The valve on your heart’s left side between the upper and lower chambers (left atrium and ventricle) is the mitral valve.Mitral Valve Regurgitation is an abnormality of the mitral valve that allows blood to flow backwards through the valve. The mitral valve normally allows blood to pass one way — from the left atrium into the left ventricle. In mitral valve regurgitation, some of the blood that is supposed to flow into the ventricle and out the aorta flows backward through the valve into the left atrium.