The Cardiac Cycle Atrial systole- Ventricles relax, Atria contractMeans 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 CLOSEDVentricular 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 backflowAlo, 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 relaxBoth relaxed. higher pressure in the aorta and pulmonary artery closes L valves SHUT./prevent backflowBlood 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.