During quiet breathing, which action defines inspiration?

• A. Rib cage muscle contraction
• B. Air is drawn from the atmosphere.
• C. Diaphragm relaxes and air leaves the lungs.
• D. Diaphragm pulls air in

Answer: (D)

During quiet breathing, the defining action of inspiration is the diaphragm contracting. When it contracts, it moves downward, increasing the vertical dimension of the thoracic cavity and lowering the pressure inside the lungs. This pressure drop relative to the outside atmosphere pulls air into the lungs. Expiration, by contrast, occurs when the diaphragm relaxes and the lungs recoil, pushing air out. Rib cage muscles can assist by expanding the chest, but the diaphragm’s downward movement is the primary driver of inspiration.