Merck Manual

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One-Way Valves in the Veins
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). If gravity momentarily pulls the blood backward or if blood begins to back up in a vein, the cusps are immediately pushed closed, preventing backward flow (shown on the right).