The left atrium is one of the four chambers of the heart, located on the left posterior side. Its primary roles are to act as a holding chamber for blood returning from the lungs and to act as a pump to transport blood to other areas of the heart. The walls of the left atrium are slightly thicker than the walls of the right atrium. Oxygen-rich blood from the lungs enters the left atrium through the pulmonary vein. The blood is then pumped into the left ventricle chamber of the heart through the mitral valve. From there, the blood is ready to be pumped into the body to deliver oxygen-rich blood to all bodily tissues. Mitral valve prolapse is a common affliction in which the mitral valve between the left atrium and left ventricle does not close properly. This condition does not typically require treatment; however, some patients with mitral valve prolapse can develop more serious conditions that require treatment. One such condition is mitral valve regurgitation, in which blood leaks back into the left atrium through the mitral valve.