Venous Hemodynamics and Normal Doppler Findings in the Venous System

The venous system guides blood back towards the right heart and stores blood not needed for current circulatory requirements. Venous blood is drained from the superficial to the deep veins and from the periphery towards the heart. The splanchnic circulation, where blood is sampled from the gastrointestinal tract and flows through the liver sinusoids towards the heart, has some different flow conditions. In addition, the veins play an important function in the orthostatic circulatory regulation. Compared with the arterial system, which has been investigated extensively by various CCDS techniques, the hemodynamic properties of venous blood flow have been studied to a lesser extent by duplex Doppler and color Doppler ultrasound. This might be due to the fact that the venous system is isolated from the pulsatile pressure in arteries by the capillary bed. Thus, venous pressure gradients and venous flow velocities are much lower, and the flow is more continuous, making disturbed flow conditions rather unusual, even in pathologic conditions. However, veins near the right heart like the superior or inferior vena cava (SVC, IVC) or liver veins demonstrate extreme pulsatility in flow velocity due to heartbeat. In addition, energy to return the blood toward the heart is derived from additional factors. These are the muscle pump, gravity, respiration, and physiologic action of the venous valves.