What are the valves of the venous system? What is their function?
The valves of the venous system are structures within the veins that permit blood to flow only in the normal way (from the tissues to the heart) and forbid it to return in the reverse way in favor of gravity. The valves close when the pressure of the fluid column above (after, regarding usual flux) is higher than the fluid pressure before them. Valves are therefore fundamental for the returning of blood to the heart.