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Vein
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A vein is defined by direction: it is the vessel that carries blood toward the heart. That is the whole definition.
Compared with an artery, its wall is thin and its lumen, the inner space the blood passes through, is large and irregular. That wider lumen has a concrete advantage: more blood fits, and the vessel puts up less resistance to it.
Where that turns into a problem is in the legs and arms. There the flow slows down, both because pressure is lower and because gravity pulls the blood downward. And that is where the valves come in: many veins, especially those in the limbs, have flaps that help the blood move in only one direction, toward the heart. It is the same idea as a door that swings open one way and jams the other way.
In brief
A vein is a blood vessel that carries blood toward the heart.
Compared with an artery, it has a thin wall and a large, irregular lumen; that wider lumen lets more blood flow with less resistance.
Many veins, especially those in the limbs, have valves that help keep blood flowing one way toward the heart, because in the limbs flow slows from lower pressure and gravity.
It is the door that swings open one way and jams the other.
Sources (1)
- OpenStax, Rice UniversityAnatomy and Physiology 2e, 20.1 Structure and Function of Blood Vessels
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