The atrioventricular valves, usually called AV valves, are the two one-way gates inside your heart that keep blood flowing in the right direction between the upper chambers (atria) and the lower chambers (ventricles). The one on the left side is the mitral valve, which has two leaflets; the one on the right is the tricuspid valve, which has three. Together they open to let blood fill the ventricles during relaxation, then snap shut when the ventricles contract so blood is pushed forward into the arteries rather than backward into the atria. That basic job description sounds simple, but the engineering behind it involves an intricate support system, surprisingly sophisticated fluid dynamics, and a vulnerability to disease that makes AV valve problems among the most common reasons people need heart surgery.
What the AV Valves Are Made Of
Each AV valve is not just a flap of tissue. It is a multi-part apparatus: the leaflets themselves, a ring of fibrous tissue called the annulus that anchors the leaflets to the heart wall, a set of cord-like structures called chordae tendineae that tether the leaflets to small muscles inside the ventricle, and those muscles themselves, the papillary muscles. The chordae tendineae are critical because they distribute the enormous forces generated when the ventricle contracts, preventing the leaflets from blowing backward into the atrium like an umbrella inverting in wind.1PubMed Central. Mechanics and Microstructure of the Atrioventricular Heart Valve Chordae Tendineae: A Review If the chordae rupture or stretch too far, blood leaks backward, a condition called regurgitation.
An interesting finding from comparative studies in porcine hearts is that the mitral and tricuspid leaflets themselves are not dramatically different in their mechanical properties or microscopic structure. Instead, the chordae tendineae on each side show distinct differences in shape, stiffness, and internal architecture that compensate for the different pressures and loading conditions in the left versus right ventricle.2PubMed Central. Comparative mechanical, morphological, and microstructural characterization of porcine mitral and tricuspid leaflets and chordae tendineae The left side of the heart pumps blood to the entire body at much higher pressures, so its chordae are built to handle heavier loads even though the leaflet tissue is essentially similar on both sides.
Within the leaflets, layers of tissue house specialized cells called valvular interstitial cells. These cells maintain the structural integrity of the valve by constantly building and breaking down proteins. Research on the mitral valve has shown that how these cells stretch and deform depends more on which layer of the leaflet they sit in than on what the valve as a whole is doing.3PubMed Central. Quantification and simulation of layer-specific mitral valve interstitial cells deformation under physiological loading That matters because it means the valve’s internal maintenance is locally regulated, which helps explain how these thin tissues can survive billions of opening-and-closing cycles over a lifetime without wearing out.
How AV Valves Actually Close
You might picture the mitral valve slamming shut the instant the ventricle starts squeezing, but it actually begins closing before that. As blood flows through the open mitral valve into the left ventricle during the filling phase, a ring-shaped vortex forms behind the valve leaflets. This swirling flow pushes the leaflets toward each other so that the valve is mostly closed by the time the ventricle contracts.4Cardiovascular Research. Fluid mechanics of a model mitral valve and left ventricle The ventricular contraction then finishes the job and seals the valve tightly.
Those vortices are not just a side effect of blood flow; they serve a purpose. Research using advanced flow-mapping techniques has shown that the inner borders of the mitral vortices act like a virtual channel, guiding the incoming blood toward the bottom of the ventricle and helping the heart fill efficiently. In patients with impaired relaxation of the ventricle, these vortices are weaker, which means the filling wave encounters resistance sooner. The atrium then has to contract harder or maintain higher pressure to achieve the same amount of filling.5PubMed Central. Vortices formed on the mitral valve tips aid normal left ventricular filling So when cardiologists talk about diastolic dysfunction, the AV valve’s flow dynamics are a central part of the story.
