The most dependant portion of the lung (zone 3) has higher arterial and venous pressures than alveolar pressure. It is therefore the most well perfused.
The most superior portion (zone 1) has higher alveolar pressure than arterial/venous pressures and is therefore the least well perfused.
Zone 2 is intermediate in that the alveolar pressure is in between arterial and venous.
The zones become important when a patient has a pathologic process in part of his lung. If an area is not well aerated due to a pulmonary contusion, for example, it may be helpful to direct blood flow to another portion of the lung that is better aerated.
This principle helps guide positioning of the patient in the ICU to maximize their oxygenation. For example, a patient with a disease process affecting their left lung, may oxygenate better when positioned right side down - thus shifting zone 3 (better perfusion) to the right lung.
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