Which structure serves as the protective outer covering of the heart?

The pericardium is the structure that serves as the protective outer covering of the heart. It consists of two layers: the fibrous pericardium, which is the tough outer layer that provides mechanical support and prevents over-expansion of the heart, and the serous pericardium, which has an inner layer that reduces friction between the heart and surrounding structures during the heart's contractions. This dual-layered setup ensures that the heart is cushioned and protected, while also allowing it to move freely within the thoracic cavity.

The other structures listed serve different purposes: the serosa refers to a layer of tissue lining certain body cavities; the endocardium lines the inner chambers of the heart and provides a smooth surface for blood flow; while the myocardium is the heart muscle itself, responsible for contracting and pumping blood. Each structure has its own distinct function, but it is the pericardium that specifically provides the protective layer around the heart.