which valve opens when the pressure in the left atrium exceeds the pressure in the left ventricle
The normal cardiac cycle, showing pressure relationships between the left-sided heart chambers. During diastole, the mitral valve (MV) is open, so that the left atrial (LA) and left ventricular (LV) pressures are equal. In late diastole, LA contraction causes a small rise in pressure in both the LA and LV (the a wave).