During isovolumetric contraction, the heart's ventricles begin to contract, but there is no immediate change in the volume of blood within the ventricles because the mitral and tricuspid valves on the atria side are already closed, and the aortic and pulmonary valves on the arterial side have not yet opened. This phase is characterized by the development of pressure within the ventricles as the muscles of the heart work to push blood out. The contraction of the ventricular myocardium increases the pressure to a point where it exceeds the pressure in the aorta and pulmonary artery, which then allows the aortic and pulmonary valves to open, initiating the ejection phase of the cardiac cycle. read more >>

In cardiac physiology, isovolumetric contraction is an event occurring in early systole during which the ventricles contract with no corresponding volume change (isovolumetrically). This short-lasting portion of the cardiac cycle takes place whilst all heart valves are closed.read more >>