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).
Main function of valve is to prevent back flow of blood. This is because blood pressure in veins is very low (compared to arteries). There is no need of valves in the arteries because the blood pressure usually varies between 70-110 mm Hg. Imagine you have just opened a tap with full flow in a washroom and you applied your hand at the outflow. That is just 50-60 mm Hg (in normal circumstances).