Heart valves open and close because of pressure differences on either side of their leaflets. Blood pushes against one side of a valve, and when the pressure on that side exceeds the pressure on the other, the valve swings open. When the pressure reverses, the valve shuts. No muscles directly pull the valves open or closed. They are passive structures, moved entirely by the force of blood flowing around them.
How Pressure Drives Every Valve Movement
Your heart has four valves, and each one acts like a one-way door. The principle is simple: blood flows from areas of higher pressure to areas of lower pressure, and the valves ride that pressure wave. When the upper chambers (atria) fill with blood returning from the body or lungs, pressure builds until it exceeds the pressure in the relaxed lower chambers (ventricles). That pressure difference pushes the valves between them open, and blood pours through. Moments later, the ventricles contract, pressure inside them skyrockets, and the same valves are forced shut so blood can’t flow backward.
The same logic applies to the valves guarding the exits of the ventricles. When a ventricle contracts hard enough that its internal pressure exceeds the pressure in the artery beyond it, the exit valve opens and blood is ejected. Once the ventricle relaxes and its pressure drops, the higher pressure in the artery pushes blood back against the valve cusps, snapping them closed. Every opening and every closing comes down to which side of the valve has more pressure at that instant.
The Two Types of Valves Work Differently
The four valves fall into two pairs, and each pair has a slightly different design suited to its job.
The mitral valve (left side) and tricuspid valve (right side) sit between the atria and ventricles. These are called atrioventricular valves, and they have thin, sail-like flaps anchored by cord-like tethers called chordae tendineae. Those cords connect to small muscles on the inner wall of the ventricle called papillary muscles. When the ventricles contract, the papillary muscles contract at the same time, pulling the cords taut. This prevents the valve flaps from blowing backward into the atrium under the enormous pressure of a contracting ventricle. Without this tethering system, the flaps would flip inside out, and blood would leak the wrong way.
The aortic valve and pulmonary valve guard the exits into the two major arteries. These valves have three cup-shaped leaflets instead of flaps. They sit in small pockets along the artery wall. During contraction, blood pushes the cusps flat against the artery wall and flows past. When the ventricle relaxes, blood briefly flows backward, fills those pockets, and presses the three cusps together to form a tight seal. The pocket design makes closure fast and reliable.
The Full Sequence in One Heartbeat
A single heartbeat moves through a precise sequence of valve events. It starts when the atria are full and their pressure exceeds the pressure in the relaxed ventricles. The mitral and tricuspid valves open, and blood flows down into the ventricles. About two-thirds of ventricular filling happens passively this way, with the atria giving a final squeeze to push the last third through.
Then the ventricles contract. Pressure inside them rises sharply, forcing the mitral and tricuspid valves shut. For a brief moment, all four valves are closed. The ventricles are squeezing, pressure is climbing, but it hasn’t yet exceeded the pressure in the arteries. This phase, called isovolumetric contraction, lasts only a fraction of a second.
Once ventricular pressure surpasses arterial pressure, the aortic and pulmonary valves pop open and blood is ejected. The left ventricle typically generates a peak pressure between 90 and 140 mmHg to push blood past the aortic valve and into the body’s circulation.
After ejection, the ventricles relax. Their pressure drops below arterial pressure, and the aortic and pulmonary valves close. Again, briefly, all four valves are closed while the ventricles continue relaxing. Once ventricular pressure falls below atrial pressure, the mitral and tricuspid valves open, and the cycle starts over. The whole thing takes less than a second at a normal resting heart rate.
What the Valve Leaflets Are Made Of
Heart valve leaflets are built from layers of connective tissue that make them both flexible and remarkably durable. They contain collagen fibers for strength and elastic fibers that allow them to stretch and snap back to their original shape with every beat. The elastic fibers are concentrated in layers that face the blood flow, giving the leaflets the resilience to open and close roughly 100,000 times per day without tearing. Over a lifetime, that adds up to billions of cycles.
Despite this durability, valve tissue does change with age. Collagen fibers gradually become more numerous and less orderly, and certain layers of the leaflet thin out. These changes can eventually stiffen the leaflets or make them less effective at sealing.
The Sounds You Can Hear
The familiar “lub-dub” of a heartbeat is the sound of valves closing. The first sound (“lub”) happens when the mitral and tricuspid valves snap shut at the start of ventricular contraction. The second sound (“dub”) comes from the aortic and pulmonary valves closing after blood has been ejected. These sounds are created by vibrations in the valve structures and surrounding blood when flow suddenly stops or changes direction. Turbulent blood flow produces louder vibrations, which is why abnormal valves often create extra sounds or murmurs that a doctor can hear through a stethoscope.
What Happens When Valves Stop Working
Valve problems generally fall into two categories. In stenosis, the valve opening narrows and can’t open fully. A healthy aortic valve opens to about three to five square centimeters. When calcium deposits or scarring stiffen the leaflets, that opening shrinks, and the heart has to work much harder to push blood through a smaller gap. Over time, this extra workload thickens and weakens the heart muscle.
In regurgitation, the valve doesn’t close completely, and blood leaks backward. This can happen if the leaflets stretch, the cords anchoring them break, or the ring of tissue supporting the valve widens. The heart compensates by pumping more volume with each beat, but this too eventually leads to enlargement and weakening if left untreated.
Both problems trace back to the same core mechanism: pressure is supposed to move the valve in one direction, but something about the valve’s structure prevents it from opening wide enough or sealing tightly enough. The pressure system itself still works. It’s the valve hardware that fails.