The valve on your heart’s left side between the upper and lower chambers (left atrium and ventricle) is the mitral valve. Mitral Valve Stenosis is a condition in which the mitral valve is narrowed. The mitral valve normally allows blood to pass from the left atrium into the left ventricle. A stenotic (stiff and narrow) mitral valve does not allow the blood to flow freely into the ventricle, causing a backup of blood in the atrium, which leads to fluid buildup in the lungs. This often causes shortness of breath and fatigue.