Which heart layer directly covers the heart's outer surface?

The epicardium is the layer that directly covers the heart's outer surface. It is a thin layer of tissue that is actually part of the pericardium, specifically the visceral pericardium. This layer plays a crucial role in providing a protective covering for the heart and is involved in the production of pericardial fluid, which reduces friction between the heart and the surrounding structures as the heart beats.

In understanding the other layers of the heart, the myocardium is the thick, muscular layer that contracts to pump blood out of the heart, while the endocardium lines the interior of the heart chambers and covers the heart valves. The pericardium, as a whole, is the fibrous sac that surrounds the heart, comprising the fibrous and serous layers, with the visceral layer being the epicardium. Thus, while the pericardium encompasses the heart, it is the epicardium that specifically refers to the layer directly on the heart's outer surface.