chest wall. When the diaphragm relaxes, it returns from a flattened to a dome shape, decreasing the thoracic cavity volume. This drop in volume raises the pressure inside the lungs above atmospheric pressure, pushing air out until pressures equalize. No active muscle contraction is needed for this phase. In contrast, contraction of the diaphragm would increase thoracic volume to draw air in, and rib cage muscles mainly change volume during inspiration, not quiet expiration. So the action happening during quiet expiration is the diaphragm relaxing.

Quiet expiration is a passive process driven by elastic recoil of the lungs and chest wall. When the diaphragm relaxes, it returns from a flattened to a dome shape, decreasing the thoracic cavity volume. This drop in volume raises the pressure inside the lungs above atmospheric pressure, pushing air out until pressures equalize. No active muscle contraction is needed for this phase. In contrast, contraction of the diaphragm would increase thoracic volume to draw air in, and rib cage muscles mainly change volume during inspiration, not quiet expiration. So the action happening during quiet expiration is the diaphragm relaxing.