Final answer:
As systole begins, ventricular pressure rises, leading to the closure of the mitral and tricuspid valves. This increase in pressure triggers isovolumic contraction, where the volume of blood in the ventricles remains the same.
Step-by-step explanation:
As systole begins, ventricular pressure rises and closes the mitral and tricuspid valves. Initially, when the muscles in the ventricle contract, there is an increase in the pressure of the blood within the ventricles. The rising pressure quickly exceeds that of the now relaxed atria, forcing blood back towards the atria and prompting the closure of the mitral and tricuspid valves. This rise in pressure is significant because it results in isovolumic or isovolumetric contraction, a period where the volume of blood in the ventricles remains constant as the ventricles are not yet ejecting blood.