Final answer:
The abnormal increase in pressure during the initial phase of ventricular systole is due to isovolumic contraction, where blood volume in the ventricles remains constant while pressure increases as the closed atrioventricular valves prevent blood ejection.
Step-by-step explanation:
The abnormal increase in pressure following ventricular systole in the cardiac cycle is due to the isovolumic contraction phase, where the ventricles contract but the semilunar valves have not yet opened because the pressure in the ventricles is not high enough to exceed the pressure in the aorta and pulmonary trunk. Consequently, the closing of the atrioventricular valves, namely the tricuspid and mitral valves, is prompted by the backflow of blood as the pressure in the ventricles surpasses that in the atria. This results in an increase in ventricular pressure but with no change in blood volume within the ventricles.
Next, during the ventricular ejection phase, the increased pressure is finally enough to open the pulmonary and aortic valves, allowing blood to be pumped out of the heart. The amount of blood ejected during this phase is known as the stroke volume, and it is typically in the range of 70-80 mL. The pressure increase is a normal part of the cardiac cycle, which is crucial for effective blood circulation.
Afterload also plays a role in the heart's ability to pump blood, representing the resistance that must be overcome for the semilunar valves to open and for blood to be ejected into the arteries. Increased afterload, as seen with conditions such as high blood pressure or hypertension, requires the heart to exert more force during systole, which can further increase ventricular pressure.