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