Question

why do veins have thin walls as compared to the arteries ?

Answer 1

i think....
veins have thin walls as compared to arteries because the arteries carry blood from heart with high pressure and therefore to withstand the pressure it has thick walls whereas Veins carry blood from different parts of the body and has less pressure.therefore it has thin walls.

Answer 2

veins have thin walls as compared to the arteries 
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the blood in the arteries flow with the very high pressure hence the arteries should have the power to withstand that high pressure so they are thick whereas the blood will not flow with very high force in veins so they have thin walls compared to that of the arteries