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The word [epicardium] is composed by the prefix [epi-], meaning "outer" or "above"; the root term [-card-], meaning "heart"; and the suffix [-ium], meaning "layer" or "membrane". Thus, the word means "outer layer of the heart".
The epicardium is part of a larger structure called the pericardium, in fact, since the pericardium is in contact with a viscus (the heart), it can also be called "visceral pericardium". It must be noted that these two terms are synonyms: "epicardium" and "visceral pericardium".
The epicardium is a type of serosa composed of an outer layer formed by mesothelial cells and an inner layer formed by loose connective tissue containing varying amounts of fat. This inner layer known as the "subepicardial layer" (a misnomer) also contains elastic fibers.
The main arterial and venous components of the coronary circulation are found in the subepicardial layer. When performing a Coronary Artery Bypass Graft (CABG), a surgeon has to open the outer layer of the epicardium, and enter the subepicardial layer to perform the graft anastomosis.
As a serosa, the epicardium is involved in the production and absorption of a clear fluid, the "pericardial fluid". This fluid acts as a lubricant for the pericardium, allowing for the effortless movement of the heart within the pericardial sac.
