The heart is surrounded by thin fluid-filled sacs called what?

The thin fluid-filled sacs that surround the heart are known as the pericardium. This structure consists of two layers: the fibrous pericardium, which is the outer layer that provides protection and stability, and the serous pericardium, which is the inner layer that produces pericardial fluid. This fluid serves several important functions, including reducing friction between the heart and surrounding structures as the heart beats, allowing for smooth movements, and providing a slight cushioning effect.

The epicardium is the outermost layer of the heart itself, making it part of the heart wall rather than a protective sac. The endocardium lines the inside of the heart chambers and valves and is involved in maintaining a smooth surface for blood flow. The myocardium is the thick muscular layer responsible for the heart's contraction and pumping ability. Understanding these distinct layers helps clarify the role of the pericardium in protecting and supporting the heart.