[Venous hemodynamics. Base equations]

The hydrodynamics in tubes with inert walls collapsed by a negative transmural pressure (inside pressure minus outside pressure) set up a fundamental and original approach in venous hemodynamics. Mechanisms of flow regulation and flow limitation can occur especially when the upstream pressure is held constant or when the average flow speed in a cross-section equals the local pressure waves speed. The three baseline equations on mechanics are required to demonstrate these properties: --the tube law, --the equation of motion, --the equation of continuity. It is shown that the behaviour of the tube varies with the nature of the flow (subcritical, critical or supercritical, subsonic, sonic or supersonic, according to the aerodynamics). Though this study does not take all the physiological and biological data into account, it includes indeed the three fundamental aspects on mechanics of the veins. The rheologic equation on the state of the venous wall, normal or pathologic, plays a major and determinant role.