When the valves in a vein do not work, they are called "incompetent".
The muscle squeezes the vein and the blood is forced up and out of the vein, as in normal veins.
However, when the pressure eases and the veins open up, the blood is allowed to fall back down the leg as the valves are not able to prevent it. This is called venous "reflux".
Venous reflux is the major cause of most venous diseases and is almost always the cause of varicose veins.
