The venous system is responsible for returning blood to the heart once the body's organs and tissues have used the oxygen and nutrients. Veins are normally only partially filled with blood, and overall, they contain about 75% of the circulating blood volume. Veins act as a blood reservoir for the body. To handle changes in volume, the veins can stretch much more easily than the thicker, more muscular arteries. Because the venous system is low pressure and much of the blood returning to the heart has to work against gravity, veins in the limbs contain valves that ensure blood movement flows in only one direction-toward the heart.
When the valves in the veins fail to work properly, blood flows backwards or refluxes. This is usually because of increased pressure within the vein, which may occur in when the vein is obstructed such as with a venous thrombus or in situations of vein compression such as tumors or injury. Reflux most often occurs because of stretching of the vein around the valve. This separates the edges of the bicuspid valves in the veins, and they no longer close tightly. This prevents blood from returning to the heart efficiently and instead causes the blood to pool in the extremities.