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.