Venules connect capillaries to larger veins. Veins transport blood slowly, under low pressure, from the capillaries in tissues to the heart.The muscle layer is relatively thin compared to arteries because veins carry blood away from tissues therefore there is no need for them to constrict or dilate to control flow of blood to the tissues.The elastic layer is relatively thin as the blood has a low pressure.The overall thickness of the wall is small and the lumen is large aiding blood flow within them.Veins have valves to prevent backflow ensure blood flows in the right direction.In the lower limbs this is also aided by muscle contractions exerting pressure on intramuscular veins and driving blood back to the heart.