WASHINGTON, DC - JANUARY 20: U.S. President Donald Trump delivers his inaugural address on the West Front of the U.S. Capitol on January 20, 2017 in Washington, DC. In today's inauguration ceremony Donald J. Trump becomes the 45th president of the United States.
One-Way Valves in the Veins. One-way valves consist of two flaps (cusps or leaflets) with edges that meet. These valves help veins return blood to the heart. As blood moves toward the heart, it pushes the cusps open like a pair of one-way swinging doors (shown on the left).
The valves act as a one way system to promote the flow of blood back to the heart. If they didn't exist then blood would clearly flow with gravity and away from the heart when you stand. However in my eyes it's not entirely clear what the sequence of events here. The two broad schools of thought are either the vein valves fail primarily allowing pressure to build up in the venous system or the vein wall dialates in a response to sustained elevations of venous pressure. There's a plethora of literature on this subject.