Thus the heart establishes and maintains a mean systemic arterial pressure that is high enough so that all of the organs in the body will be properly perfused. In this lecture, the cardiac cycle is presented in this context. The graphical representation of these events as a function of time, is referred to as the Wiggers Diagram, see Figure 1 . Figure 1 - Wiggers Diagram for the left side of heart. Throughout ventricular filling, pressure in the atrium exceeds pressure in the ventricle - although at times the pressure difference is imperceptibly small.
Ventricular filling occurs in three stages.
Initially, filling is rapid as the blood which has accumulated in the atrium during contraction of the ventricle drains passively into the ventricle and with it atrial pressure dissipates. As the accumulated blood in the atrium dissipates, the rate of filling of the ventricle slows. In some individuals the contraction of the atrium can be detected as the fourth heart sound. Note that throughout the ventricular filling phase, pressure in the aorta is slowing decreasing. Depolarization of the ventricles, as evidenced by the QRS complex, brings on ventricular contraction.