The Sound of a Closing AV Valve
The first heart sound you hear through a stethoscope, the classic “lub,” is produced mainly by the AV valves closing. It has two components: M1 from the mitral valve and T1 from the tricuspid valve. M1 almost always comes first, because electrical activation of the left ventricle slightly precedes the right, and the left side fires at higher pressures. Intracardiac micromanometer studies have confirmed that each component corresponds to a pressure wave (called a C wave) in its respective atrium, essentially the brief shudder that occurs when the valve leaflets abruptly halt.6PubMed. The contribution of tricuspid valve closure to the first heart sound. An intracardiac micromanometer study
Computational models have put numbers on this: M1 leads T1 by roughly 15 milliseconds, and left-sided components are louder.7PLOS Computational Biology. Hemodynamics-driven mathematical model of first and second heart sound generation That delay is usually too short to hear as a true split, but in conditions like left bundle branch block or mitral stenosis, the timing flips or widens enough that the first heart sound sounds different, sometimes giving a clinician the first clue that something is off.8PubMed. The contribution of tricuspid valve closure to the first heart sound. An intracardiac micromanometer study
The Fibrous Skeleton and Electrical Insulation
Both AV valves are anchored into a dense band of connective tissue at the base of the ventricles called the cardiac fibrous skeleton. This structure does double duty. Mechanically, it provides a firm attachment point for the valve leaflets so they do not get pulled out of shape by the powerful contractions on either side. Electrically, it acts as an insulating barrier between the atria and ventricles, ensuring that the only normal electrical pathway between upper and lower chambers runs through the AV node and the bundle of His.9PubMed. Fibrous Skeleton of the Heart: Anatomic Overview and Evaluation of Pathologic Conditions with CT and MR Imaging When that insulation breaks down, whether from calcification, fibrosis, or a congenital gap, abnormal electrical connections can form, leading to arrhythmias such as pre-excitation syndromes.
How AV Valves Form Before Birth
During embryonic development, the heart starts as a simple tube. The AV valves begin as swellings of tissue called endocardial cushions that appear in the region between the future atria and ventricles. These cushions form through a process where endocardial cells lining the heart undergo a dramatic identity shift, transforming from flat lining cells into mobile, stem-cell-like mesenchymal cells. This transformation is essential for generating the progenitor cells that will go on to build both the valve leaflets and the septa that separate the four chambers.10PubMed Central. Endocardial and epicardial epithelial to mesenchymal transitions in heart development and disease
Growth of those cushions requires tight molecular control. The signaling molecule β-catenin, for instance, is needed for the cells to multiply and migrate properly during cushion formation, and it works in part by suppressing a cell-cycle brake called p21.11Life Science Alliance. β-Catenin regulates endocardial cushion growth by suppressing p21 Tiny RNA molecules called microRNAs also fine-tune the process; overexpression of miR-23b and miR-199a can block the transformation of endocardial cells into mesenchymal cells and impair a large subset of genes involved in cushion development.12PubMed. MiR-23b and miR-199a impair epithelial-to-mesenchymal transition during atrioventricular endocardial cushion formation Defects at any stage of this tightly choreographed process can lead to congenital valve malformations, ranging from minor leaflet abnormalities to complete AV canal defects that require surgical correction early in life.
What Goes Wrong With AV Valves
AV valve disease falls into a few broad categories, each with a different root cause and a different set of consequences.
Mitral Valve Prolapse
Mitral valve prolapse (MVP) is the most common structural AV valve abnormality in developed countries, affecting a few percent of the population. In MVP, one or both leaflets bulge backward into the left atrium during ventricular contraction. The underlying problem in most cases is myxomatous degeneration, a disorder of connective tissue in which the extracellular matrix becomes disorganized through the activity of enzymes that break down structural proteins like collagen and elastin.13PubMed. Mitral valve prolapse Many people with mild MVP live their entire lives without symptoms or complications. In more severe cases, the prolapsing leaflets allow blood to leak backward, and if that regurgitation worsens over time, surgical repair or replacement becomes necessary.
Rheumatic Valve Disease
Globally, rheumatic heart disease remains one of the most devastating threats to AV valves. It begins with a strep throat infection that triggers an abnormal immune response, and the resulting inflammation attacks the valve tissue.14PubMed Central. Rheumatic Heart Valve Disease Pathophysiology and Underlying Mechanisms The mitral valve is hit most often. Over years, persistent immune activation and fibrosis thicken and stiffen the leaflets, sometimes fusing them together.15PubMed Central. Chronic Mitral Valve Fibrosis in Rheumatic Heart Disease: From Immune Trigger to Inflammatory and Mechanical Progression Mechanical stress on the already-damaged valve then accelerates the scarring, creating a vicious cycle. The result can be stenosis (the valve cannot open fully), regurgitation (it cannot close fully), or both. In low-resource settings where access to antibiotics and cardiac surgery is limited, rheumatic AV valve disease remains a major killer, especially of young adults.
