Your heart has four chambers: two upper chambers called atria, where blood enters the heart, and two lower chambers called ventricles, where blood leaves the heart. It also has a total of four valves -- two entryways between each atrium and ventricle, and two exit valves between the ventricles and the outside of the heart. These valves are small muscular rings that have small leaflets attached. When closed, these leaflets fit tightly together, preventing the flow of blood. When pressure builds behind the valves, they open, allowing blood to pass. The valves are one-way doorways, forcing blood in one direction so that it doesn't leak backward after passing through a valve.
The mitral valve, in the left side of your heart, is the exit gate leading out of your lower ventricle. (The mitral valve is also known as the bicuspid valve, because unlike the other valves, the mitral valve only has two leaflets, instead of three.) In this article, we'll look at a condition called mitral valve prolapse (MVP), which is when the mitral valve closes but the leaflets billow backward into the upper atrium. Mitral valve prolapse can cause mitral valve regurgitation, which is when your blood leaks through a poorly closed valve. In this case, some of the blood that's already passed from the atrium to the ventricle leaks back into the atrium. And that's not good.
There are five things you need to know about MVP, and the first one is on the very next page.