The pericardium is a protective, double-walled sac that surrounds the heart and the proximal portions of the major blood vessels. This structure anchors the heart within the chest and provides a barrier against friction during continuous movement. To understand its function, it is helpful to clarify the exact placement of its components, specifically the parietal pericardium. This article will define the anatomical context of the entire sac and clarify the precise location and purpose of the parietal layer.
Defining the Pericardium
The pericardial sac is situated within the thoracic cavity, in the middle mediastinum. This places the heart centrally between the two lungs, slightly offset to the left side of the sternum. The pericardium creates a defined compartment, separating the heart from other mediastinal structures like the esophagus and trachea.
The sac is anchored firmly to surrounding structures to maintain the heart’s position. Inferiorly, the broad base of the pericardium is fused directly to the central tendon of the diaphragm, forming a stable floor. Superiorly, the sac blends with the connective tissue layers of the great vessels (aorta, pulmonary trunk, and vena cavae), securing the heart at its roots. This fixation prevents excessive movement during changes in body position or physical activity.
Identifying the Pericardial Layers
The overall structure of the pericardium is composed of two distinct parts: a thick outer layer and a thin inner layer. The external component is the Fibrous Pericardium, a dense layer of non-stretching connective tissue that provides structural protection and helps prevent the heart from over-expanding with blood. Enclosed within this strong outer shell is the Serous Pericardium, which is a thin membrane designed for lubrication.
The Serous Pericardium is further organized into two separate sheets, which are continuous with one another at the points where the great vessels enter and exit the heart. The outer sheet is known as the Parietal Serous Pericardium, and the inner sheet is the Visceral Serous Pericardium. The visceral layer is so tightly adhered to the heart muscle that it is also referred to as the epicardium, forming the outermost layer of the heart wall itself.
Precise Location and Attachment of the Parietal Layer
The parietal pericardium is the external lamina of the serous membrane, serving as the inner lining of the pericardial sac. Its location is defined by its direct fusion to the internal surface of the fibrous pericardium. This makes it the innermost layer of the tough, non-elastic outer shell.
This layer forms one boundary of the narrow pericardial cavity. This potential space is situated between the parietal layer and the visceral layer (epicardium), which covers the heart’s surface. Bound to the fibrous sac, the parietal layer remains static while the visceral layer moves freely with every heartbeat. Superiorly, the parietal pericardium extends to the roots of the great vessels, where it folds back to become the visceral layer.
Primary Role of the Parietal Layer
The primary function of the parietal layer is the production of fluid within the pericardial cavity. The mesothelial cells of this membrane, along with those of the visceral layer, secrete pericardial fluid. This thin, watery substance fills the space between the two serous layers.
This lubricating fluid reduces the friction generated by the heart’s constant pumping motion. The fluid allows the visceral layer, which moves with the heart, to slide smoothly against the stationary parietal layer. This mechanism ensures the heart can contract and relax continuously without causing abrasive damage against the surrounding fibrous sac.