The outer wall of the heart is surrounded by which sac?

The outer wall of the heart is surrounded by the pericardium, which is a double-walled sac that provides protection and cushioning to the heart. The pericardium has two layers: the fibrous pericardium which is tough and helps anchor the heart in the chest, and the serous pericardium which provides a smooth, lubricated surface to reduce friction as the heart beats. This structure is essential for the heart's proper function and maintains it in its position within the thoracic cavity.

In contrast, diastole and systole are terms related to the phases of the heart's pumping cycle, describing the heart's relaxation and contraction, respectively. The endocardium is the innermost layer of the heart wall which lines the chambers and helps prevent blood from sticking to the inside of the heart. While each of these terms relates to the heart, only the pericardium specifically refers to the protective outer sac surrounding it.