Zones of Pulmonary Blood Flow.

Zone 1

Zero perfusion

Does not exist in normal lungs.

Occurs when the Pulmonary arterial pressure becomes less than alveolar air pressure.

Pulmonary capillaries become collapsed.

Zone 2

Intermittent blood flow

This occurs during systole when the pulmonary arterial pressure raises more than alveolar air pressure.

In normal lungs, this zone occurs from apex to hilum of the lungs.

Zone 3

Continuous blood flow

Here pulmonary arterial pressure is greater than alveolar air pressure during the entire cardiac cycle.

In normal lungs, this zone occurs from the middle zone of lungs to base.