The filling stage is the stage where the ventricle is filling with blood that has just returned to the heart from a vein. As filling begins, the heart is relaxed (not contracting). If blood is entering the ventricle from the atria, then the pressure in the ventricle must be lower than that in the atria (remember, blood must flow from high to low pressure). That also means that the AV valves must be open (otherwise, how can blood flow into the ventricle!). These valves are open because the blood in the atria (under higher pressure) pushes open the valves to enter the ventricle. The valves are constructed so that blood trying to enter the ventricle from the atria will be able to push the valve open and allow blood to flow. If the pressure in the ventricle was higher than that in the atria, then the blood would try to move into the atria. The blood would push on the walls of the valve leaflets and this would force the valves to close, preventing blood from flowing in the wrong direction.
The pressure in the ventricle is much lower than that in the artery. Therefore the blood wants to flow from the artery into the ventricle (the wrong way!), however, as the blood tries to move into the ventricle, it pushes on the valve leaflets of the semilunar valves closing the valve and preventing the backflow of blood.
Approximately 70-80% of the blood that enters the ventricle during the filling in a resting individual does so passively, without the benefit of atrial contraction. Near the end of filling, however, the atira does contract, pushing more blood into the ventricle.
So in the filling stage, the Pvent < Patria
so the AV valves are open and
the Pvent < Partery so the semi-lunar
valves are closed.