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How do heart valves prevent blood from flowing backward?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

Heart valves ensure one-way blood flow through the heart by using specialized structures to block backflow. There are two primary mechanisms based on valve type:

Atrioventricular (AV) Valves: The tricuspid valve and mitral valve consist of flaps that are pushed upward when the ventricles contract. To prevent these flaps from billowing back into the atria, they are restrained by chordae tendineae (tendinous cords) connected to papillary muscles. These structures hold the flaps in place, forming a complete partition.12
Semilunar Valves: The aortic valve and pulmonary valve are pocketlike, half-moon-shaped structures. When blood flow reverses, these pockets fill with blood, causing the flaps to press against each other and seal the orifice, thereby blocking reentry into the ventricles.13

These valves are composed of connective tissue lined with endocardium, and their rhythmic closure is responsible for the audible sounds of the heartbeat.23