Large Medium Vein Venule

Veins progress from venules collecting capillary blood, merging into small then medium veins with increasing smooth muscle and valves preventing backflow in limbs against gravity, finally large veins like venae cavae with minimal muscle relying on nearby structures. Venules are porous for exchange, medium have paired arteries, large conduct low-pressure flow to heart, all lined by endothelium with varying connective tissue support.