F6iii: Explain V/Q matching & mismatching

  • V/Q is so important because it’s affecting how much O2 gets in & CO2 gets out
  • Ventilation & perfusion performs as ratio
    • If CO ∆, it will ∆ V/Q of all lung units
    • If MV ∆, it will ∆ V/Q of all lung units

(Given the other is constant)

  • A lung with V/Q inequality cannot maintain high PO2 & low PCO2 as well as a homogenous lung
  • Ventilation is greater @ base → greater compliance
  • Perfusion is greater @ base → ↑hydrostatic P
  • ↑perfusion > ventilation from apex → base
  • Normal V/Q scatter is responsible for the natural A – a gradient

→ Even though PO2 is 40mmHg higher at apex cf. base, most perfusion is coming from base where PO2 is low

→ The sigmoid ODC also means that even units with high V/Q the O2 content of blood doesn’t increase much (plateau)

    • V/Q (16mL O2/100mL blood) = 1/10
    • V/Q = 10/10 = 19.5mL O2/100mL blood
    • V/Q = 10/1 = 20mL O2/100mL blood

→ So when blood is mixed from all of these different V/Q ratio alveoli, it brings the PO2 down


  • Supine → V/Q scatter is less because the vertical gradient is so much less
    • ∴ ↓dead space
  • Lateral → V/Q scatter is same, just instead of apex → base it occurs from sup → inf lung