Functional Mitral Regurgitation
Sometimes the mitral valve leaks even though the leaflets and chordae are structurally normal. This is called functional mitral regurgitation, and it happens because the ventricle itself has remodeled. In people with ischemic heart disease or dilated cardiomyopathy, the left ventricle enlarges and changes shape. The papillary muscles get pulled outward and downward, which tethers the leaflets and prevents them from closing properly.16PubMed. Determinants of the degree of functional mitral regurgitation in patients with systolic left ventricular dysfunction: A quantitative clinical study At the same time, the annulus dilates, widening the gap the leaflets need to bridge.17PubMed. New Perspective on Pathophysiology and Management of Functional Mitral Regurgitation This is a frustrating clinical problem because fixing the valve alone does not fix the underlying ventricular dysfunction, and the regurgitation may recur after repair.
Drugs That Can Damage AV Valves
A lesser-known threat to AV valves comes from certain medications. The connection was first noticed in the mid-1960s with ergot-derived drugs whose structures resemble serotonin, a signaling molecule. Patients using these drugs developed valve thickening strikingly similar to what is seen in carcinoid heart disease, a condition in which tumors release high levels of serotonin into the bloodstream. The exact mechanism remained unclear until the late 1990s, when the diet drug combination fenfluramine-phentermine (Fen-Phen) caused a wave of valve disease cases. Researchers traced the damage to a specific serotonin receptor, the 5-HT2B receptor, which when activated drives fibrotic changes in valve tissue.18PubMed Central. Serotonin receptors and heart valve disease–it was meant 2B Since then, several other drugs that activate this receptor have been flagged, including some Parkinson’s disease medications. Screening new drugs for 5-HT2B activity has become part of the pharmaceutical safety toolkit.
How Doctors Assess AV Valve Function
Echocardiography, or cardiac ultrasound, remains the first-line tool for evaluating AV valves. Standard two-dimensional echocardiography (2DE) provides a reasonable picture, but it has significant limitations when it comes to quantifying how much a valve leaks or how much an annulus has dilated. Three-dimensional echocardiography (3DE) substantially improves accuracy. For tricuspid regurgitation, 3DE agrees with cardiac MRI roughly 87% of the time (within one grade of severity), compared with only about 68% for 2DE. Two-dimensional echo frequently overestimates how severe the leak is: more than half of cases were overestimated by 2DE versus about a fifth by 3DE.19PubMed. 3D Echocardiographic and CMR Imaging for the Assessment of Right Ventricular Function and Tricuspid Regurgitation Severity
For the tricuspid annulus specifically, real-time 3D echo provides measurements comparable to MRI and can guide surgical decision-making about whether the annulus needs to be tightened during valve repair.20PubMed Central. Value of assessment of tricuspid annulus: real-time three-dimensional echocardiography and magnetic resonance imaging Cardiac MRI itself is considered the gold standard for measuring chamber volumes and valve function, but it is more expensive, less widely available, and cannot be done on patients with certain implanted devices. In practice, the choice between imaging methods depends on the clinical question and what is available at a given center.
Repair, Replacement, and Catheter-Based Options
When an AV valve fails badly enough to cause symptoms or heart damage, the options are surgical repair, surgical replacement, or increasingly, catheter-based intervention. Repair is generally preferred over replacement for the mitral valve because it preserves the patient’s own tissue and avoids the need for lifelong blood thinners (which mechanical replacement valves require) or the limited lifespan of biological replacement valves.
