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).
It only flows through arteries in one direction due to the pressure being exerted by the heart. Veins have small valves along their length that prevent backwards flow. As to why it only travels one way through the heart, again it comes down to valves. It's like having a set of doors only one of which is open at a time. For example, blood enters the right atrium through the vena cava.