https://www.reddit.com/r/askscience/comments/23n98f/why_are_our_veins_asymmetrical/ "Veins aren't exactly formed randomly, but there are random elements in how they are formed. Identical twins will have different vasculature, for example. This is because the body doesn't have a blueprint in the traditional sense, it is procedurally generated. Limb buds set up gradients of hormones that tell bones where to stop and start. Many blood vessels are set in stone, like the aorta and vena cava, but farther out the vessels are procedurally generated, so left and right end up similar but different. Veins do not move based on need."