The numbers in this description correspond with those on the graph of the various events occurring during the cardiac cycle. The ventricle is filling with blood. The pressure is rising in both the atrium and ventricle, but the pressure is higher in the atrium. The pressure is rising because the muscle is being stretched as the volume of blood rises. This signal originated in the S.A. Node.
Depolarization is the signal that initiated contraction by the atrial myocardium.
An increase in the blood pressure in the atrium results from the contraction of the atrial myocardium. A slight increase in the rate of filling of the ventricle results from the contraction of the atrium. Blood pressure in the ventricle rises rapidly due to ventricular contraction. This causes ventricular pressure to rise above atrial pressure. Closing of the valve is caused by the reversal of the pressure gradient between the atrium and the ventricle.