Which structure is the sac around the heart?

• A. Myocardium
• B. Endocardium
• C. Epicardium
• D. Pericardium

Answer: (D)

The heart is enclosed by a protective sack called the pericardium. This double-layered membrane surrounds the heart and the origins of the great vessels, with a small amount of lubricating fluid in the pericardial cavity to reduce friction as the heart beats. The outer fibrous pericardium provides protection and anchors the heart, while the serous pericardium splits into a parietal layer lining the fibrous sac and a visceral layer that covers the heart itself (the epicardium). To contrast, the myocardium is the thick muscular layer of the heart wall, the endocardium lines the interior chambers and valves, and the epicardium is the outer surface of the heart wall. So, the sac around the heart is the pericardium.