Two heart valves positioned between the upper collecting chambers and the lower pumping chambers open during ventricular diastole to let blood fall downward into the ventricle, then snap shut during ventricular systole so blood cannot leak back into the atrium. Picture two one-way doors guarding the only doorways between the upper and lower floors of a four-room house. Because the left ventricle pumps at far higher pressure than the right, those two doors are built from slightly different materials and held by a different number of anchors, which is why anatomy students are asked to memorize them in detail.
This walkthrough explains where each one sits, how its parts work together, and why clinicians pay close attention to the sounds it makes during the cardiac cycle.
The Two Gatekeepers Between Atria and Ventricles
Your heart contains four valves in total, but only two of them belong to the atrioventricular family. The right atrioventricular orifice, between the right atrium and right ventricle, is sealed by the tricuspid valve, so named because its leaflets form three cusps. The left atrioventricular orifice, between the left atrium and left ventricle, is sealed by the mitral valve, which carries two cusps and is therefore also called the bicuspid valve.
Think of each valve as a one-way door hanging from the fibrous ring of the cardiac skeleton. When the door swings open, blood pours from the upper chamber into the lower one. When pressure below pushes up, the door closes and the cords beneath it lock the leaflets in place so nothing flows backward.
That locking action depends on more than just the leaflets themselves, which is where the supporting apparatus comes in.
Quick memory hook: tri-cuspid goes tri-ght (three on the right), while the bi-cuspid mitral valve stands bi-side (two on the left).
Anatomy of Each Valve and Its Supporting Apparatus
Each atrioventricular valve is more than just flaps of tissue. Three structures work together: the leaflets themselves, the chordae tendineae, and the papillary muscles. Surrounding them, the fibrous rings of the cardiac skeleton keep the doorframe from stretching under pressure.
Leaflet Shape and Naming
The tricuspid valve has three cusps named for where they sit: anterior, septal, and posterior. The mitral valve has only two, the anterior and posterior leaflets, and the anterior leaflet is roughly twice the height of its posterior partner. Both sets of leaflets are lined by a thin, smooth layer of endocardium, the same tissue that lines the inside of every chamber.
Chordae Tendineae and Papillary Muscles
Attached to the free edge of every leaflet are the chordae tendineae, tough string-like tendons that look like the cords of a parachute. Those cords anchor into the papillary muscles, finger-shaped muscular projections that rise from the inner wall of the ventricle. Together they stop the leaflets from flopping backward into the atrium when the ventricle contracts.
The Cardiac Skeleton as Anchor Frame
Surrounding all four valves is the fibrous skeleton of the heart, sometimes called the annulus fibrosus. This rigid scaffold of collagen forms rings around each valve opening and prevents the orifices from dilating every time the ventricles squeeze. Without that scaffold, the atrioventricular valves would stretch open and leak during every heartbeat.
With the anatomy mapped, the real picture emerges when you watch these parts coordinate during a single beat.
| Structure | Tricuspid valve (right) | Mitral valve (left) |
|---|---|---|
| Leaflet count | 3 (anterior, septal, posterior) | 2 (anterior, posterior) |
| Chordae tendineae | Thinner, more numerous | Thicker, fewer |
| Papillary muscles | 3 groups | 2 groups (anterolateral and posteromedial) |
| Fibrous ring | Part of the cardiac skeleton | Part of the cardiac skeleton |
How Leaflets, Cords, and Muscles Work as a Single Unit
Those three components do not function on their own. They behave like a parachute system, in which every piece only works if the others pull their weight.
Opening During Diastole
As the ventricle relaxes, pressure inside it drops below the pressure in the atrium above. The leaflets, which were sealed shut, swing downward into the ventricle like saloon doors opening, and blood pours through. No muscle contraction is required. The fall in ventricular pressure alone is enough to push them open.
Closing During Systole
When the ventricle begins to contract, pressure inside it rises sharply. Blood pushes up against the underside of the leaflets, forcing them to drift back toward the atrium. Just before they can flip inside out, the papillary muscles contract and pull the chordae tendineae taut. The cords hold the leaflet edges firmly against each other in the midline, creating a watertight seal.
That sealed closure is the moment that generates the first heart sound and drives the rest of the cycle.
Picture the leaflets as the parachute canopy, the chordae as the suspension lines, and the papillary muscles as the riser lines pulled by the jumper. If any one of them fails to pull, the canopy collapses.
AV Valves in the Cardiac Cycle and the Heart Sounds They Produce
The full cardiac cycle has two main phases: ventricular diastole, when the heart relaxes and fills, and ventricular systole, when the ventricles contract and eject blood. The atrioventricular valves behave differently in each phase.
| Phase | Ventricular pressure | AV valve state | Resulting sound |
|---|---|---|---|
| Ventricular diastole | Low (falls below atrial pressure) | Open | Silent filling |
| Isovolumetric contraction | Rising rapidly | Shut, leaflets coapting | S1, the lub |
| Ventricular systole | High | Closed and sealed by chordae | Silent ejection |
| Isovolumetric relaxation | Falling | Shut until pressure drops below atrial pressure | S2, the dub |
The first heart sound, S1 or the lub, is produced by the sudden closure of both atrioventricular valves at the start of systole, along with the opening of the semilunar valves and the vibration of blood in the ventricles. The second heart sound, S2 or the dub, comes later, when the semilunar valves snap shut at the end of systole and the ventricles begin to relax.
