At the end of the isometric relaxation phase
At the end of Isovolumetric relaxation, the intraventricular pressures drop to less than the atrial pressure resulting in the opening of Atrioventricular valves Ref : Ganong 25th ed/page 539
Atrioventricular valves open
Atrioventricular valves close
Corresponds to "C" wave in JVP
Corresponds to T wave in ECG
Cardiovascular system