The wall consists of three layers.
Tunica interna – the inner layer consisting of endothelial cells.
Tunica medica – the middle layer consisting of smooth muscle cells.
Tunica adventitia (externa) – the outer layer consisting of connective tissue.
The leg veins have venous valves in form of sails directed towards the heart, dividing the vein into segments of various length.
The importance of venous valves
The venous valves open when blood is pushed upwards, in the opposite direction of the force of gravity.
The valves close right after blood has passed, preventing the backflow of blood.
In the deep veins
A prerequisite for the normal function of venous valves is the appropriate tone of vein walls. When the tone of the outer vein is reduced, the vein is enlarged and because of that the venous valves cannot close completely. This enables reflux of blood into legs (venous insufficiency) and its flow to superficial veins, where it may clot and build an obstruction. The volume of blood the veins have to receive grows and this affects the development of varicose veins.
In the case of weakened venous valves in the superficial veins, the deep veins have to take over their function and convey larger volumes of blood, which additionally enlarges and weakens them. The enlargement prevents the venous valves to close completely, so blood travels to the heart with delay.