Veins are the tubes that conduct the used blood to the heart. They are superficial unlike the arteries and they can be seen prominently bulging out from below the skin in lean persons. The greenish tubes seen on the skin of the fair colored persons are actually these veins and are not the 'nerves' as many mistake them to be. They carry the blood from the distal parts of the limbs up towards the heart. They have valves in them that permit the flow of blood towards heart only but not backwards! To see this action of veins do the following:
You can place a finger tip on a distal portion in a stretch of a vein and press it down gently to block the flow of blood in it.
Then with another finger tip pressing down on it next to the previously fixed pressing finger, move over the length of the vein pushing up the blood in it (emptying) proximally.
Next take out this second pressing down finger tip. Now you shall see the previously prominent vein as an empty, dry, flat and green streak! The emptied blood does not flow back as a valve in the vein prevents it! Now if you release the pressure by the first applied finger tip, blood from the distal part of the vein will flow forwards and fill it up making it prominent once again!
It is a wonder action of the vein valves that helps to circulate the blood inside our body properly!
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