Deep versus superficial veins
With the venous system a distinction is made between superficial veins, connecting veins (perforating veins) and deep veins. The superficial venous system lies close to the surface of the skin and carries the blood in one direction only from the upper skin layers via the connecting veins deep into the venous system. From here 90 % of the blood is transported back to the heart. The deep veins lie deep in the musculature (upper thigh, lower thigh), close to the bone and possess very elastic venous walls. They need to have, as the transport of blood back to the heart is not their only task. A further task is storing large blood reserves. The veins hold around five to seven litres of blood.
The venous structure
The venous wall consists of three layers:
- Intima (=inner layer)
- Media (= middle layer) and
- Adventitia (= outer layer)
The vascular wall of the veins is thinner than that of the arteries. The thin muscular layer lends the vein its elasticity. To protect against excessive stress the venous wall has unyielding connective tissue fibres.
The veins of the leg possess at various points numerous sail-shaped valves pointing in the direction of the heart, the so-called venous valves. They divide the vein into sections of varied length. The venous valves open as soon as the blood is pressed from below upwards against gravity in a central direction. In the case of healthy legs the immediate closure of the valves prevents the blood from flowing back.