What events occur during the cardiac cycle?

Within the heart the atria contract creating a high pressure and squeezing blood into the ventricles. The semi lunar valves are closed so pressure within the ventricles builds up.Pressure within the ventricles becomes greater than in the atria which closes the atrioventricular valves. The cardiac muscle of the ventricle walls contract which causes the pressure in the ventricles to exceed the pressure within the atria and pulmonary artery forcing the semi lunar valves to open and expelling blood from the heart.