Mechanics of Passive Expiration

Steps:

• Expiration is the exhalation phase where air leaves the lungs.
• The diaphragm relaxes, moving upward to its resting position.
• This reduces the thoracic cavity volume.
• Decreased volume raises intrapulmonary pressure above atmospheric pressure, expelling air.

Why B is correct:

• Diaphragm relaxation decreases thoracic volume, increasing pressure per Boyle's law (pressure inversely proportional to volume), then pressure falls as air exits.

Why the others are wrong:

• A: Diaphragm contracts during inspiration, not expiration.
• C: Diaphragm contraction increases thoracic volume for inspiration.
• D: Pressure falls then rises and volume increase describe inspiration.

Final answer: B