<P> In a healthy young adult, blood enters the atria and flows to the ventricles via the opened atrioventricular valves (tricuspid and mitral valves). Atrial contraction rapidly follows, actively pumping about 20% of the returning blood . As diastole ends, the ventricles begin depolarizing and, while ventricular pressure starts to rise owing to contraction, the atrioventricular valves close in order to prevent backflow to the atria . At this stage, which corresponds to the R peak or the QRS complex seen on an ECG, the semilunar vales (aortic and pulmonary valves) are also closed . The net result is that, while contraction causes ventricular pressures to rise sharply, there is no overall change in volume because of the closed valves . The isovolumetric contraction phase lasts about 0.03 s, but this short period of time is enough to build up a sufficiently high pressure that eventually overcomes that of the aorta and the pulmonary trunk upon opening of the semilunar valves . This process, therefore, helps maintain the correct unidirectional flow of blood through the heart and circulatory system . </P> <P> The word contains the prefix iso -, derived from the Ancient Greek ἴσος (ísos), meaning equal . Therefore, an isovolumetric contraction is one in which the volume of fluid remains constant . </P>

Isovolumetric ventricular contraction phase of the cardiac cycle