During quiet inspiration, which occurs?

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

Answer: (C)

Quiet inspiration is driven by the diaphragm contracting. When the diaphragm contracts, it moves downward and flattens, increasing the volume of the thoracic cavity. This expansion lowers the pressure inside the lungs (intrapulmonary pressure) below atmospheric pressure, so air flows into the lungs. The rib cage can expand as well because the external intercostal muscles assist, but the initiating action is the diaphragm’s contraction. If the diaphragm relaxed, expiration would begin.