Exhalation Mechanics: Diaphragm, Rib Cage & Lung Dynamics
Quiet exhalation is a passive process driven by elastic recoil, not active muscle contraction.
Step-by-Step
- Inspiratory muscles relax: Diaphragm returns to dome shape (elastic recoil); external intercostals relax.
- Rib cage descends: Gravity + elastic recoil of chest wall lower ribs → ↓ thoracic volume (anteroposterior & lateral).
- Intrapleural pressure rises (becomes less negative) → alveolar pressure > atmospheric pressure.
- Lungs recoil: Elastic fibers + surface tension collapse alveoli → air flows out.
- Airway diameter narrows slightly (dynamic compression) but remains patent in health.
Forced Exhalation (Exercise, Cough, Disease)
- Internal intercostals pull ribs down forcefully.
- Abdominal muscles (rectus, obliques, transversus) ↑ intra-abdominal pressure → push diaphragm up.
- Generates high pleural pressure → rapid flow (cough) or sustained expiration (exercise).
Clinical Note
In COPD/asthma, loss of elastic recoil + airway collapse → air trapping, hyperinflation, prolonged expiration. Pursed-lip breathing creates back-pressure to stent airways open.
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