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A lung contains branching airways and an extensive thin surface for gas exchange with blood. In humans, the right lung usually has three lobes and the left two. Airway transport, alveolar exchange and vascular flow are interacting functions rather than interchangeable measures of lung performance.^[ref]

The pleura is a serous membrane with a visceral layer covering a lung and a parietal layer lining the adjacent thoracic wall. The intervening pleural cavity normally contains a thin fluid film. Sliding at this interface permits breathing movements while preserving mechanical coupling between the lung and thorax.^[ref]