The role of gravity and non-gravitational factors in determining pulmonary ventilation perfusion distribution

West's zonal model has traditionally been used to explain blood and gas flow distribution in the lung. Vertical gradients in intrapleural and hydrostatic pressures due to gravity are central to this model. Therefore, gravity had long been regarded as the major factor determining the distribution of blood flow and ventilation in the lung. However, when examined in the light of recent evidence it fails to provide an adequate explanation for heterogeneity of flow at horizontal levels, postural flow variations and persistence of heterogeneity in the absence of gravity. The structure of the pulmonary vessels and airways is being increasingly recognised as being the major determinant of pulmonary blood and gas flow patterns. This emerging paradigm, particularly the effects of posture and PEEP on the architecture of the lung parenchyma and pulmonary blood vessels, has important applications in selecting the appropriate ventilatory strategies in patients with ALI.