During expiration, what happens to the diaphragm and intercostal muscles?

• A. They ascend and compress the chest
• B. They contract and flatten
• C. They relax and expand
• D. They remain contracted

Answer: (C)

Expiration is a largely passive process. The diaphragm relaxes and moves upward into a dome shape, and the external intercostal muscles relax so the rib cage drops inward and downward. This relaxation allows the lungs and chest wall to recoil to their resting size, reducing thoracic volume and increasing air pressure inside the lungs to push air out. So the diaphragm and intercostal muscles relaxing—and returning toward their resting length—best describes what happens during expiration. (If expiration is forced, other muscles can actively help push air out, but in quiet breathing it’s primarily a passive recoil.)