During expiration, which occurs?

• A. Diaphragm descends
• B. Intra-abdominal pressure decreases
• C. Intra-thoracic pressure decreases
• D. Diaphragm ascends

Answer: (D)

Expiration is driven by the diaphragm relaxing and rising into the thoracic cavity, which reduces the volume of the thoracic space and raises intrathoracic pressure to push air out. The diaphragm descends during inhalation, so that option describes the opposite phase. Intrathoracic pressure actually increases, not decreases, during expiration to drive exhalation. As the diaphragm moves upward, the abdominal contents are compressed, which tends to increase intra-abdominal pressure rather than decrease it. In quiet breathing this upward diaphragm movement is passive, while in forced expiration the abdominal muscles can contribute by pushing the diaphragm higher and boosting intra-abdominal pressure.