The mitral valve separates the left atrium from the left ventricle in the heart. It is a one-way valve that allows a certain volume of blood into the left ventricle in tandem with the heartbeat. Mitral prolapse occurs when the flaps of the valve do not close properly, allowing for blood to regurgitate backward into the left atrium. While the condition is mostly harmless, some cases may require surgical correction. Mitral prolapse can be distinguished by a unique heart murmur.