REGIONAL BLOOD FLOW DISTRIBUTION

A. Upright Posture

In the upright posture, pulmonary vascular hydrostatic pressure decreases at increasing heights above the lung because of the effect of gravity. Thus, blood flow is highest in the base of the lungs and decreases toward the apex

Zone 1: Alveolar air pressure greater than either pulmonary arterial of venous pressures, so vessels collapsed and flow close to zero (Note: does not exist in normal lung, but can occur in pulmonary hypotension)

Zone 2: Alveolar air pressure less than pulmonary arterial pressure but greater than pulmonary venous pressure, so vessels partially collapsed and flow is low

Zone 3: Alveolar air pressure less than both pulmonary arterial and venous pressures, so vessels open for full length and flow highest

B. Prone or supine Posture (lying down)

In the prone (face down) or supine (face up) posture, all lung regions are near heart level, so the effect of gravity is much less and the pulmonary flow is more uniform