Which of the following describes the function of the pericardial sac?

The correct choice highlights the primary role of the pericardial sac, which is to allow the heart to expand and contract with minimal friction. The pericardial sac is a double-walled structure that encases the heart, consisting of an outer fibrous layer and an inner serous layer. The serous layer produces pericardial fluid, which acts as a lubricant, reducing friction as the heart beats and moves within the thoracic cavity. This lubrication is essential for the heart's efficient functioning, as it minimizes wear and tear on the heart and surrounding tissues during its constant rhythmic contractions.

In contrast, while the pericardial sac does help to anchor the heart, its primary function is facilitating movement more than providing structural support. It does not generate electrical impulses, which are produced by specialized cardiac tissues. Furthermore, it does not serve as a reservoir for blood; that function is primarily carried out by the heart chambers and associated blood vessels. This highlights the specific and critical role of the pericardial sac in cardiovascular health.