F3i: Describe the inspiration & expiration process involving the chest wall, diaphragm, pleural & lung parenchyma

  • Air moves from region of High P → Low P
  • ∴requires a pressure gradient to be established b/w atmosphere & alveoli


  • Diaphragm (phrenic n) descends
  • External IC contract to increase AP & lateral diameters
  • Collectively expand thoracic cavity which ↑its volume

BOYLE’S LAW: in a closed system where gas molecules number is constant, and at constant temp, pressure exerted by gas varies inversely with volume of gas

NB: ↑ vol gas = ↓ pressure gas exerts

  • Lungs are not attached to CW, but ∆ vol acc to ∆ vol of thoracic cavity
  • Pleural (visceral) covers lungs & pleura (parietal) lines thorax & coupled with thin fluid layer

∴ Intrapleural pressure ↓ from – 5 to –8cm H2O

  • Alveolar P ↓ to subatmospheric
  • ∴pressure gradient for flow of air into lungs
  • Air flows in & lung volume ↑ until alveolar P rises to atmospheric (0 cmH2O)
  • Then air flow ceases, as the P gradient no longer exists


  • Cycle reverses
  • Insp muscles relax
  • Inward elastic recoil of lungs → deflate
  • Airflow out of lungs ceases when alveolar P = atmos P

NOTE: At FRC, lungs have tendency to recoil inwards, chest has tendency to recoil outwards

NOTE 2: In order for lungs to expand, the insp muscles must overcome 2 key factors

1. Lung Compliance

2. Airway Resistance

NB: Muscle contraction → ∆ V → ∆ P → Air flow

Muscle Contraction