During expiration, which action occurs?

• A. The diaphragm contracts and moves downward
• B. The rib cage expands
• C. The lungs exhale air
• D. The diaphragm relaxes and moves upward

Answer: (D)

During expiration the body decreases the size of the thoracic cavity to push air out of the lungs. The key action is the diaphragm relaxing and moving upward, which reduces vertical space in the chest and raises the pressure inside the lungs, forcing air to flow out. At rest, expiration is largely a passive process driven by this elastic recoil of the lungs and chest wall. The rib cage typically moves inward and downward during expiration, not outward or upward, and the act of the lungs exhaling air is a result of these pressure changes, not the primary muscular action.