During ventricular contraction the av valves are closed, and that simple fact makes all the difference in how the heart pumps blood. The “lub” comes from the atrioventricular (AV) valves snapping shut when the ventricles squeeze. If you’ve ever listened to a heartbeat, you’ve heard the “lub‑dub” sound. It’s a tiny mechanical event, but it’s the reason your body gets oxygen‑rich blood with every beat.
What Is Ventricular Contraction?
Ventricular contraction is the phase of the cardiac cycle when the heart’s lower chambers — the ventricles — tighten and push blood out to the lungs and the rest of the body. It follows atrial systole, the period when the upper chambers fill with blood and then contract to give the ventricles a final fill. The whole process is timed by electrical signals that travel through the heart’s conduction system, but the mechanical action is what we’re talking about here.
In plain terms, think of the ventricles as two muscular balloons. When they’re relaxed, they’re like deflated balloons that can be filled. When they contract, they become tight fists that squeeze the blood out through the great vessels — the pulmonary artery and the aorta. The AV valves, which sit between each atrium and ventricle, are the gatekeepers that open when the atria push blood in and close when the ventricles push back But it adds up..
Why It Matters
You might wonder why anyone should care about the exact state of the AV valves during that split‑second squeeze. The answer is simple: if those valves stay open, blood will flow backward into the atria instead of moving forward where it needs to go. That backflow, called regurgitation, forces the heart to work harder, reduces overall efficiency, and can lead to symptoms like shortness of breath, swelling, or even heart failure over time.
When the AV valves are closed during ventricular contraction, the pressure inside the ventricles rises sharply. It also keeps the atria from emptying prematurely, preserving the filling cycle for the next beat. Worth adding: that pressure pushes blood out through the semilunar valves (the aortic and pulmonary valves) into the arteries. In short, the closure of the AV valves is what makes the “forward” flow possible.
How It Works
Pressure Changes in the Heart
The key driver of valve behavior is pressure. As the ventricles contract, the pressure inside them climbs from a few millimeters of mercury at rest to about 20–25 mm Hg in a healthy heart. That rise is faster than the pressure in the atria, which stays relatively low because the atria are still filling or have just finished a gentle contraction. On top of that, when ventricular pressure exceeds atrial pressure, the AV valves are forced shut. It’s a bit like a one‑way door that only opens when the outside pressure is lower than the inside That's the part that actually makes a difference..
If you ever feel a sudden “thump” in your chest when you stand up quickly, that’s the moment the ventricles contract and the AV valves close, sending a brief surge of pressure through the system. It’s a normal, essential part of the rhythm.
The Mechanics of Closure
The AV valves aren’t just flaps that snap shut; they’re complex structures with a job to do. Each valve consists of leaflets — thin, flexible tissue that can open and close like a curtain. When the ventricles relax, the pressure inside them drops, allowing blood to flow from the atria into the ventricles. Here's the thing — the leaflets are held open by the pressure difference, and they’re tethered to the ventricular walls by tiny cords called chordae tendineae. Those cords prevent the leaflets from ballooning back into the atria when the ventricles contract.
When the ventricles start to squeeze, the pressure on the ventricular side of the leaflets spikes. The result is a rapid, secure closure that stops any backflow. And the leaflets bend toward the atria, and the chordae become taut, pulling the leaflets shut. Because the closure happens so quickly, you hear the “lub” sound that marks the beginning of ventricular systole.
Common Mistakes
A lot of popular health articles get this part wrong, and it’s worth setting the record straight.
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Mistake 1: “The AV valves open during ventricular contraction.”
In reality, the AV valves are closed while the ventricles are pushing blood out. They only open when the ventricles relax and the pressure in the atria becomes higher than the ventricular pressure The details matter here.. -
Mistake 2: “The AV valves are the same as the semilunar valves.”
The semilunar valves (aortic and pulmonary) open when ventricular pressure exceeds arterial pressure, allowing blood to leave the heart. The AV valves serve a completely different purpose — they keep blood moving in the right direction between atria and ventricles Worth keeping that in mind.. -
Mistake 3: “If the AV valves are leaky, the heart can’t contract effectively.”
While severe regurgitation does force the heart to work harder, the ventricles can still contract. The problem is that some of the blood meant for the arteries leaks back, reducing the net amount that actually gets pumped out.
Understanding these nuances helps you avoid oversimplified explanations and appreciate the elegance of the heart’s design.
Practical Tips
If you’re a student, a clinician, or just someone who wants to understand their own body better, here are a few concrete ways to keep this concept clear:
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Listen to your heartbeat.
The “lub” sound is the AV valves closing. Count the beats and notice that the “lub” always precedes the “dub.” That timing tells you when contraction starts. -
Visualize the pressure curve.
Imagine a graph where the x‑axis is time and the y‑axis is pressure. The ventricular pressure line shoots up during contraction, while the atrial pressure stays low. The point where the two lines cross is when the AV valves close. -
Use a simple model.
Grab two balloons (one for the ventricle, one for the atrium) and a piece of flexible fabric for the valve. Inflate the ventricle balloon, then let the pressure rise. The fabric will naturally fold inward, mimicking valve closure. -
Check for symptoms.
If you experience persistent chest discomfort, swelling, or unusual fatigue, it could signal that the AV valves aren’t closing properly. A quick visit to a healthcare professional can catch problems early Turns out it matters.. -
Don’t skip the filling phase.
Even though the AV valves close during contraction, the preceding atrial systole is crucial. Make sure you give your heart enough time to fill — adequate hydration, good posture, and regular movement all support optimal filling.
FAQ
What exactly are the AV valves?
The AV valves are the tricuspid valve (between the right atrium and ventricle) and the mitral (bicuspid) valve (between the left atrium and ventricle). They act as one‑way gates that allow blood to flow from the atria into the ventricles and then close to keep that blood from leaking back.
Why do the AV valves close before the semilunar valves open?
Because the ventricles must first build enough pressure to push blood past the arterial pressure. The pressure rise forces the AV valves shut, and only after the ventricular pressure exceeds the pressure in the aorta or pulmonary artery do the semilunar valves open.
Can the AV valves close too early?
If the ventricles contract before the atria have finished filling, the AV valves may close prematurely, reducing the amount of blood that can be pumped out. This is why the timing of the electrical signal (the QRS complex) is so tightly coordinated with atrial contraction.
Is there any condition where the AV valves stay open during contraction?
Yes. Conditions like mitral regurgitation or tricuspid regurgitation cause the AV valves to leak, meaning they don’t seal completely when the ventricles contract. This leads to reduced cardiac output and can cause symptoms It's one of those things that adds up..
How does exercise affect the timing of valve closure?
During intense exercise, the heart rate increases, and the time between beats shortens. The ventricles still contract, so the AV valves close just as they do at rest, but the interval between closure and the next opening becomes shorter, which is why the “lub” and “dub” sounds become faster.
Closing Thoughts
Understanding that during ventricular contraction the AV valves are closed isn’t just a textbook fact — it’s the foundation of how the heart delivers oxygen to every cell in the body. The pressure shift that forces those valves shut is a tiny, elegant dance that keeps the circulatory system running smoothly. Still, when you listen to your own heartbeat, you’re hearing the sound of that dance in action. Keep that rhythm in mind, and you’ll have a clearer picture of how remarkable — and fragile — your heart truly is That's the whole idea..
