The pericardium is the thick, membranous, fluid-filled sac that surrounds the heart and the roots of the vessels that enter and leave this vital organ, functioning as a protective membrane . The pericardium is one of the mesothelium tissues of the thoracic cavity, along with the pleura which cover the lungs. The pericardium is composed of two layers, an outer fibrous pericardium and an inner serous pericardium.

Membranes of the Thoracic Cavity

A transverse section of the thorax, showing the contents of the middle and the posterior mediastinum. The pleural and pericardial cavities are exaggerated since normally there is no space between parietal and visceral pleura and between pericardium and heart.

This transverse diagram of the thorax indicates the transverse thoracis, internal mammary vessels, left phrenic nerve, heart, rib, pleural cavity, pulmonary arteries, sympathetic trunk, thoracic duct, vagus nerves, azygos vein, pulmonary pleura, costal pleura, lungs, sternum, and thoracic aorta.

Fibrous Pericardium

The fibrous pericardium is the outer layer of the pericardium. It is composed of dense connective tissue which anchors the heart to the mediastinum of the chest wall. It prevents the heart from overfilling with blood and protects it from nearby infections by completely separating it from the rest of the thoracic cavity. It is continuous with the outer fibrous layer of the neighboring great blood vessels.

Serous Pericardium

The serous pericardium, the inner layer of the pericardium, is composed of two different layers. The outer layer, the parietal layer, is completely adhered to the fibrous pericardium. The inner layer is known as the visceral layer, which covers and protects the great vessels and heart. The space between the parietal and visceral layers is called the pericardial cavity.

The visceral layer is referred to as the epicardium in the areas where it is in direct contact with the heart. The space between these two serous layers, the parietal and the visceral, is the pericardial cavity, which contains pericardial fluid. The serous pericardium, with its two membranes and the fluid-filled pericardial cavity, provides protection to the heart and a lubricated sliding surface within which the heart can move in response to its own contractions and to the movement of adjacent structures such as the diaphragm and the lungs.

Function of the Pericardium

The pericardium is important because it protects the heart from trauma, shock, stress, and even infections from the nearby lungs. It supports the heart and anchors it to the medastinum so it doesn't move within the body. The pericardium lubricates the heart and prevents it from becoming too large if blood volume is overloaded (though it will not prevent chronic heart enlargement). 

Despite these functions, the pericardium is still vulnerable to problems of its own. Pericarditis is the term for inflammation in the pericardium, typically due to infection. Pericarditis is often a severe disease because it can constrict and apply pressure on the heart and work against its normal function. Pericarditis comes in many types depending on which tissue layer is infected.