venous return
Venous return is the flow of blood coming back to the heart through the veins—the ‘inflow’ that refills the pump so it can keep ejecting. Over time it must equal the heart’s output: you cannot pump out more than comes back, and what comes back is what gets pumped.
Blood returns from a high-pressure side to a low-pressure side: the small pressure left in the venules and veins (driven ultimately by the heart and adjusted by venous tone) pushes blood toward the nearly zero pressure in the right atrium. Several helpers boost it—the skeletal muscle pump squeezing leg veins, one-way venous valves, and the respiratory pump, where inhaling lowers chest pressure and draws blood toward the heart.
Venous return is tightly linked to preload, the filling that stretches the ventricle before it contracts; by the Frank–Starling mechanism, more return generally means a bigger stroke volume. This is why standing up suddenly (which lets blood pool in the legs and reduces return) can briefly drop output and make you feel lightheaded, and why lying down or raising the legs increases it.
Raising a patient’s legs increases venous return and preload, temporarily boosting cardiac output—sometimes used to assess fluid responsiveness.
Passive leg raising is a reversible way to test how the heart responds to more filling.
In steady state, venous return equals cardiac output—they are two views of the same circulating flow.