Zones of West

                                                  Zones of West😎

 Intravascular pressure is thought to decrease linearly with vertical height, being lowest at the top of the lung where flow is zero during diastole and very low during systole. In zone 1 near the top of the lung, alveolar pressure exceeds arterial pressure and the collapsible blood vessels close, stopping flow. 

In zone 2, arterial pressure exceeds alveolar pressure, which exceeds venous pressure. At any given height, the flow is determined by the difference between arterial and alveolar pressure (venous pressure doesn't count).
This is called a "Starling resistor." 

In zone 3, both arterial and venous pressures exceed alveolar pressure. Although the driving (arterial–venous) pressure is equal with vertical position, the increase in flow down the lung was explained by vessel distension, decreasing vascular resistance, going down the lung. In the initial zone presentation (15), a model was proposed with three zones.
 http://physiologyonline.physiology.org/content/14/5/182