Excitation of the atria occurs, P wave in the EKG results. Impulse continues to AV node. The a-wave of venous pressure pulse. Impulse reaches ventricular conduction system and ventricle just before the peak of atrial contraction, initiating QRS complex. Note that during the filling phase, aortic pressure is continuously falling.
Ventricular contraction begins at the peak of atrial contraction.
AV valve closes as pressure in ventricle exceeds pressure in atrium. Closure of AV valves marks the beginning of isovolumetric contraction phase. Closure of AV valves causes the first heart sound. Closure of AV valves marks the end of ventricular diastole, beginning of systole. 7. pressure in atrium dissipates as walls adjust to the pressure change.