3.2 Fibrous Pericardium

Fibrous Pericardium is the tough, dense connective tissue outer layer of the pericardial sac that surrounds and protects the heart. It forms a strong, fibrous envelope that anchors the heart within the mediastinum, prevents excessive movement during changes in body position, and limits overfilling of the heart by providing a relatively inelastic barrier. The fibrous pericardium is continuous with the central tendon of the diaphragm inferiorly and merges superiorly with the outer layers of the great vessels entering and leaving the heart.

Gross Anatomy and Shape

General Configuration

The fibrous pericardium is shaped like a truncated cone or a flask, with its broad base fused to the central tendon of the diaphragm and its apex blending with the adventitia of the great vessels. This conical shape provides stability and ensures that the heart remains in a fixed position despite bodily movement or changes in thoracic pressure.

Surfaces and Relations

• Anterior Surface: Lies directly behind the sternum and upper costal cartilages, separated by loose areolar tissue and, in some individuals, a small amount of fat. The anterior surface may also be crossed by the phrenic nerves and pericardiacophrenic vessels.
• Posterior Surface: Closely related to the structures of the posterior mediastinum, including the esophagus, descending thoracic aorta, and thoracic duct.
• Lateral Surfaces: In proximity to the mediastinal pleura of each lung, separated by the phrenic nerves and pericardiacophrenic vessels.
• Base: Firmly attached to the central tendon of the diaphragm.
• Apex/Superior Continuity: Merges with the adventitia of the ascending aorta, pulmonary trunk, and superior vena cava.

Histological Structure and Composition

Collagenous Framework

The fibrous pericardium is composed primarily of dense, irregular connective tissue rich in collagen fibers. This provides tensile strength and resistance to stretching, key features that prevent acute cardiac distension.

Cellular and Vascular Content

Fibroblasts and scattered elastic fibers are present within the collagenous matrix. The fibrous pericardium is relatively avascular compared to the adjacent serous pericardium but receives some blood supply via small branches of the pericardiacophrenic and bronchial arteries.

Attachments and Continuities

Inferior (Base) Attachment

The fibrous pericardium is firmly fused to the central tendon of the diaphragm. This connection ensures that the movements of the diaphragm during respiration are transmitted to the pericardial sac, and thus to the heart.

Superior Continuity

At its apex, the fibrous pericardium blends seamlessly with the outer connective tissue (adventitia) of the main great vessels, including the ascending aorta, pulmonary trunk, superior vena cava, and, to a lesser extent, the pulmonary veins and inferior vena cava.

Anterior and Posterior Relations

The anterior portion is attached to the posterior surface of the sternum via the superior and inferior sternopericardial ligaments, stabilizing the pericardial sac within the thoracic cavity. Posteriorly, the fibrous pericardium is in close proximity to the main thoracic structures, but has no strong ligamentous attachments.

Functional Significance

Mechanical Protection

The fibrous pericardium acts as a barrier against physical shocks and sudden increases in intracardiac volume, limiting excessive acute dilation of the heart.

Anchoring and Support

By anchoring the heart to the diaphragm and sternum, the fibrous pericardium ensures positional stability within the mediastinum and coordinates movements during respiration and changes in body posture.

Limitation of Overfilling

Its tough, non-distensible nature prevents the heart from overexpanding, maintaining optimal cardiac geometry and function.

Spatial Envelope and Cardiac Relations

The fibrous pericardium forms the outer boundary of the pericardial cavity, within which the heart and the serous pericardium (parietal and visceral layers) are found. This envelope separates the heart from other mediastinal structures and defines the pericardial space that contains a thin film of lubricating serous fluid.

Clinical Considerations

Pericardial Tamponade

Because the fibrous pericardium is non-elastic, rapid accumulation of fluid or blood within the pericardial cavity (such as in pericardial tamponade) can compress the heart and impair its function, since the fibrous layer does not expand to accommodate increased volume.

Pericarditis

Inflammation of the pericardium may involve the fibrous layer, leading to pain, friction rub, and potentially fibrous thickening or constrictive pericarditis.

Surgical Importance

Understanding the attachments and boundaries of the fibrous pericardium is critical during cardiac and thoracic surgical procedures to avoid inadvertent injury to the heart, phrenic nerves, or great vessels.

Summary Table: Key Features of the Fibrous Pericardium

Conceptual Summary

The fibrous pericardium is the tough, protective outer sac surrounding the heart and roots of the great vessels. Its anatomical attachments anchor the heart in the thorax, and its inelastic, collagen-rich wall prevents sudden cardiac dilation, forming a crucial barrier within the mediastinum. Understanding its structure and relations is vital for interpreting both normal cardiac physiology and a variety of pathological conditions.