In West Zone 2, perfusion is determined by which pressure?

In West Zone 2, perfusion is influenced primarily by the arterial-alveolar pressure gradient. This zone is characterized by a situation where the pressure within the pulmonary arteries (arterial pressure) exceeds the pressure in the alveoli but is less than the pressure in the pulmonary veins. As a result, perfusion is dependent on the difference between the arterial pressure and the alveolar pressure.

This pressure gradient allows for the flow of blood through the pulmonary capillaries during this phase, as the arterial pressure is high enough to overcome the pressure in the alveoli, facilitating gas exchange. Understanding this is crucial for recognizing how different mechanisms affect perfusion in the lungs, particularly in varying conditions or zones of the lungs.

The other options reflect different aspects of pressure dynamics but do not directly apply to the mechanism defining perfusion in West Zone 2. Mean arterial pressure relates to systemic circulation, while intrathoracic pressure plays a broader role in pulmonary mechanics. Capillary hydrostatic pressure is vital in determining filtration and absorption but is not the primary determinant of perfusion in this specific zone of the lung.