Attached to the inner walls of the leg veins is a series of delicate valves that have an essential function in the peripheral pump. Paper-thin flaps (or cusps) are paired to form a valve that ensures a one-directional effect on the flow of blood. The free end of each valve flap contacts its mate in the center of the vein. The valves join the vein at the lower part of a bulbous widening. With the flap edges together, the valve is closed.
When pressure in the vein below the valve is increased, the flaps are forced apart, the valve opens, and blood freely flows toward the heart. When the valves flaps are forced open, they fit snugly into the bulge of the vein, thus streamlining the flow of blood.
On release of the pressure, the weight of the venous column pushes the edges of the flaps together again and closes the valve. The adjacent edges of the valves prevent the backward flow of blood.
In both the deep and superficial veins, these (
unidirectional,) one-way valves are oriented so that blood flows only in a heart-ward direction. In the perforating veins, the valves are oriented so that venous flow is directed from the superficial veins to the deep veins. These perforator valves—as in the superficial and deep veins—are located from foot to thigh. They are closer together at the more distal locations.
One function of valves in the veins is to break up the column of blood into a series of short columns. When an individual is standing, the valves of the legs are forcibly closed by the weight of the blood column in the veins above. Thus, the column is broken up into many short segments in which the weight of blood in each is relatively small. The valves are positioned closer together at descending levels of the leg as the pressure is increased by the height of the venous column. Without benefit of valves, the pressure approximates that of a long vertical tube. Thus, the valves serve to relieve the high venous pressure in the lower tissues of the legs, even with motionless standing.
Observers of the action of valves by ultrasound concluded that the forward movement of blood forces the valves against the shallow pockets in the venous wall . In this opened stage, a vortex occurs behind the valve cusps, preventing stasis. On closing, the cusps return to their closed, apposing position.
This action of valves can be demonstrated in a simple experiment. Hold the forearm at waist height and note that the ...