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The structure of the
venous system - the leg vessels |
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1. Venous
valve
2. Muscle
3. Vein
4. Artery |
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In contrast to the arterial system, blood circulation in
the veins is scarcely assisted by the pumping function of the heart. The
small residual pressure is no longer sufficient to transport the blood
against gravity to the heart. An ingenious system drives the column of blood
weighing kilograms "uphill":
Venous valves are built into the leg veins at intervals of a few centimetres.
They seal the vein sections so that blood can only flow towards the heart.
The muscles in which the deep veins are embedded act as a pump. On movement,
every muscle contraction has the effect of pumping blood back to the heart.
It is via these deep veins, also called axial veins, that 85% of venous
blood returns. In addition to the muscle pump, there are two further factors
which ensure that blood is transported back: |
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1. |
Firstly, during breathing negative
pressure is created in the chest which draws up venous blood by suction.
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2. |
Secondly, owing to the suction effect of
the heart, venous blood returns to the heart even if the muscle pump is
not activated.
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3. |
The arteries also provide indirect
assistance. The more powerful, muscular arteries are located close
alongside the veins. If the pulsating bloodstream distends the artery, the
vein becomes compressed, the venous valves open and the venous blood is
driven in the direction of the heart. Since the arteries have the same
effect as a contracted muscle, this form of transporting blood back to the
heart is also referred to as the arterial pump. |
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Outside the musculature,
under the skin, lies a second, superficial venous system. Its numerous
veins converge into two main branches, known as the great and small
saphenous veins. These flow directly into the deep axial vein above the
knee or in the groin. There are, however, numerous other connecting veins
between the superficial and deep venous system through which the blood is
aspirated to the axial veins. |
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