( A ) The SA Node generates an action potential which stimulates both the atria to undergo contraction; first right and then left together called the atrial systole.
( B ) The atria undergoes relaxation (diastole), coinciding with the ventricular systole.
( C ) The action potential is conducted to the ventricular side by the AV bundle usually bypassing AV Node
( D ) Atrial systole increases the flow of blood into the ventricles by about 50 per cent.
