The heart is enclosed by which double-layered membranous sac?

The heart is enclosed by the pericardium, a double-layered membranous sac that provides protection and helps to anchor the heart within the thoracic cavity. The pericardium consists of the outer fibrous pericardium and the inner serous pericardium. The serous pericardium is further divided into the parietal layer, which lines the fibrous pericardium, and the visceral layer, which is also known as the epicardium, and covers the actual surface of the heart.

The role of the pericardium is crucial as it facilitates smooth movement of the heart during contraction and relaxation while minimizing friction. Additionally, the pericardium can also help prevent overexpansion of the heart during filling. Each of the other choices refers to different layers or specific components of the heart:

• The endocardium is the inner lining of the heart chambers, providing a smooth surface for blood to flow over.

• The myocardium is the muscular layer responsible for the heart's contractions.

• The epicardium is the outer layer of the heart wall, which is actually part of the serous pericardium but not a separate protective sac like the pericardium itself