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Old 28 September 2007, 04:33 PM
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Spamamander Spamamander is offline
Join Date: 01 January 2006
Location: Central WA
Posts: 4,383

My completely uneducated guess would have to do with the thickness of the vessel walls. Arteries do not need to have thick walls, as blood is forced through them by the heart, so you can see the color of the blood within. Veins have thick walls to force the blood back to the heart, so they are not transparent and you instead see the color of the venous walls.
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