For patients too sick or too elderly for open-heart surgery, catheter-based approaches have changed the landscape. The MitraClip device, for example, is delivered through a vein and clips together portions of the mitral leaflets to reduce regurgitation. Clinical trials and registries have shown it is safe for high-surgical-risk patients and produces meaningful reductions in regurgitation along with reverse remodeling of the left ventricle.21PubMed Central. Transcatheter mitral repair: MitraClip technique Catheter-based approaches for the tricuspid valve are newer and evolving quickly, as the tricuspid valve was historically undertreated, sometimes called “the forgotten valve” by cardiologists.
For functional ischemic mitral regurgitation, some surgeons use a technique that brings the displaced papillary muscles closer together, directly addressing the tethering that prevents the leaflets from closing.22Annals of Thoracic Surgery. Papillary Muscle Approximation for Functional Ischemic Mitral Regurgitation This is typically combined with annuloplasty, a ring sewn around the annulus to restore its size. The challenge remains that the ventricle may continue to remodel after surgery, so long-term outcomes depend on treating the heart failure as well as the valve.
AV Valves in Athletes
Vigorous training causes the heart to enlarge, and the AV valves remodel right along with it. Athletes show larger mitral and tricuspid annular areas and increased leaflet tenting compared with non-athletes.23PubMed Central. Mitral and Tricuspid Valve Disease in Athletes What is striking is that the annular enlargement is disproportionate to the increase in chamber size. In one 3D echocardiographic study of elite athletes, left ventricular volume increased by about 35% compared with sedentary controls, but the mitral annular area increased by roughly 63%.24PubMed. Geometrical remodeling of the mitral and tricuspid annuli in response to exercise training: a 3-D echocardiographic study in elite athletes Athletes who developed functional mitral regurgitation also had a significantly larger tricuspid annulus, suggesting that both AV valves are affected by the same remodeling process.
A clinical curiosity seen in some trained athletes is mid-diastolic tricuspid regurgitation, a brief backward leak through the tricuspid valve that occurs in the middle of the filling phase rather than during contraction. This appears to result from the combination of sinus bradycardia (the slow resting heart rates athletes typically have, which stretches out the filling period) and a large, highly compliant right ventricle that readily transmits pressure swings from breathing.25European Heart Journal – Cardiovascular Imaging. Mid-diastolic tricuspid regurgitation: a novel echocardiographic marker for an athlete’s heart? This finding can alarm a clinician who is unfamiliar with it, but it appears to be a benign marker of athletic adaptation rather than a sign of disease.
Tissue-Engineered AV Valves
Current replacement valves, whether mechanical or made from animal tissue, have real drawbacks. Mechanical valves last a long time but require lifelong anticoagulation, bringing bleeding risks. Bioprosthetic valves avoid blood thinners but wear out after a decade or two and cannot grow, which is a particular problem in children and young adults who would face multiple reoperations over their lifetime. Tissue-engineered heart valves are being developed to overcome both problems. The concept is to implant a scaffold that the patient’s own cells gradually infiltrate, transforming it into a living valve capable of growth, self-repair, and remodeling.26PubMed Central. Designing Biocompatible Tissue Engineered Heart Valves In Situ: JACC Review Topic of the Week Early versions have been tested mostly in the pulmonary position (the easiest valve to access and lowest-pressure environment), but the long-term goal includes AV valve applications. The technology is still in its early stages, and significant hurdles remain around getting the scaffold to remodel into tissue with the right mechanical properties, but the potential payoff of a truly lifelong, living replacement valve is enormous.
Leonardo da Vinci and the First Accurate Valve Drawings
The anatomy of the heart’s valves has fascinated observers for centuries, but accurate depiction was slow to arrive. Leonardo da Vinci, driven by his interest in the relationship between form and function in the human body, was the first person to produce accurate drawings of the heart and its valves.27ScienceDirect. Discovery of the cardiovascular system: from Galen to William Harvey His anatomical sketches, made from direct dissections in the late 1400s and early 1500s, captured the geometry of the valve leaflets and the swirling flow patterns behind them with a level of detail that was not matched by the medical profession for generations. Remarkably, his intuition about the vortices behind the aortic valve was only confirmed experimentally in the twentieth century, and similar vortex dynamics behind the AV valves, as described earlier in this article, remain an active area of fluid-dynamics research today.