The heart is a powerful muscular pump that circulates blood throughout the body. To perform this function, the heart is protected by a specialized, multi-layered external structure known as the pericardium. This protective sac encases the entire organ and the roots of the major blood vessels entering and leaving it.
The Pericardium: Defining the Protective Sac
The pericardium is a double-walled, fibro-serous sac surrounding the heart and the proximal portions of the great vessels. It forms a boundary separating the heart from surrounding structures, such as the lungs. This sac provides a physical barrier against infection and blunt forces, serving several mechanical purposes that regulate the heart’s function and position.
One primary function is anchoring the heart within the chest, limiting excessive movement during vigorous activity. The pericardium also acts as a physical restraint. Due to its inextensible nature, it prevents the heart from over-distending or rapidly overfilling with blood, ensuring efficient pumping action is maintained.
The Distinct Layers of the Pericardium
The pericardial sac is comprised of two components: the tough outer fibrous layer and the inner serous layer. The fibrous pericardium is the most superficial component, formed of dense connective tissue that provides rigidity and strength. This tissue is firmly attached to the central tendon of the diaphragm and connects to the sternum via sternopericardial ligaments, securing the heart’s position.
Deep to the fibrous layer lies the serous pericardium, a thin membrane divided into two layers. The outer portion is the parietal layer, which is fused directly to the internal surface of the fibrous pericardium. The inner portion is the visceral layer, which adheres directly to the surface of the heart muscle.
Between these two serous layers—the parietal and the visceral—is a potential space called the pericardial cavity. This cavity contains a small amount of pericardial fluid, which is a lubricating serous secretion. This fluid functions like an oil to minimize the friction generated as the heart moves within the sac.
The Epicardium and Its Function
The visceral layer of the serous pericardium is anatomically synonymous with the epicardium. The epicardium is considered the outermost layer of the heart wall, serving as the transition point between the protective sac and the muscular organ. It is a composite structure, consisting of a single layer of epithelial cells (mesothelium) overlying thin connective tissue.
The epicardium houses the heart’s major circulatory network, including the coronary blood vessels and nerves that supply the underlying myocardium (heart muscle). These vessels are embedded within the connective tissue, providing immediate access for the delivery of oxygen and nutrients to the contracting muscle. A significant component of the epicardium is epicardial adipose tissue (EAT), a specialized type of visceral fat.
This adipose tissue acts as a localized metabolic buffer, providing a ready source of free fatty acids for the heart muscle to use as energy. Epicardial fat also secretes bioactive molecules, known as adipokines, which locally influence the function of the coronary arteries and the heart muscle. This fat offers mechanical cushioning and may help protect the heart against hypothermia.