Key messages

• In the healthy, quietly breathing subject, both ventilation VA and perfusion Q increase down the lung.

• In addition, non-gravitational distribution of perfusion has been described.

• On average, VA/Q » 1, with higher ratios in upper lung regions and lower ratios in lower lung regions.

• Important factors influencing VA/Q are the gradient of the vertical pleural pressure, the shape of the thoracic cavity and the lung, forces exerted by abdominal organs, airway closure in dependent lung regions, gravitational forces, length of pulmonary arterial vessels, and cardiac output.

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