What structure separates the right and left sides of the heart?

The septum is the correct structure that separates the right and left sides of the heart. It is a muscular wall that plays a crucial role in maintaining the separation of oxygen-rich blood in the left side of the heart from oxygen-poor blood in the right side. This division is vital for proper circulation, as it allows the heart to function effectively, ensuring that blood flows correctly through the pulmonary and systemic circuits.

The pericardium, while related to the heart, is the fibrous sac that surrounds it, providing protection and anchorage, but it does not serve to separate the chambers of the heart. The endocardium is the inner lining of the heart chambers and valves; it does not provide separation between the heart's sides. The epicardium, which is the outer layer of the heart wall, also does not play a role in separating the heart's internal structures. Therefore, understanding the specific functions of these components helps clarify the unique role the septum plays in cardiac anatomy and physiology.