Vein

Veins are blood vessels that transport the oxygen-poor blood from the capillary networks to the heart. Every day, about 7000 litres of blood flow back from the body periphery to the heart. In addition to its resistance function, the venous system also has a large storage function. The venous system has a significantly higher absorption capacity than the arterial system. At 50-60%, the extrathoracic veins contain more than half of the total blood volume. The arteries account for only about 15%. The veins run parallel to the arteries (and nerves) in the great circulation.

A basic distinction is made between the epifascial (superficial), deep vein system and the connecting perforating veins, the perforating veins.

Epifascial venous system:

• The epifascial venous system lies in the superficial compartment. It is limited in depth by the muscle fascia (therefore epifascial) and on the surface by the dermis.

Deep venous system:

• Intermuscular veins: these deep veins run between the muscles as accompanying veins of the arteries. They are mainly in pairs or in several and are connected to each other like rope ladders. They can surround the arteries as a venous network
• Intramuscular veins: these run as drainage systems within the muscles. The intramuscular venous system is crucial for muscle pump function.

Perforating veins:

• The perforating veins connect the epifascial with the deep venous system. Up to 150 perforating veins are formed in the entire lower extremity. Perforating veins are equipped with venous valves and play an important role in venous drainage of the lower limb. They conduct the blood from the superficial epifascial to the deep venous system. Their valve system prevents reflux from the deep venous system into the epifascial venous system during muscle contractions. The perforating veins of the lower leg are dividedinto 4 main groups: medial, anterior, lateral and posterior.

The blood pressure in the veins is significantly lower than in the arteries. Veins, together with the capillaries and venules, belong to the low pressure system of the blood circulation. While the venous pressure of the extremities is about 10mmHG when lying down, it rises to 90-100mmHg on the lower legs when standing.

Blood flow in the veins is ensured by foot and leg muscles(muscle pump) and by the abdominothoracic two-phase pump. Due to the constant tension of the muscles of the extremities as well as the breathing activity, the blood is pumped from the periphery of the vessels against gravity in the direction of the heart. The direction of the blood flow is defined by valve systems in the veins, the so-called venous valves. A muscle contraction, such as that caused by a tip-toe movement, results in the displacement of about 200ml of blood.

Venous valves: Unlike arteries, many small and medium-sized veins are equipped with venous valves. Venous valves ensure that the blood does not flow back down again when the muscle relaxes, following the force of gravity. If the veins dilate (varicosis), the valve function of the veins is no longer or only insufficiently guaranteed. This results in valve insufficiency with a bi-directional blood flow. The consequences are changes in the wall structure of the veins, edema formation and other signs of chronic venous insufficiency.

Structure of the veins: Veins have thinner walls than arteries due to a less developed musculature. However, the basic structure of both vascular systems is largely identical, with a characteristic three-layer wall.

The innermost layer of the vein is called the tunica interna (intima). Outwardly, the tunica media (media) follows, and finally the outer layer of the vein is the adventitia.

Intima: The intima of the vein consists of a single-layer endothelium with a basal membrane of varying thickness. The endothelium fulfils several important tasks:

• Barrier function between blood and interstitium
• Antithrombosis function (production and excretion of prostaglandins)
• Formation of venous valves in medium-sized peripheral veins
• Regulation of vascular tone (release of so-called vasokinins such as endothelinin-1, PAF, prostacyclins)
• Repair of endothelial damage

The repair of endothelial damage is carried out by the release of von Willebrand factor (vWF) and factor VIII from the so-called Weibel Palade corpuscles. These are special organelles of the endothelium. The von Willebrand factor initiates the aggregation of activated thrombocytes by attachment to the GPllb-/GPIIIa-receptors of the thrombocytes. In a further step, the von Willebrand factor mediates the adhesion of the thrombocytes to the exposed collagen fibrils of the injured endothelium.

Media: The muscles of the media are only weakly developed in the veins in relation to the arteries. The circular stratification is missing. Its thickness is determined by the anatomical location. The media of the lower leg veins are richer in muscle cells than those of the arm veins.

Adventitia: The media of the veins merges without sharp edges into the adventitia, through which the veins are anchored to their surroundings. In the Adventitia there are plenty of sensitive and vegetative nerves as well as the supplying Vasa vasorum.

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