Coverings of the Heart
The heart is enclosed in a double walled sac called the pericardium. The looser, outside of the sac is called the fibrous pericardium, this is a dense connective tissue that protects the heart, anchors the heart to its surroundings, and prevents the heart from overfilling with blood. Next is the serous pericardium, which is a thin slippery two-layer membrane. This membrane is connected to the large arteries that are exiting the heart. One layer deeper is the pericardial cavity. This cavity keeps the heart working in a friction free environment.
Layers of the heart wall
The heart wall, richly supplied with blood vessels, is composed of three layers. The first layer is the Epicardium. This often gets infiltrated with fat, especially in older people. The middle layer is the myocardium. This layer is made up of mostly cardiac muscle, the cardiac muscle is the layer that contracts to make the heart pump. This muscle is arranged in circular bundles, these bundles interlace all the parts of the heart. The fibrous skeleton of the heart reinforces the myocardium internally and anchors the cardiac muscle fibers. The third layer is the endocardium. This is a sheet of endothelium resting on a thin connective tissue layer. This lines the hearts chambers and covers the fibrous skeleton of the valves. This layer is what keeps the blood inside of the heart and keeps the heart from acting more of like a sponge.