F6ii: Describe West’s zones of the lung & explain the mechanisms responsible for them

Hydrostatic Pressure

  • Pulm. BP is low → mPAP = 15mmHg
  • Erect posture = 30cmH2O (23mmHg) difference between apex & base
  • Due to hydrostatic P
  • arterial & venous P ↑from apex → base
  • Different areas of BF KA West’s Zones

Starling Resistor

  • Capillaries act as Starling Resistors
  • Thin vessel wall
  • Susceptible to Transmural P
  • Surrounded by Alveolar gas
    • ↑Alv P > Cap P → vessel collapse → no BF
  • Subject to elastic lung forces:
    • ↑lung vol → radial traction = ↑BF

West’s Zones

  • Zone 1 → PA > Pa > PV
  • Zone 2 → Pa > PA > PV
  • Zone 3 → Pa > PV > PA
  • Zone 4 → v. low volume

Zone 1: PA > Pa > Pv

  • Pressure in alveoli > pulm. artery pressure
  • Caps collapse → nil BF
  • Alveolar ‘DS’

Zone 1 doesn’t happen in health, but when

  • ↑PA g. PEEP
  • ↓Pa g. haemorrhage

Zone 2: Pa > PA > Pv

  • ↑hydrostatic P ∴↑Pa & Pv
  • But PA > PV
  • ∴ BF dependent on pressure difference between PA and Pa
  • Recruitment effects dominate here
  • PV no effect on BF

Zone 3: Pa > Pv > PA

  • Further ↑hydrostatic P
  • Both PV & Pa > PA
  • ∴ BF depends on difference between Pa & PV
  • Distention effects dominate here
PVR

Zone 4

  • @ very low lung volumes
  • ↓radial traction
  • ↑PVR
  • No BF

Exercise/Posture

  • Exercise → ↑CO → difference b/w apex & base less
  • Posture → dependent part of lung subject to ↑hydrostatic P