The valveless veins are veins that lack venous valves. Most veins contain valves (known as the valvula venosa in the TA) to prevent backflow, i.e. ensuring that blood flow is always towards the heart 1.
It has been shown that veins that were previously thought to be valveless actually do have valves, including the facial and superior ophthalmic veins 2.
Valveless veins are common in the central nervous system 3.
List of valveless veins
common iliac veins (>90% individuals) 5
external iliac veins (~75% individuals) 5
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