SYSTOLE
The contraction of ventricles is called systole. Each period of systole is followed by diastole when the heart is at rest. The ejection fraction is the measure of cardiac output every systole. Systole corresponds to the phase 0 which is the point of stimulation. The membrane at this point is depolarized which means that there is no net charge. This time cell cannot contract. During repolarization, sodium goes back the cell and muscles is now ready for another contraction.