1. Functional residual capacity may be higher, thereby reducing end-expiratory airspace closure (1, 2). This benefit is important relative to gas exchange as it will particularly affect the dorsal lung, and this region receives the greatest proportion of perfusion in all postures (3, 4). 2. The generally dorsal-to-ventral orientation of the major airways may allow more efficient drainage of secretions (5). 3. Regional ventilation and regional ventilation-to-perfusion relationships are more uniform (6–9) and gas exchange is improved (10–12) as a result of the anatomy of the diaphragm (13), postural differences in chest wall mechanics (14), and/or because the lungs fit into the thorax with less distortion from the heart, mediastinum, and diaphragm (15–18). 4. The effect of recruiting maneuvers on oxygenation is both increased and prolonged (19). 5. Ventilator-induced lung injury may be reduced (20, 21).