Prone positioning is a lung-protective strategy for severe acute respiratory distress syndrome (ARDS) that exploits gravitational physiology and lung mechanics to improve ventilation-perfusion (V/Q) matching and oxygenation. In ARDS, diffuse alveolar damage from inflammatory injury causes surfactant dysfunction, alveolar flooding with protein-rich edema fluid, and formation of hyaline membranes — the lungs become heavy (normal lung weight ~800g; ARDS lungs may weigh 1500-2000g) and develop a gradient of aeration from the non-dependent (anterior, in supine position) to dependent (posterior, in supine position) regions. In the supine position, the weight of the edematous lung compresses dependent dorsal alveoli, causing atelectasis and consolidation in the posterior lung zones while relatively sparing the anterior zones — this creates a 'baby lung' concept where only a small fraction of the total lung (often only 200-500g of aerated tissue) is available for gas exchange. Critically, pulmonary blood flow is gravitationally distributed predominantly to the dependent (posterior) regions, but in supine ARDS these regions are atelectatic and consolidated — blood flows past non-ventilated alveoli without participating in gas exchange, creating intrapulmonary shunt (the primary cause of...