At its initiation the mitral and tricuspid valves close.
Ventricular muscles initially shorten relatively little, but intraventricular pressure rises quickly as the myocardium presses on the blood in the ventricle.
The period of isovolumetric ventricular contraction lasts about 0.05 sec until the pressures in the left and right ventricles exceed the pressures in the aorta and pulmonary artery and the aortic and pulmonary valves open.
During isovolumetric contraction the AV valves bulge into the atria, causing a significant rise in atrial pressure.