Three Phases of Breathing: Inspiration, Gas Exchange, Expiration
Respiration comprises three continuous phases:
- Inspiration (Inhalation) – Diaphragm contracts (flattens), external intercostals lift ribs → thoracic volume ↑ → intrapulmonary pressure ↓ → air flows in.
- Gas Exchange (External Respiration) – At alveolar‑capillary membrane: O₂ diffuses into blood (binds Hb), CO₂ diffuses out (driven by partial pressure gradients).
- Expiration (Exhalation) – Diaphragm relaxes (domes up), intercostals relax → volume ↓ → pressure ↑ → air flows out. (Passive at rest; active with forced expiration using internal intercostals/abdominals.)
Cycle repeats 12–20 times/min at rest.
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