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Ventilation-Perfusion Zones of the Upright Lung

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West and colleagues divided the upright lung into three zones based on the relationship between pulmonary arterial (Pa), alveolar (PA), and venous (Pv) pressures. In Zone I, usually near the top of the lung, PA exceeds Pa, so no blood flow occurs. In Zone II, Pa exceeds PA and is much higher than Pv so the difference between Pa and PA is a main factor in determining the resistance to blood flow. In Zone III , at the lower regions (dependent part) of the lung, Pa is considerably higher than PA and Pv is greater than PA. Thus, in Zone III, the main factor in determining the resistance to flow is the difference between Pa and Pv. Gas exchange does not occur to any extent in Zone I and is optimal in Zone III. Normally, little if any of the lung is in Zone I, wheras gas exchange is optimal when the lung is in Zone III.