Final answer:
The ventricles are the parts of the heart that contract and force blood out during the ventricular systole phase of the cardiac cycle.
Step-by-step explanation:
The part of the heart responsible for contracting and forcing blood out of the heart is the ventricles. During the cardiac cycle, following a phase of relaxation known as atrial diastole, the ventricles enter a phase called ventricular systole where they contract to pump blood out of the heart. This contraction is vital for maintaining the circulatory system's function, ensuring that oxygen-rich blood reaches the body and oxygen-depleted blood is sent to the lungs for re-oxygenation.