Valves and the heartbeat sound

The "lub" of a heartbeat is the tricuspid and mitral valves shutting; the "dub" is the pulmonary and aortic valves shutting.

One-way doors

Any pump that pushes fluid needs valves, or the fluid would slosh back the way it came. The heart has four. Each valve is a set of thin flaps, called cusps or leaflets, attached to a ring of tissue. The flaps open to let blood flow out of a chamber and close so the chamber can refill without blood leaking back. They are pushed open and shut by the blood itself: when the pressure behind a valve is higher, it opens; when the pressure in front is higher, it slams shut.

Two pairs

The valves between the atria and the ventricles are called the atrioventricular valves. On the right side it is the tricuspid valve, which has three flaps. On the left side it is the mitral valve, which has two flaps and is also called the bicuspid valve. The other two valves sit where the big arteries leave the ventricles. The pulmonary valve lies between the right ventricle and the pulmonary artery, and the aortic valve between the left ventricle and the aorta. These two are called semilunar valves, after their half-moon shaped cusps.

What happens in one beat

Follow the cycle in the model with slow motion on. While the heart rests, the atrioventricular valves stand open and blood flows from the atria into the ventricles. Then the atria contract and top up the ventricles. Next the ventricles begin to squeeze. The pressure inside them rises above the pressure in the atria, and the tricuspid and mitral valves snap shut. For a brief moment all four valves are closed while the pressure keeps climbing. When it rises above the pressure in the pulmonary artery and the aorta, the semilunar valves are pushed open and blood rushes out. When the ventricles relax, the pressure inside them drops, the pulmonary and aortic valves close, and soon after the tricuspid and mitral valves open again to start the next fill.

Lub and dub

The sounds you hear through a stethoscope are not the muscle squeezing. They are vibrations in the tissues and blood caused by valves closing. The first sound, often written "lub", comes from the tricuspid and mitral valves closing as the ventricles begin to contract. The second sound, "dub", comes from the aortic and pulmonary valves closing at the end of ventricular contraction. So the gap between lub and dub is the time the ventricles spend pushing blood out.

Doctors listen to these sounds because they tell a lot about the valves. An unusual whooshing heart sound is called a murmur. Heart valve diseases can make a valve leak so blood flows backward, or narrow a valve so blood flows through it slowly. Only a health professional can tell what a particular sound means for a particular person.

Watching the valves in the model

In the cut view the four valves sit at the doorways. The tricuspid and mitral flaps swing down into the ventricles when they open and lift back into a closed dome when they shut. The pulmonary and aortic cusps swing up into their arteries when they open. Watch the blood dots too: while a valve is closed they gather behind it, and as soon as it opens they move through. If you raise the heart rate, the whole sequence speeds up, but the order never changes.

You can feel the rhythm without a stethoscope. Say "lub, dub" and then leave a pause before the next "lub". The short gap is the ventricles pushing blood out; the longer pause is the heart resting and refilling. At a normal resting rate the refilling part is the longer of the two.