Final answer:
During the isovolumic contraction phase of ventricular systole, the ventricles contract without changing volume as the blood pressure within the chambers rises but does not open the semilunar valves, leading to the designation 'isovolumic contraction'.
Step-by-step explanation:
During the isovolumic contraction phase of ventricular systole, the muscles in the ventricles contract, causing an increase in the blood pressure within these chambers. However, this pressure is not sufficient to open the semilunar (pulmonary and aortic) valves for the blood to be ejected from the heart. Consequently, the volume of blood within the ventricles remains constant, as the atrioventricular valves (tricuspid and mitral) have closed in response to the rising pressure. This period is characterized as isovolumic contraction and is distinct because there is no change in volume despite the contraction of the ventricular muscles.
In contrast, during the isovolumic ventricular relaxation phase of ventricular diastole, the ventricles begin to relax, and the pressure within drops. When this pressure falls below that of the pulmonary trunk and aorta, the semilunar valves close to prevent backflow into the heart. At this moment, the atrioventricular valves are also closed, thus maintaining a constant volume within the ventricles, hence the term isovolumetric relaxation.