Distribution of Perfusion in Normal Lung

• West’s Zones and Perfusion:

  • lung divided into 4 zones
  • better perfusion at the lower lungs
  • due to to the interplay between alveolar pressure (PA), arterial pressure (Pa), venous pressure (PV), tissue pressure (PT)

  
• West’s Zones and Perfusion - lower zones:

  • alveoli become smaller due to more positive pressure
  • so less pressure on vasculature
  • so more blood flow (perfusion)
  • optimal perfusion is in zone 3
  • in zone 4 the weight of the lung starts to squash the vasculature - decrease in perfusion

  
• West’s Zones and Perfusion - upper zones:

  • bigger alveoli at apices due to negative pressure opening alveoli up
  • so puts pressure /squashes on vasculature
  • so less blood flow (perfusion) occurs

  
• • there is between blood supply (perfusion) at the middle to bottom aspects of the lungs

  
• • Distribution of perfusion is affected by the interaction of alveolar pressure, arterial pressure and tissue (lung) pressure AND fractional geometry (branching) of pulmonary artery
  • Gravity also has some limited effect
• V/Q Perfusion Summary:

  • Blood flow increases down upright lung - due to there being more positive pleural pressure on the way down, so less alveolar pressure squashing the vasculature - increase in perfusion
  • Non-dependent to dependent regional perfusion is influenced by:
  • interaction of pulmonary artery, alveolar & tissue pressures
  • pulmonary artery vascular geometry
  • Distribution affected by posture & exercise - in side lying the lung on the bottom will have more positive pleural pressure, so less alveolar pressure squashing the vasculature - increase in perfusion