The heart is the central organ of the cardiovascular system. It lies in the middle compartment of the thorax between the lungs, and is covered by the pericardium. 

The pericardium (also known as the pericardial sac) is a fibroserous double-walled sac that encloses the heart, the pericardial fluid and the roots of the great vessels that exit and enter the heart. 

It is situated within the middle part of the inferior mediastinum (middle mediastinum), forming the anterior and posterior borders of the middle mediastinum. 

The main function of the pericardium is to protect and lubricate the moving surfaces of the heart.

Layers of perdicardium

The pericardium consists of two layers:

• Fibrous layer (fibrous pericardium) - outermost layer of the pericardium;
• Serous layer (serous pericardium) - the inner layer of the pericardium.

Fibrous pericardium

The fibrous layer of the pericardium consists of dense connective tissue. It is attached to the central tendon of the diaphragm via the pericardiacophrenic ligament whose fibers merge with the tunica adventitia of the vessels which enter and exit the heart. 

The pericardial sac also attaches via ligamentous fibers to the sternum. Due to these attachments it is affected by the movements of the heart, great vessels, sternum, and diaphragm.

Serous pericardium

The serous pericardium consists of two layers:

• Parietal layer - fused and inseparable from the fibrous pericardium;
• Visceral layer (a.k.a epicardium) - attached to the outer surface of the heart itself.

Pericardial cavity

The pericardial cavity is a potential space found between the two layers of the serous pericardium. It contains lubricating serous fluid known as pericardial fluid.