Lung collapse is a pressure-dependent phenomenon. Each acinus has a critical closing pressure, i.e., the minimum transpulmonary pressure (Ptp) below that the acinus begins to collapse. While airway pressure is homogeneously distributed within all lung units, Pleural pressure increases along the vertical gravitational vector because of the lung's weight. As a consequence, the decreased Ptp in the dependent zones promotes collapse. This means that patients in the supine position suffer from increasing closing pressures in the ventral to dorsal direction.