What causes the sound of a heartbeat?

From childhood on, your heart remains similar in size to your balled-up fist. Inside of it, there are four different chambers: two atria stacked on top of two ventricles. Each atrium is paired with a ventricle, and a wall separates them into two different shafts. On both the left side and the right side of the heart, blood enters the upper atrium, files through a valve into the ventricle and then exits through another valve on the way out of the heart.

­When the heart beats, an electrical signal passes from the top of the heart, near the atria, down through the ventricles, and the chambers contract in that order. So when the upper atria contract, the atrioventricular valves sandwiched between the atria and the ventricles open, and the blood in each atrium flows through its respective valve down into a ventricle.

On the left side, where oxygenated blood is coursing through, this gateway to the left ventricle is called the mitral valve. On the right side, where oxygen-depleted blood is passing into the right ventricle, it's known as the tricuspid valve.

Once both ventricles simultaneously fill with blood, the atrioventricular valves slam shut, preventing blood from moving back into the atria. This shutting of the atrioventricular valves creates the first sound of your heartbeat -- "Lub!"

By this time, the heart's electrical signal has passed from the atria into the ventricles, so while the atria relax, the ventricles contract. Now, on either side of the heart, the second set of valves opens. These valves leading out of the ventricles represent the heart's exit doors, and together they're known as the semilunar valves.

­These valves direct blood from either ventricle to its next destination. Oxygen-depleted blood in the right ventricle leaves the heart through the pulmonary valve that connects to the pulmonary artery leading to the lungs. Oxygen-rich blood in the left ventricle, meanwhile, departs through the aortic valve that connects the heart to the aorta, the body's major expressway for the delivery of freshly oxygenated blood. Once the passing electrical current contracts the ventricles, the blood inside them is forced through the open semilunar valves, which then slam shut. This is the second half of your heartbeat -- "Dub!"

Now, we'll look at the valves themselves a little closer. If you can picture a tambourine, you're halfway there. The frame of the tambourine represents the annulus, or the tough ring of tissue to which the valve's flaps are attached. Now imagine punching through the top of a tambourine and it splitting into three equal-sized pieces across the opening. These pieces represent the valve's flaps or cusps. All of the valves have three cusps, except the mitral valve, which has two, but operates the same as the others.

It is the sound of these cusps coming back together to form a temporary seal that makes the sound we recognize as a heartbeat. But what makes a lub-dub become a lub-dub-whoosh? Find out next.

Not all hearts go lub-dub, lub-dub. Some may go lub-dub-whoosh, while others may go lub … ssss … dub, or any number of crazy noises. Remember the turnstiles the concertgoers were filing through? Think of these crazy noises as the turnstiles being uncoordinated or even malfunctioning.

While an abnormal rhythm might result from heart disease or a malfunctioning in the heart's electrical system, little hisses, whooshes and other odd noises are usually the result of faulty valves in the heart. These abnormal sounds are called heart murmurs, and doctors detect them using a stethoscope or an echocardiogram.

Heart murmurs are common. About half of all kids have hearts that thump to a different beat [source: University of Chicago]. Fortunately, most of these childhood murmurs are known as "innocent murmurs," meaning they're harmless and don't require medical attention. Many innocent murmurs disappear on their own by adulthood.

Murmurs can, however, signify a serious problem. There are two main types of troublesome valves -- valves that don't seal and allow some blood to move backward (known as regurgitation), and valves that don't open wide enough to let the right amount of blood move forward (stenosis). When either scenario is present, the heart must work harder to do its job.

Luckily, doctors can fix valves. For example, when a valve isn't opening fully, surgeons insert a catheter with a tiny balloon into a vein in the body and push it through until it reaches the constricted or stenotic valve inside the heart. Then they inflate the balloon, knocking the valve's flaps fully apart. This normally solves the problem, allowing the proper amount of blood to pass through the repaired valve. This simple procedure doesn't usually require a hospital stay. Surgeons can also patch holes, remove calcium buildup, or reshape the valve or cusps as needed in more complex procedures.

Sometimes valves can't be fixed, and you need a new one. When doctors give you a new one, they look to machine, man or pig for your replacement.

Mechanical valves are plastic or metal and precisely mimic a human valve. They attach to the surrounding tissue exactly like the natural valve they replace, and they last as long as you last. One major drawback is that mechanical valves are prone to attracting clots, so recipients have to take blood thinners daily for the rest of their lives. If a clot does form, the mechanical valve may need to be swapped out.

Besides mechanical valves, a biological valve may be transplanted. These biological valves may come from either a human or an animal, often a pig. Transplanted human valves get along famously in their new environment, which looks a lot like their old neighborhood. The biggest problem? They're hard to come by, because nobody wants to (or is able to) surrender a perfectly good valve before they give up the ghost.

Animal valves have an advantage over mechanical valves, because the recipient only needs to take blood thinners daily for a few months and can then stop. The downside to having a strange piece of pig wedged into your heart is that biological valves wear away after 10 or 15 years and need to be replaced again.

Now we know more about the heart's turnstiles. Some turnstiles may need a little extra attention, but the important thing is that the show goes on and that customers continue pouring through the gates. See the next section for lots more information about the human heart.