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The blood vessels -
arteries and veins |
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1. Venous
valve
2. Vein
3. Artery |
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There are two types of blood vessels: the arteries
distribute the blood to all tissues and organs of the body, the veins
collect it and transport it back to the heart. In the arterial system
relatively high pressure predominates since it is directly exposed to the
pumping action of the heart. The vessel walls of the arteries are therefore
thicker and more muscular, they regulate blood pressure by dilation and
constriction. The volume of blood which they transport is, however,
relatively small, it accounts for only about 15% of total blood volume.
In the venous system, by contrast, the pressure is relatively low. The
thin-walled veins can easily be stretched. They store about 85% of the total
blood volume of the organism. The flow of venous blood back to the heart
proceeds very evenly. The venous system also plays an important role in
regulating body heat. In a warm environment, the veins are dilated and
filled with blood, give off heat to the outside and thus keep body
temperature constant. If the environment is cool, they contract and
distribute a large amount of blood to the chest and stomach. Heat release is
minimised and hypothermia prevented. |
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