What happens during the isovolumic relaxation phase of the cardiac cycle?

During the isovolumic relaxation phase of the cardiac cycle, the ventricles are indeed relaxed, but importantly, there is no blood entering or leaving them. This phase occurs right after the ventricles have ejected blood into the arteries, at which point the semilunar (SL) valves close to prevent backflow. The atrioventricular (AV) valves remain closed during this phase as the ventricles transition from contraction to filling.

The key characteristic of isovolumic relaxation is that the volume of blood within the ventricles remains constant (hence "isovolumic"), even though the pressure within the ventricles decreases as the muscle fibers relax. This phase is crucial for allowing the heart to prepare for the next filling cycle, as the pressure drop allows for subsequent opening of the AV valves to initiate ventricular filling. Understanding this phase helps clarify the dynamics of blood flow and pressure changes that occur throughout the cardiac cycle.