Basic principles of fluid mechanics may be used to analyze the relationships of flow, pressure and cross-sectional area of any collapsible tube, including pulmonary blood vessels. The cross-sectional area of a collapsible tube is a direct function of transmural pressure. Alterations in pressure inside the tube may result from changes in fluid velocity, downstream constriction in the tube, or direct increase in internal pressure, as with a manometer. This paper reviews the basic equations applicable to flow in collapsible tubes and illustrates the principles of such flow with a physical model. Depending upon the circumstances, changes in size of the tube may or may not reflect changes in flow or changes in fluid velocity.