Final answer:
The isovolumic contraction phase refers to the initial phase of ventricular systole, where the ventricles contract with no change in blood volume, as both the semilunar and atrioventricular valves remain closed, preventing blood ejection. This phase occurs right after atrial relaxation and precedes ventricular ejection.
Step-by-step explanation:
Phase 3: Isovolumic Contraction of Ventricles
During the cardiac cycle, phase three is known as isovolumic contraction. This phase occurs after the atria have relaxed and the ventricles begin to contract. At the start of this phase, the pressure within the ventricles rises, but the semilunar valves (pulmonary and aortic) remain closed because the pressure is not high enough to push them open. At the same time, the increased pressure within the ventricles closes the atrioventricular valves (tricuspid and mitral), preventing blood from flowing back into the atria. Since the semilunar valves are still closed and the atrioventricular valves have just closed, there is no ejection of blood from the ventricles and thus the volume of blood within the ventricles remains constant, which defines this as the isovolumic phase.
The isovolumic contraction is a critical part of ventricular systole, preparing the heart for the subsequent ventricular ejection phase, where the blood is propelled into the aorta and pulmonary trunk. This phase is crucial for maintaining the efficiency of the heart's pumping action, and it is represented on an ECG by the QRS complex. Understanding this process is fundamental to grasping heart physiology and the cardiac cycle's intricacies.