The Cardiac Cycle

The ventricles are relaxed, the atria contract which increase the pressure inside the chamber
Blood is pushed into the ventricles due to the increased pressure. The AV valves are open, the SL valves are closed

The atria relax, the ventricles contract which decreases their volume and increases the pressure in the ventricles.
The pressure becomes higher than in the atria which forces the AV valves to shut as to prevent back flow.
As the pressure in the ventricles is higher than in the aorta and pulmonary artery aswell, the SL valves are forced opened to let the blood push through into the arteries.

Both ventricles and atria relax, the high pressure in the aorta and pulmonary artery closes the SL valves as to prevent backflow into the ventricles.
Blood returns to the heart and the atria fill again, thus increasing the pressure in the atria.
As the ventricles relax, the pressure falls below that of the atria which means that the AV valves open. This allows blood to flow without being pushed by atrial contraction into the ventricles from the atria.
The atria contract and the process repeats itself.