Venous return is the volume of blood flowing back to the heart through the veins. Although the heart pumps blood through the blood vessels, circulation through the veins is also moved along by other factors. These include skeletal muscle contractions during exercise, falling pressure in the chest when one breathes in, and the expansion of the upper chambers of the heart, or atria, each time the heart beats. The study of the factors affecting blood flow is known as hemodynamics.

When the heart contracts, it ejects blood from the lower chambers, or ventricles. The closed valves between the ventricles and the atria are pulled downward. This increases the amount of space in the atria and blood is drawn into them from the great veins leading into the heart. Venous return increases and if the heart beats faster, the effect becomes greater.

Upon breathing in, pressure in the chest falls and this negative pressure is transferred to the great veins causing blood to move along. This is because blood tends to move from high pressure areas to low pressure areas. At the same time, downward movement of the diaphragm causes pressure in the abdomen to rise, pushing venous blood back to the heart. The blood cannot flow backward because veins contain valves that only allow blood to flow in one direction.

While exercising, the veins that run through skeletal muscles become compressed due to muscle contraction. Eventually, the contractions become strong enough to stop blood flow completely, but in between each one an increased flow of blood occurs. Pulsating arteries situated nearby can also compress veins to some extent.

Remaining in a standing position lowers venous return, as blood pools in the veins of the legs. Veins have a great capacity due to their ability to dilate. In a person who is at rest, about half of the circulatory blood will be present in the veins.

Working the leg muscles while standing up helps pump blood back to the heart. Remaining still decreases venous return, with a subsequent drop in cardiac output and inadequate blood flow to the brain. This leads to fainting, but luckily a faint usually ends with the person lying on the floor. Venous return then increases, and the output from the heart improves, restoring blood flow to the brain.

In people with varicose veins, the valves that prevent backflow of blood into the veins cease to function. This means that venous return is lowered and such problems as swelling of the ankles may result. The veins may be treated in a cardiology clinic using laser technology to effectively remove them, diverting blood to healthier veins.