During systole the thick muscular walls of the ventricles contract. This happens to both sides of the heart at about the same time. The diagram shows systole in the left side of the heart. The contraction of the ventricular muscle raises the pressure in the ventricle.
What happens during a heartbeat?
Each side of your heart uses an inlet valve to help move blood between the atrium and ventricle. During diastole, the atria and ventricles of your heart relax and begin to fill with blood. At the end of diastole, your heart's atria contract (atrial systole) and pump blood into the ventricles.
What happens to the heart during atrial diastole?
Phase 1: Atrial systole: Blood under low pressure flows into the left and right atria from the pulmonary vein (left) and vena cava (right). As atria fill pressure against atrioventricular valves pushes them open and blood starts leaking into the ventricles. Atria and ventricles relax during diastole.