During atrial systole, the atria contract and force blood into the ventricles, according to the medical library of the University of Utah. Atrial systole typically increases ventricular blood volume by 10 percent, but that volume can rise to as much as 40 percent when the heart rate is elevated, notes RnCeus.com.
Atrial systole is one stage of the complete cardiac cycle, according to Wikipedia. The cycle starts with the end of one heartbeat and ends with the start of another, and it is measured in beats per minute. In the first stage of the cardiac cycle, which is known as ventricular filling, the heart relaxes, the semilunar valves close and the atrioventricular valves open. Blood passively flows into the atria and ventricles, expanding the two chambers.
The second stage is atrial systole, according to Wikipedia. The atrioventricular valves open, the semilunar valves open and the expanded atrium contracts, forcefully ejecting blood into the ventricles. In the third stage, known as isovolumic contraction, both the atrioventricular and semilunar valves close, and the engorged ventricles start to contract, causing a sharp rise in ventricular blood pressure.
In the fourth stage, the atrioventricular valves close, the semilunar valves open and the ventricles contract, forcing blood out of the chambers. For this reason, this stage is known as ventricular ejection, as described by Wikipedia. In the fifth and final stage of the cardiac cycle, which is known as isovolumic relaxation, both atrioventricular and semilunar valves close, the ventricles relax and intraventricular pressure falls. The entire process is coordinated through electrical signals that are generated by specialized heart cells.