Isovolumetric contraction

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.

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 0.05 seconds,[1] 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 artery upon opening of the semilunar valves.

This process, therefore, helps maintain the correct unidirectional flow of blood through the heart and circulatory system.

[2] The word contains the prefix iso-, derived from the Ancient Greek ἴσος (ísos), meaning equal.