Which structure consists of a double-walled connective tissue sac that surrounds the heart and protects it from trauma and infection?

The structure that surrounds and protects the heart is the pericardium. This double-walled connective tissue sac has both an outer fibrous layer and an inner serous layer, which serves to encapsulate the heart and provide it with a protective environment. The fibrous layer helps anchor the heart and prevents excessive movement within the thoracic cavity, while the serous layer secretes a lubricating fluid that reduces friction between the heart and surrounding tissues as the heart beats. This arrangement not only protects the heart from trauma but also helps prevent infections from spreading to the organ due to the barriers created by its layers.

In contrast, the endocardium is the thin membrane lining the heart's chambers and valves, the epicardium is the outer layer of the heart itself, and the myocardium is the muscular layer responsible for the heart's contractions. While all these structures are essential for the heart's overall function, they do not provide the protective encapsulation characteristic of the pericardium.