Final answer:
The isovolumic ventricular relaxation phase is the early phase of ventricular diastole where the ventricle pressure falls below artery pressure, causing semilunar valves to close, shown as the T wave on an ECG and associated with a dicrotic notch, with no change in ventricle blood volume.
Step-by-step explanation:
The isovolumic ventricular relaxation phase is the initial phase of the ventricular diastole during which the pressure in the ventricles falls below that in the two major arteries, the pulmonary trunk, and the aorta. This results in the semilunar valves closing to prevent blood from flowing back into the heart ventricles, marking the end of ventricular systole and the beginning of diastole. This relaxation is represented by the T wave of the ECG and coincides with the presence of the dicrotic notch—a small dip in the blood pressure tracing—indicating the closure of the semilunar valves. At this point, since the atrioventricular valves are still closed, no blood enters or leaves the ventricles, thus the volume of blood in the ventricles remains constant, hence the term 'isovolumic' or 'isovolumetric.'
The duration of the ventricular diastole is approximately 430 ms, entailing both the isovolumic relaxation phase and the phase when the ventricles fill with blood as the atrioventricular valves open.