Final answer:
During isovolumetric contraction, the pressure within the ventricles increases because the heart muscles contract but the volume remains constant as the semilunar valves have not yet opened.
Step-by-step explanation:
During isovolumetric contraction, the pressure in the ventricles increases. This phase is characterized by the initial contraction of the ventricles where tension and pressure rise but no blood is ejected, as the volume within the ventricles remains constant. The pressure quickly surpasses that of the atria, leading to the closure of the tricuspid and mitral valves. Despite the pressure increase, the semilunar valves remain closed until the pressure is sufficient to open them, thus no blood is pumped out of the heart at this stage.