The atrioventricular valves are two valves inside your heart that sit between the upper chambers (atria) and lower chambers (ventricles), acting as one-way gates that keep blood flowing in the right direction. There are exactly two: the mitral valve on the left side and the tricuspid valve on the right. Together, they open to let blood fill the ventricles, then snap shut to prevent it from leaking backward when the ventricles contract.
The Two Valves: Mitral and Tricuspid
The mitral valve connects the left atrium to the left ventricle and has two leaflets, sometimes called the anterior and posterior leaflets. Because of this two-flap design, it’s occasionally referred to as the bicuspid valve. The mitral valve handles the higher-pressure side of the heart, where oxygen-rich blood returning from the lungs gets pumped out to the rest of the body.
The tricuspid valve sits between the right atrium and right ventricle and has three leaflets: anterior, posterior, and septal. It manages the lower-pressure side, where oxygen-depleted blood returning from the body gets directed toward the lungs. Despite handling less pressure, the tricuspid valve has a slightly larger opening. A healthy mitral valve has an opening area of about 4 to 6 square centimeters, while the tricuspid valve averages around 4.8 square centimeters.
How They Open and Close
The atrioventricular valves are passive structures. They don’t have muscles of their own. Instead, they respond entirely to pressure differences between the chambers above and below them.
When the ventricles relax after a contraction, the pressure inside them drops below the pressure in the atria. That pressure difference pushes the valve leaflets open, and blood flows downward to fill the ventricles. This filling phase is called diastole.
Once the ventricles are full and begin to contract, the rising pressure in the ventricles quickly exceeds the pressure in the atria. This reversal of pressure forces the leaflets shut. The closure of both atrioventricular valves at nearly the same instant produces the first heart sound, known as S1. If you’ve ever listened to a heartbeat and heard the familiar “lub-dub,” the “lub” is S1, the sound of these two valves closing together. That closure marks the beginning of systole, the contraction phase when the ventricles push blood out into the arteries.
The Support System: Chordae and Papillary Muscles
Closing a valve under high pressure creates a real engineering challenge. Without something holding the leaflets in place, the force of a contracting ventricle would blow them backward into the atria, like an umbrella flipping inside out in the wind. The heart solves this with a tethering system made of two components: chordae tendineae and papillary muscles.
The chordae tendineae are thin, cord-like strands that attach to the underside of each valve leaflet and anchor them to small columns of muscle (papillary muscles) protruding from the ventricle walls. When the ventricles contract, the papillary muscles also contract, pulling the chordae taut. This keeps the leaflets from flipping into the atria, a condition called prolapse. The chordae also distribute the mechanical stress of closure across the leaflet surface so no single point bears too much force.
The tricuspid valve’s papillary muscles are more variable and numerous than those of the mitral valve, reflecting the more complex geometry of the right ventricle.
What the Leaflets Are Made Of
Each valve leaflet has a layered internal structure designed to handle the repetitive stress of opening and closing roughly 100,000 times a day. There are three main layers. The fibrosa is the thickest, composed of densely packed collagen bundles that give the leaflet its strength. The spongiosa is a middle cushioning layer rich in gel-like molecules that absorb shock during closure. The atrialis faces the atrial side and contains thin elastic fibers running in parallel, giving the leaflet flexibility to stretch and recoil with each heartbeat.
This combination of strength, cushioning, and elasticity allows the valves to endure billions of cycles over a lifetime without wearing out under normal conditions.
What Goes Wrong: Stenosis and Regurgitation
The two main problems that affect atrioventricular valves are stenosis and regurgitation, and they represent opposite mechanical failures.
Stenosis occurs when the valve opening becomes narrowed and stiff, preventing it from opening fully. For the mitral valve, problems typically begin when the opening shrinks below 2 square centimeters, roughly half its normal minimum. At that point, blood backs up behind the valve, increasing pressure in the atrium and eventually in the lungs. The most common cause of mitral stenosis worldwide is rheumatic heart disease, which develops after untreated strep throat infections cause scarring on the valve leaflets.
Regurgitation happens when the valve doesn’t close completely, allowing blood to leak backward into the atrium during ventricular contraction. This can result from leaflet prolapse (where the leaflets bulge too far upward), from stretched or torn chordae tendineae, or from a dilated valve ring that prevents the leaflets from meeting in the middle. The heart has to work harder to compensate for the leaked volume, which over time can lead to heart failure.
Common Causes of Valve Disease
- Aging: Calcium deposits build up on the leaflets over decades, making them thicker and stiffer.
- Rheumatic disease: Scarring from untreated strep infections remains the leading cause of valve disease globally.
- Endocarditis: A severe bloodstream infection that damages the valve lining, sometimes associated with intravenous drug use.
- Heart attack: Damage to the papillary muscles can cause a valve to stop closing properly.
- Congenital defects: Some people are born with valve malformations.
- Connective tissue disorders: Conditions like Marfan syndrome can weaken the valve structure.
How Atrioventricular Valves Differ From Semilunar Valves
Your heart has four valves total. The two atrioventricular valves (mitral and tricuspid) sit between the atria and ventricles. The other two, the aortic and pulmonary valves, sit between the ventricles and the major arteries leaving the heart. These are called semilunar valves because their leaflets are shaped like half-moons.
The key structural difference is that semilunar valves have no chordae tendineae or papillary muscles. They rely on their cup-like shape to catch blood that tries to flow backward, pressing the leaflets together to form a seal. Atrioventricular valves, by contrast, need the full tethering apparatus because they face much greater backward pressure during ventricular contraction. The semilunar valves also have three small, symmetrical leaflets each, while the atrioventricular valves have the asymmetric two-leaflet and three-leaflet arrangements described above.