Mitral Valve Regurgitation
Mitral valve regurgitation (leaking backwards) is very common. As blood travels through the chambers of the heart, the 4 major valves including the mitral valve, keep blood flowing in the right direction. The mitral valve sits between the left atrium (contains oxygenated blood returning from the lungs) and the left ventricle. Blood passes through the valve and then the left ventricle contracts (squeezes or pumps) and the valve closes to prevent blood going back towards the lungs.
If the valve does not close properly, the blood regurgitates or goes backwards leading to congestive heart failure, pulmonary hypertension, atrial fibrillation, and other problems. Mitral regurgitation can be acute (happening suddenly), chronic (long-term), or both.
The most common cause of mitral regurgitation is mitral valve prolapse. Mitral valve prolapse is where valve leaflet tissue billows up into the left atrium preventing the two leaflets from coapting properly. Other causes include infection, coronary artery disease including heart attacks, rheumatic heart disease (from untreated strep throat in the past), congenital heart defects, high blood pressure, age-related wear and tear, a failing left ventricle from other causes, connective tissue disorders such as Marfan's disease, and others.
This is a great video from Khan Academy that nicely reviews what mitral regurgitation is: