Updated: 10/6/2015

Breathing Cycle

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Breathing Cycle: Normal, Quiet Breathing 
  • Rest
    • period between breathing cycles
    • diaphragm is at equilibrium position
    • alveolar pressure = atmospheric pressure → alveolar pressure = 0
      • no pressure gradient between mouth/nose (atmospheric pressure) and alveoli
        • no airflow into or out of lungs
    • intrapleural pressure < 0
      • caused by opposing forces of lungs
        • collapsing force on lungs and expanding force on chest wall
      • keeps lungs inflated
    • volume = functional residual capacity (FRC)
      • volume of gas that remains in lungs following normal tidal volume (TV) expiration
      • equilibrium volume of lungs
  • Inspiration
    • diaphragm contracts → ↑ intrathoracic volume → ↓ intrathoracic pressure
    • ↓ intrathoracic pressure → ↓ alveolar pressure
      • alveolar pressure < atmospheric pressure → alveolar pressure < 0
        • pressure gradient between mouth/nose (atmospheric pressure) and alveoli drives airflow into lung
    • inspired volume = TV
    • end-inspiration volume = FRC + TV
  • Expiration
    • passive process
    • diaphragm relaxes → ↓ intrathoracic volume → ↑ intrathoracic pressure
    • ↑ intrathoracic pressure → ↑ alveolar pressure
      • alveolar pressure > atmospheric pressure → alveolar pressure > 0
        • reverse pressure gradient between mouth/nose (atmospheric pressure) and alveoli drives airflow out of lung
    • expired volume = TV
    • end-expiration volume = FRC

       

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