F6i: Describe the concepts of global and regional ventilation and perfusion and the factors that affect these

V/Q ratio determines partial pressures of all gases in the lungs

  • Alveolar ventilation = 4L/min
  • Pulmonary blood flow = 5L/min

∴ if uniform, every alveolus would have a perfect ratio of 0.8

Perfect V/Q matching is present when the amount of O2 delivered by ventilation is exactly equal to the amount of O2 required to return O2 content of mixed venous blood back to normal

Obviously, that is not the case!

Regional V/Q Differences

  • In upright lung
  • Ventilation increases top to bottom
  • Perfusion increases top to bottom
  • BUT perfusion increases much more
Ventilation Perfusion
  • As a consequence:
    • High V/Q apex of lung, low V/Q at bottom
    • PO2 decreases from apex to base
    • PCO2 increases from apex to base, but much less PO2 ∆ (so pH ↓ also)
    • Volume at apex is less cf. base
    • V/Q ↓down the lung → gas exchange results from this
Ventilation 1