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 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