Which structure in the heart divides the left and right chambers?

The septum is the wall of tissue that divides the left and right chambers of the heart, specifically separating the left atrium from the right atrium and the left ventricle from the right ventricle. This structure plays a crucial role in maintaining the separation between the oxygenated blood in the left side of the heart and the deoxygenated blood in the right side. By ensuring that blood does not mix between the two sides, the septum facilitates efficient circulation, allowing the heart to function properly and deliver oxygen-rich blood to the body while directing oxygen-poor blood to the lungs for oxygenation.

The other choices do not serve this specific purpose. The ventricles are the lower chambers of the heart responsible for pumping blood, the atria are the upper chambers that receive blood, and valves are structures that ensure unidirectional blood flow but do not separate the chambers. Thus, the septum is the correct answer as it is the anatomical structure that divides the heart's left and right sides.