Problem: Explain more on whats occurring atria and ventricles and pressure gradient during the following cycle. 1. Atrial Systole (Atrial Contraction) What's Occurring w/Atria & Ventricles? Atria contract to top off the ventricles with blood (vents are relaxed). Pressure Gradient: Atrial pressure > Ventricular pressure Valves Open/Close: AV valves (tricuspid & mitral): Open Semilunar valves (pulmonary & aortic): Closed. Need Assignment Help?