When ventricular pressure rises above the pressure in the two major arteries, blood pushes open the two semilunar valves and moves into the pulmonary trunk and aorta in the ventricular ejection phase. Following ventricular repolarization, the ventricles begin to relax, and pressure within the ventricles drops.

Why does ventricular pressure increase during ejection?

Because the semilunar valves are open, the continued contraction of the ventricles increases the pressure in the pulmonary artery and aorta. The conclusion of ventricular ejection causes the pressure in the ventricles to fall below those of the pulmonary artery and aorta.

What happens to pressure during ventricular filling?

The atria contract to eject blood into the ventricles. About 25 % of the ventricular filling volume is ejected from the atrium to the ventricle. As the ventricular myocardium is relaxed, the ventricular pressure does not change significantly. The blood pressure in both ventricles is almost zero.