Pericardium

The pericardium is a fibroserous sac that encloses the heart and the roots of the great vessels. It provides mechanical support, limits overexpansion of the heart, and reduces friction during cardiac movements. Located within the middle mediastinum, the pericardium occupies a central position in the thoracic cavity, posterior to the sternum and anterior to the vertebral column.

Layers of the Pericardium

The pericardium consists of two main layers: the outer fibrous pericardium and the inner serous pericardium. These layers enclose the pericardial cavity, a potential space filled with a small amount of lubricating serous fluid.

1. Fibrous Pericardium

This is the outermost layer, composed of tough, dense connective tissue. It is non-elastic and functions as a protective outer layer that anchors the heart in place. The fibrous pericardium is continuous with:

• The tunica adventitia of the great vessels superiorly
• The central tendon of the diaphragm inferiorly (via the pericardiophrenic ligament)
• The sternum anteriorly (via the sternopericardial ligaments)

This layer maintains the position of the heart within the mediastinum and prevents excessive dilation during high venous return.

2. Serous Pericardium

The serous pericardium is a double-layered membrane that lines the inner surface of the fibrous pericardium and reflects onto the surface of the heart.

• Parietal layer: Lines the inside of the fibrous pericardium
• Visceral layer (epicardium): Covers the surface of the heart and forms part of the heart wall

Between these two layers lies the pericardial cavity, which contains 15–50 mL of serous fluid to reduce friction during cardiac cycles.

Pericardial Cavity

The pericardial cavity is the potential space between the parietal and visceral layers of the serous pericardium. It is filled with a thin film of serous fluid secreted by mesothelial cells. This fluid allows the heart to beat within a low-friction environment and prevents adhesions between the layers during movement.

Sinuses of the Pericardium

The reflections of the serous pericardium around the great vessels form recesses or sinuses, which are important anatomical landmarks:

1. Transverse Pericardial Sinus

A horizontal passage posterior to the ascending aorta and pulmonary trunk, and anterior to the superior vena cava. It is clinically important as a landmark during cardiac surgery to isolate the arterial vessels.

2. Oblique Pericardial Sinus

A blind recess located posterior to the left atrium, between the pulmonary veins and the inferior vena cava. It provides a space for atrial expansion and can be visualized during posterior dissection of the pericardium.

Attachments

The pericardium is anchored within the thoracic cavity by several ligaments and structural attachments:

• Inferiorly: Attached to the central tendon of the diaphragm via the pericardiophrenic ligament
• Anteriorly: Anchored to the posterior surface of the sternum via the sternopericardial ligaments
• Superiorly: Continuous with the outer layers of the great vessels (aorta, pulmonary trunk, SVC, pulmonary veins)

Contents of the Pericardium

Within the pericardial sac, the following structures are enclosed:

• The heart
• The ascending aorta (proximal portion)
• The pulmonary trunk and proximal pulmonary arteries
• The superior vena cava (terminal portion)
• The pulmonary veins (proximal portions)
• The inferior vena cava (terminal portion)

Innervation

• Fibrous and parietal serous pericardium: Supplied by the phrenic nerve (somatic sensory)
• Visceral pericardium (epicardium): Supplied by autonomic fibers from the cardiac plexus (visceral afferents)

Due to its somatic innervation, pain from the parietal pericardium is well localized and can be referred to the shoulder (C3–C5 dermatomes) via the phrenic nerve.

Blood Supply

Arterial Supply

• Pericardiacophrenic artery: A branch of the internal thoracic artery; runs with the phrenic nerve
• Musculophrenic artery: Also a branch of the internal thoracic artery
• Bronchial, esophageal, and superior phrenic arteries: Provide minor contributions

Venous Drainage

• Pericardiacophrenic veins drain into the internal thoracic or brachiocephalic veins

Relations

• Anteriorly: Sternum, anterior mediastinal fat, thymus remnants
• Posteriorly: Esophagus, descending thoracic aorta
• Laterally: Lungs and pleural cavities
• Inferiorly: Diaphragm

The pericardium lies deep to the sternum and is in close proximity to major thoracic structures such as the trachea, lungs, phrenic nerves, vagus nerves, and great vessels, making it a central landmark in thoracic anatomy.