The mitral valve controls the flow of oxygen-rich blood between the two left chambers of your heart. It opens to let blood pass from the upper chamber (left atrium) into the lower chamber (left ventricle), then snaps shut to prevent blood from leaking backward. This one-way gating action happens with every heartbeat, roughly 100,000 times a day, keeping blood moving efficiently toward the rest of your body.
Where the Mitral Valve Sits
The mitral valve is one of four valves in the heart. It sits between the left atrium and the left ventricle, the two chambers on the heart’s left side. These left-side chambers handle blood that has just picked up fresh oxygen from the lungs. After traveling through the pulmonary veins into the left atrium, blood passes through the mitral valve into the left ventricle, which then pumps it out through the aorta to supply the entire body.
Because the left ventricle generates the highest pressure of any heart chamber, the mitral valve needs to form a tight seal every time it closes. A healthy mitral valve opening measures about 4 to 6 square centimeters, large enough to allow blood to flow freely during the filling phase of each heartbeat.
How the Valve Opens and Closes
The mitral valve’s action is tied to the two phases of each heartbeat. During the filling phase (diastole), the left ventricle relaxes and pressure drops inside it. That pressure difference causes the valve’s flaps to swing open, and blood flows down from the atrium. During the pumping phase (systole), the ventricle contracts forcefully. The rising pressure pushes the flaps shut, sealing off the atrium so blood is driven out through the aortic valve and into the body instead of backward.
When the mitral valve closes at the start of each contraction, it produces the louder portion of the first heart sound, the familiar “lub” your doctor hears through a stethoscope. The left ventricle contracts slightly earlier than the right, so the mitral valve’s closure is the leading component of that sound. Changes in how loud or soft the first heart sound is are largely driven by what’s happening at the mitral valve.
The Parts That Make It Work
Unlike the other three heart valves, which each have three flaps, the mitral valve has only two. These flaps are called the anterior and posterior leaflets. The anterior leaflet is larger, longer, and thicker. The posterior leaflet is crescent-shaped with a wider base but shorter overall height. Together, they meet along a seam line to form a seal when the valve closes.
Holding the leaflets in place is a support system called the subvalvular apparatus. Thin, cord-like structures called chordae tendineae connect the underside of each leaflet to small finger-like muscles (papillary muscles) anchored in the wall of the left ventricle. Each valve typically has 10 to 20 individual cords that branch out before fanning into the leaflets. Some cords attach at the leaflet edges, preventing the flaps from flipping inside out under pressure. Others attach farther back on the leaflet body, providing structural reinforcement.
When the ventricle contracts, the papillary muscles contract at the same time. This keeps the distance between the valve ring and the muscle tips constant even as the surrounding heart wall is squeezing inward. The effect is like a shock absorber: the cords stay taut, the leaflets stay in position, and blood can’t push them backward into the atrium.
What Happens When It Stops Working
Mitral valve disease takes two main forms, depending on whether the valve leaks or becomes too narrow.
- Regurgitation (a leaky valve): The leaflets don’t close tightly, so blood leaks backward into the left atrium each time the ventricle pumps. The heart has to work harder to push enough blood forward, and over time the extra volume can stretch both the atrium and the ventricle.
- Stenosis (a narrowed valve): The leaflets become thick, stiff, or fused together, shrinking the opening. Once the valve area drops below about 2 square centimeters, it creates a significant bottleneck. The heart must generate more force to push blood through the smaller gap, and pressure builds up in the left atrium and eventually backs into the lungs.
Globally, about 15.5 million people live with degenerative mitral valve disease, a wear-and-tear form that becomes more common with age. Among people 70 and older, roughly 1 in 47 has the condition. Rheumatic heart disease, caused by untreated strep infections, is another major driver of mitral valve problems worldwide.
Mitral Valve Prolapse
Mitral valve prolapse is one of the most common valve abnormalities. In this condition, one or both leaflets have extra tissue or are stretchier than normal. During each contraction, instead of forming a flat seal, the leaflets balloon backward into the left atrium like a parachute. Many people with prolapse have no symptoms at all and never need treatment. In some cases, though, the bulging leaflets don’t meet properly, allowing blood to leak backward. That leak (regurgitation) is what can eventually require intervention if it becomes severe enough to strain the heart.
Repair and Replacement
When mitral valve disease becomes severe enough to cause symptoms or weaken the heart, surgery may be recommended. There are two broad approaches: repairing the existing valve or replacing it with a prosthetic one.
Repair is generally preferred whenever it’s feasible. Degenerative mitral valve disease, the most common type in developed countries, tends to respond well to repair because the tissue, while stretched or torn, is still pliable enough to reshape. Surgeons can trim excess leaflet tissue, reinforce the ring around the valve, or reattach broken cords. A successful repair preserves the natural valve and avoids the need for lifelong blood-thinning medication that some replacement valves require.
Replacement becomes the better option when the valve is too damaged to reconstruct reliably. This is sometimes the case with rheumatic disease, which can stiffen and calcify the leaflets beyond repair. Younger patients who need a replacement typically receive a mechanical valve, which is extremely durable but requires daily blood thinners. Older patients, generally over 60, are more often given a tissue valve made from animal tissue, which doesn’t require blood thinners but wears out over time and may eventually need to be replaced again.