In the human respiratory system, what is the primary driving mechanism responsible for normal quiet exhalation at rest?
Answer and explanation
Correct answer: A. Passive elastic recoil of the lungs and chest wall
- Step 1: Distinguish active inhalation from passive exhalation during normal breathing.
- Step 2: Normal quiet inhalation requires active diaphragm contraction. Quiet exhalation is purely passive, driven by the relaxation of the diaphragm and elastic recoil of stretched alveolar elastic fibers and thoracic tissues.
Answer
Passive elastic recoil of the lungs and chest wall.
Why not the others: Internal intercostals and abdominal muscles contract only during forced active expiration; atmospheric pressure does not push air out.
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