The heart is located between the lungs, at the center of the chest, just under the rib cage. The heart is a muscular organ that is approximately the size of a human fist (see Figure 1.1). It contains three layers: the epicardium, the myocardium, and the endocardium.

The epicardium is the outermost layer of the heart. It is composed of a single layer of cardiac cells and connective tissue. It also contains the heart’s nerves and coronary vessels. The myocardium comprises many layers of cardiac muscle fiber that wrap around the atria and the ventricles. The endocardium contains the branches of the heart’s electrical conducting system. 

The entire heart is enveloped in a fibroelastic sac known as the pericardial sac, which contains approximately 50 mL of lubricating fluid.

Figure 1.1. The Heart

Heart Anatomy and Cardiac Blood Flow