AbstractThe concept of the waterfall phenomenon in Zone 2 in the pulmonary vasculature is well known from West's lung model. It is believed that the flow through this zone is determined by the pressure difference between the pulmonary artery and alveoli, and the left atrial pressure is not transmissible to the alveolar capillaries. However, it is impossible to see whether alveolar capillaries are really displaying the waterfall phenomenon or not. In this review, the interrelation between the flow and geometry of the alveolar capillaries in the waterfall phenomenon is analyzed based on physiological studies using a model system and isolated lung lobe experiments. Further, extending the concept to the analysis of ventilatory changes of the inferior vena cava (IVC) configuration, it is ascertained that the waterfall phenomenon normally occurs in the IVC during inspiration just before it enters the thorax and the waterfall phenomenon in the IVC disintegrates with elevation of the central venous pressure. Because these configurations of the IVC in normal and abnormal conditions are visible with ultrasonography, the technique is very useful as a noninvasive approach to diagnose right heart failure.