Describe the gravity dependent processes which affect pulmonary blood flow. What changes take place when the pressure increases in the pulmonary vessels? (97A2)
Pulmonary blood pressures (both arterial and venous) are low
=> mean pulmonary blood pressure = 15mmHg
In an erect person, there is about 30cmH20 (23mmHg) difference between apex and base due to hydrostatic pressure.
=> both pulmonary arterial and venous pressure increases from apex to base
=> blood flows at different levels change as per West's zone
PA - pressure in alveoli
Pa - pressure in pulmonary artery
Pv - pressure in pulmonary
Pressure in alveoli is > than pulmonary arterial pressure
=> capillary is squashed flat
=> no perfusion
=> "alveolar dead space"
Zone 1 doesn't happen in normal person but happens when
At very low volume
=> reduction in radial traction
=> extra-alveolar vessels narrow
=> pulmonary vascular resistance increase
=> decrease in blood flow
NB: PVR is lowest at FRC
(Also see PVR)
When pulmonary blood pressure increases (e.g. due to hydrostatic pressure), PVR would decrease because:
Both mechanisms contribute to increased perfusion, but:
When Pa is much higher than PA and the difference exceed oncotic pressure
=> transudation (movement of plasma from capillaries into alveoli) occurs
cardiac output increase and blood pressure increase
=> Zone 1 and 2 reduces in size, and zone 3 expands
=> greater blood flow
=> difference between apex and base due to gravity not as great
the dependent part of the lung will have higher Pa and Pv due to hydrostatic pressure
=> lower PVR and better blood flow
(as long as there is no significant alveolar collapse (zone 4))
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Created | 20050227 |
Updated | 20050227 |