Mitral valve prolapse is that mitral valve can not close completely due to some change in its structure. It's thought to be inherited and commonly occur with other connective tissue disorder, mostly Marfan syndrome.

Mitral valve prolapse allows blood to enter into the left atrium. This often results in the valve not forming a tight seal.

Heart attacks that damage the structures that support the mitral valve are a common cause of mitral valve insufficiency. Myxomatous degeneration can cause a "floppy" mitral valve that leaks.

The mitral, or bicuspid valve

The mitral valve is closed when the left ventricle is contracting.