ABSTRACT
This chapter refers to an evaluation of venous haemodynamics prepared on behalf of the Union Internationale de Phlebologie. It discusses fluid hydrodynamics, venous physiology in general, and lower limb veins, the vein wall, lymphatics and the microcirculation. Veins provide passive conduits for blood flow at low pressure and velocity. Pulsatile right atrial contractions cause transient increase in venous pressure and arrest of flow which is restored as the atrium relaxes, although the effect is only observed in central veins. The resultant intraluminal lateral pressure is determined by the volume of blood within the veins. Inspiration decreases intrathoracic pressure increasing the pressure gradient and venous drainage from veins and lymphatics of the upper limbs to central veins. Veins collapse at a low transmural pressure to form an ellipse and there is minimal stretch of the vein wall so that large changes in venous volume result in only small increments in transmural pressure.
