F5ii: Describe the gravity dependent processes which affect pulmonary blood flow. Describe the changes that result from an increase in pressure in the pulmonary vessels

  • Pulmonary BF = CO 5L/min, but in a low pressure system

Gravity in Lung

  • RV contacts → impacts kinetic E to blood in PA
  • Energy dissipates as blood climbs vertical gradient of lungs
  • In upright lung, BF ↓ linearly from bottom to top
  • The cause is gravity, producing a hydrostatic pressure gradient
  • For any column of liquid, hydrostatic P ↑ with ↑ depth
  • Every 1cm up lungs = ↓PAP 1cm H2O
  • PA higher @ base because of hydrostatic P of gravity and as blood is going up the vertical column, the pressure is getting more negative (or less)
  • The pressure difference between top & bottom of lung is ~23mmHg
  • significant because pulmonary circulation operates as a low pressure system

(PAP is just sufficient to reach top of lungs)

  • The P gradient creates differences in BF throughout pulmonary circulation

West’s Zones describe differences in BF in upright lung

  • 4 vertical regions based on relationship between PALVEOLUS, PARTERIES, PVEINS, PPULM INTERSTITIUM

ZONE 1 → PALV > Pa > PV

  • PAP is very low
  • Capillaries are squashed → nil BF
  • No gas exchange → Alveolar dead space
  • Normally does not occur → only if ↓Pa with hypotension or ↑PALV (i.e. PEEP)

ZONE 2 → Pa > PALV > PV

  • BF determined by arterial-alveolar difference
  • Cyclical & caps flutter between open/closed state (with breathing)
  • Present from lung apex → 10cm above heart

ZONE 3 → Pa > PV > PALV

  • ∴BF determined by arterial – venous difference
  • Caps open enabling constant BF
  • From 10cm above heart → lung base

ZONE 4 → Pa > Pinterstitium > PV > PALV

  • A positive interstitial P due to fluid transudate from capillary → interstitium
  • Can be due to ↑↑↑PAP (vol of load, MS, PE) or a very – ve intrapleural P (laryngeal obstruction)
  • BF determined by arterial – interstitial P diff.
  • PATHOLOGICAL → no V or Q

Consequence of ↑ Pulmonary Pressure

  • Pulmonary circulation = low P circuit
  • ↑PAP → distention & recruitment of pulmonary caps → ↓PVR → ↑pulm BF
  • Recruit = opening of previously closed caps → Zone 1 & 2
  • Distention = ↑radius of already opened caps → Zone 3
  • OVERALL → ↓PVR → ↑Pulmonary BF
  • But, if there’s very high pulmonary pressures, these processes become exhausted & Zone 4 will develop
  • ↑RVP → ↑RAP → venous congestion