<P> In an electrocardiogram, electrical systole initiates the atrial systole at the P wave deflection of a steady signal; and it starts contractions (systole) of the ventricles at the Q deflection of the QRS complex . (Completing the P wave represents the end of the ventricular diastole and the start of the ventricular systole--see cycle diagram .) </P> <P> The cardiac cycle involves four major stages of activity: 1) "Isovolumic relaxation", 2) Inflow, 3) "Isovolumic contraction", 4) "Ejection". (See Wiggers diagram, which presents the stages, label-wise, in 3, 4, 1, 2 order, left - to - right .) Moving from the left along the Wiggers diagram shows the activities within four stages during a single cardiac cycle . (See the consecutive panels labeled, at bottom - right, "Diastole" then "Systole".) </P> <P> Stages 1 and 2 together--"Isovolumic relaxation" plus Inflow (equals "Rapid inflow", "Diastasis", and "Atrial systole")--comprise the ventricular "Diastole" period, including atrial systole, during which blood returning to the heart flows through the atria into the relaxed ventricles . Stages 3 and 4 together--"Isovoumic contraction" plus "Ejection"--are the ventricular "Systole" period, which is the simultaneous pumping of separate blood supplies from the two ventricles, one to the pulmonary artery and one to the aorta . Notably, near the end of the "Diastole", the atria begin contracting, then pumping blood into the ventricles; this pressurized delivery during ventrucular relaxation (ventricular diastole) is called the atrial systole, aka atrial kick . </P> <P> The time-wise increases and decreases of the heart's blood volume (see Wiggers diagram), are also instructive to follow . The red - line tracing of "Ventricular volume" provides an excellent track of the two periods and four stages of one cardiac cycle . Starting with the Diastole period: the low - volume plateau of "Isovolumic relaxation" stage, followed by a rapid rise and two slower rises, all components of the "Inflow stage"--increasing to the high - volume plateau of the "Isovolumic contraction" stage; (find the label at left side of diagram). Then, the Systole, including the high "Isovolumic contraction" stage to the rapid decrease in blood volume (i.e., the vertical drop of the red - line tracing) which signifies the emptying of the ventricles during the "Ejection" stage of the completed cycle--all equal to one heartbeat . </P>

Which phase of the cardiac cycle marks the beginning of systole