The human heart has four valves. Each one acts as a one-way gate, opening to let blood through and snapping shut to prevent it from flowing backward. Together, they keep blood moving in a single direction through the heart’s four chambers and out to the lungs and body.
The Four Valves and Where They Sit
The four valves are arranged in two pairs based on their position in the heart.
The first pair sits between the upper chambers (atria) and the lower chambers (ventricles):
- Tricuspid valve: Located on the right side, it lets blood flow from the right atrium into the right ventricle.
- Mitral valve: Located on the left side, it lets blood flow from the left atrium into the left ventricle.
The second pair guards the exits from the ventricles:
- Pulmonary valve: Opens to let the right ventricle pump blood into the pulmonary artery, which carries it to the lungs to pick up oxygen.
- Aortic valve: Opens to let the left ventricle push oxygen-rich blood into the aorta and out to the rest of the body.
How the Valves Are Built
Each valve is made of thin, strong flaps of tissue called leaflets (sometimes called cusps). The number of leaflets varies by valve. The tricuspid valve has three leaflets, which is where it gets its name. The pulmonary and aortic valves also have three leaflets each. The mitral valve is the outlier: it has only two leaflets, which is why it’s sometimes called the bicuspid valve.
The two valves between the atria and ventricles (tricuspid and mitral) have extra structural support. Thin, cord-like strings of tissue connect their leaflet edges to small muscles on the ventricle wall. These cords act like anchor lines on a parachute. When the ventricle contracts, the muscles tighten the cords just enough to keep the leaflets from flipping inside out under pressure. Without them, the valve leaflets would bulge backward into the atrium and leak.
The pulmonary and aortic valves don’t need this anchoring system. Their crescent-shaped leaflets are reinforced by the pressure of blood itself pushing back against them once the ventricle finishes contracting.
How Valves Open and Close
Heart valves are entirely passive. They don’t have muscles of their own. Instead, they respond to pressure differences on either side of the leaflets.
Take the mitral valve as an example. When the left ventricle relaxes after a contraction, the pressure inside it drops suddenly below the pressure in the left atrium above. That pressure difference pushes the mitral valve open, and blood rushes down into the ventricle. As the atrium contracts and then the ventricle begins to squeeze, the pressure reverses. Blood briefly pushes back against the leaflets, filling them like sails catching wind, and the valve snaps shut. The same principle governs all four valves: higher pressure on one side opens them, higher pressure on the other side closes them.
This opening and closing happens with every heartbeat, roughly 100,000 times a day. Over a lifetime, each valve cycles billions of times.
What Happens When Valves Don’t Work Properly
Valve problems generally fall into two categories. In stenosis, a valve stiffens or narrows and doesn’t open fully, forcing the heart to work harder to push blood through a smaller opening. In regurgitation (sometimes called a “leaky valve”), the leaflets don’t close tightly, allowing blood to flow backward with each beat.
These problems are common. Roughly 24 million people worldwide have mitral regurgitation, making it the most frequent valve disorder. About 9.4 million people have calcific aortic valve disease, a condition where calcium deposits gradually stiffen the aortic valve leaflets over years. Rheumatic heart disease, caused by untreated strep throat damaging valve tissue, affects approximately 41 million people, mostly in developing countries.
Bicuspid Aortic Valve
Some people are born with an aortic valve that has two leaflets instead of the normal three. This is the most common congenital heart defect. A two-leaflet aortic valve can function well for decades, but it’s more prone to stiffening or leaking later in life because the two leaflets handle more stress per beat than three would.
How Valve Problems Are Detected
Healthy valves open and close almost silently. When a valve is narrowed or leaky, blood flow becomes turbulent, producing a sound called a heart murmur that a doctor can hear through a stethoscope. Not all murmurs indicate a problem, but they’re often the first clue that a valve isn’t working normally.
If a murmur sounds concerning, an echocardiogram (an ultrasound of the heart) can show exactly how well each valve opens and closes, how much blood leaks backward, and whether the heart is straining to compensate. Mild valve disease often needs nothing more than monitoring over time. Severe cases, where the valve is so tight or leaky that the heart begins to enlarge or weaken, may eventually require repair or replacement.
Symptoms of significant valve disease tend to creep in gradually: shortness of breath during activity, unusual fatigue, swelling in the ankles, or a fluttering sensation in the chest. Because the heart compensates for a long time before symptoms appear, many people live with moderate valve disease for years without knowing it.