Mitral regurgitation

Mitral regurgitation, also called mitral insufficiency, means the mitral valve does not close all the way. The mitral valve is the door between the left atrium and the left ventricle of the heart. When the ventricle contracts, in systole, some of the blood escapes backward into the atrium instead of all going out to the body. The door does not quite close, and with each push a little escapes backward.

That door has two flaps, called leaflets, one anterior and one posterior, mounted inside a ring called the annulus. Because the blood that flowed back piles up in the atrium, the ventricle takes that extra volume back in diastole, which is when it fills. That is more preload, meaning more blood waiting inside before each beat, and it ends up causing volume overload of the left ventricle.

There is a degenerative form, and in that form the most common cause is myxomatous degeneration of the valve: the tissue of the leaflets turns soft and redundant, so it bulges into the atrium. That last part is mitral valve prolapse.