The heart is a hollow organ composed of three layers of tissue. Starting with the outermost layer, these are as follows:
- Epicardium-Outermost layer of the heart wall, also called the visceral layer of the pericardium (visceral pericardium). Coronary arteries, blood and lymph capillaries, fat, and nerve fibers are found in this layer.
- Myocardium-The thick middle layer of cardiac muscle that pumps blood through the blood vessels. This layer is responsible for the contraction of the heart.
- Endocardium-The smooth layer of epithelial cells that line the inside of the heart (myocardium) providing a smooth surface for easy blood flow. It also covers the heart valves.
The heart is protected by a strong double-walled sac, called the pericardium. The layers of the pericardium, starting with the outermost layer are as follows:
- Fibrous pericardium-Surrounds the heart as a protective sac anchoring it to the diaphragm located inferiorly and to the sternum located anteriorly
- Parietal pericardium-A serous layer that lines the inner surface of the fibrous pericardium
- Pericardial cavity-Between the double-walled pericardium is the pericardial cavity, which contains a small amount of serous (lubricating) fluid to prevent friction during heart contraction.
- Visceral pericardium-Covers the surface of the heart muscle; also called the epicardium
Myocardial tissue is a special type of contractile tissue found only in the heart. Although it is similar in appearance to skeletal muscle tissue, myocardial tissue has some unique structural and electrical properties. These properties are described more fully in the discussion of electrophysiology.
