The Cardiac Cycle Atrial systole- Ventricles relax, Atria contract

Means there's a decreased volume of the atria-so an increased pressure inside the chamber. blood is pushed into the ventricles. Therefore, as blood enters there's a slight increase in pressure and volume.
-AV valves OPEN and SL valves CLOSED

Ventricular systole- Atria relax-, Ventricles contract

the contracting ventricles(decreased in volume) increases their pressure. so pressure becomes higher in the ventricles than the atria. forces AV valves SHUT./prevent backflow
Alo, the pressure in the ventricles is also higher in the Aorta and Pulmonary artery, causing SL valve to OPEN and blood to be forced out of the ventricles.

Diastole- Ventricles relax, Atria relax

Both relaxed. higher pressure in the aorta and pulmonary artery closes L valves SHUT./prevent backflow
Blood returns to the heart, and atria fill up again, due to higher pressure in the vena cava and pulmonary vien.
as expected, artial pressure increases again -AV valves OPEN again .

Allows the blood to flow passivley - without atrial contraction into the ventricles from the atria.
Atria contract- whole things re starts.

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AQA- The circulatory system

The Cardiac Cycle