Final answer:
During isovolumetric contraction, the volume inside the ventricles remains unchanged because the ventricles contract and build pressure without opening the semilunar valves, thereby preventing blood ejection and volume alteration.
Step-by-step explanation:
In isovolumetric contraction, there is no change in ventricular volume because the ventricular muscles contract and raise the pressure inside the ventricles, but not enough to open the semilunar valves (pulmonary and aortic). Consequently, as the blood pressure inside the ventricles increases, it surpasses the pressure in the relaxed atria, causing the atrioventricular valves (tricuspid and mitral) to close. Therefore, during this phase of the cardiac cycle, no blood is ejected from the ventricles, and the volume remains the same. The isovolumetric contraction is an initial phase of ventricular systole characterized by an increase in tension and pressure but no blood pumping or ejection from the heart.