What is the primary function of the pericardial sac that surrounds the heart?

The primary function of the pericardial sac that surrounds the heart indeed revolves around allowing the heart to expand and contract with minimal friction. This fibrous and serous membrane structure provides a protective layer that reduces friction as the heart beats and fills with blood. The heart's rhythmic contractions create significant movement, and the pericardial fluid within the sac serves as a lubricant, ensuring smooth movement during these processes.

While the pericardial sac provides cushioning and protection, its role in minimizing friction is fundamental to the heart's efficiency and overall function. The heart's ability to contract and fill properly is crucial for effective circulation, and the pericardial sac plays a vital part in maintaining that functionality.

The other options, while they may refer to different aspects of cardiac physiology or anatomy, do not accurately describe the primary role of the pericardial sac. For instance, electrical insulation is primarily provided by other cardiac structures, not the pericardium. The coronary arteries are indeed in proximity to the heart but are not housed within the pericardial sac. Additionally, the storage of oxygenated blood occurs in large vessels and specific heart chambers, not within the pericardial sac.