During ventricular ejection, the atrioventricular valves stay firmly closed while the semilunar valves open, letting blood leave through the pulmonary artery on the right and the aorta on the left. Once ejection is complete and ventricular pressure falls again, the cycle resets.
Atrioventricular Versus Semilunar Valves: Key Distinctions
The semilunar valves, the aortic and pulmonary valves, share the heart with the atrioventricular valves, but their location, structure, and trigger for opening set them in a separate category.
| Feature | Atrioventricular valves | Semilunar valves |
|---|---|---|
| Location | Between atrium and ventricle | At the great artery outlets |
| Structure | Leaflets with chordae tendineae and papillary muscles | Three pocket-like cusps, no cord apparatus |
| Opening trigger | Falling ventricular pressure (diastole) | Rising ventricular pressure (systole) |
| Closing trigger | Rising ventricular pressure (systole) | Falling arterial pressure (back-pressure) |
| Heart sound produced at closure | S1 (lub) | S2 (dub) |
The it need their chord-and-muscle support because the ventricles contract forcefully right behind them. The semilunar valves, by contrast, sit at artery outlets where their crescent-shaped cusps can seal themselves using only the backward pressure of blood in the aorta or pulmonary artery.
Common Clinical Conditions That Affect the AV Valves
When the it fail, blood moves in the wrong direction or not at all. The two core failure modes show up across nearly every valve problem.
Stenosis and Regurgitation
Stenosis is the narrowing of a valve opening, usually from scarring or calcification, which forces blood through a smaller hole. Regurgitation is backflow, in which the leaflets fail to seal completely and blood leaks backward into the atrium during systole. Both conditions are graded by severity, from mild through moderate to critical, and both reduce the heart’s overall efficiency.
Mitral Valve Prolapse and Rheumatic Heart Disease
Mitral valve prolapse affects roughly 2 to 3 percent of adults and happens when the leaflets billow upward into the left atrium during systole because the chordae are slightly too long. Many people with prolapse never notice symptoms, though some develop mitral regurgitation over time. Rheumatic heart disease, by contrast, follows untreated streptococcal infection and scars the valve leaflets, especially the mitral valve, which is the most frequently affected of the it in that disease.
Memory Hooks for the Long Term
- Tricuspid = right side. Three letters in tri, three leaflets, three on the right.
- Mitral = left side, two leaflets. Bi in bicuspid matches the two cusps.
- Lub = AV closure. S1 marks the start of systole when the it snap shut.
- Dub = semilunar closure. S2 marks the end of systole when the aortic and pulmonary valves snap shut.
The mitral valve is the most commonly affected of the two it in clinical practice, which is why mitral valve prolapse and mitral regurgitation appear so often in board-style questions and bedside examinations.
Bottom Line
it are the two one-way doors that keep blood moving from the atria down into the ventricles during diastole and prevent backflow during systole. Once you can picture leaflets swinging on a fibrous frame, held in place by parachute-like cords and papillary muscles, the rest of the cardiac cycle falls into place.
FAQ
What are the atrioventricular valves?
The it are the two heart valves that sit between the atria and the ventricles. The tricuspid valve guards the right side and the mitral valve, also called the bicuspid valve, guards the left side.
What is the function of the atrioventricular valves?
Their function is to let blood flow from each atrium into its ventricle during diastole and to seal shut during systole so blood cannot leak backward. Their closure at the start of systole produces the first heart sound, S1.
What is the difference between atrioventricular and semilunar valves?
it sit between the atria and ventricles and rely on chordae tendineae plus papillary muscles for support. Semilunar valves sit at the outlets of the pulmonary artery and aorta and use three pocket-like cusps with no cord apparatus.
What structures hold the atrioventricular valves in place?
Three structures keep them functional: the leaflets themselves, the chordae tendineae that tether the leaflet edges, and the papillary muscles that anchor the chordae. The fibrous rings of the cardiac skeleton keep the valve openings from stretching.
What happens if the atrioventricular valves fail?
Valve failure shows up as stenosis (narrowing) or regurgitation (backflow). Both reduce cardiac efficiency and can cause shortness of breath, fatigue, or fluid buildup, which is why a qualified cardiology team should evaluate any suspected valve problem.
Which atrioventricular valve is most often affected by disease?
The mitral valve is the most commonly affected, showing up in mitral valve prolapse, mitral regurgitation, and rheumatic mitral stenosis more often than its right-sided counterpart.
