Pericardium is a fibroserous sac, which encloses the heart and roots of great vessels. It has two primary functions. Firstly, it restricts excessive movements of the heart so that injury to this vital organ of human body is avoided. Secondly, it serves as a lubricated container in which different parts of the heart can contract.
The pericardium lies within the middle mediastinum, posterior to body of sternum and anterior to fifth to eighth thoracic vertebrae.
The pericardium consists of two parts: fibrous pericardium and serous pericardium.

Fibrous pericardium:

It is the strong fibrous part of pericardium, which is attached firmly to the central tendon of diaphragm below. It fuses with the outer coats of great blood vessels that pass through it. These blood vessels are: Aorta, Superior vena cave, Inferior vena cave, Pulmonary trunk, Pulmonary veins. Fibrous pericardium is attached in front to the sternum be means of sternopericardial ligaments.

Serous pericardium:

This part of pericardium coats the heart and lines the fibrous pericardium on the inside. It is composed of two layers: Parietal and Visceral.
Parietal layer of serous pericardium:
It lines the fibrous pericardium and is reflected around the roots of great blood vessels. At the region of reflection, it becomes continuous with the visceral layer of serous pericardium, which covers the heart closely.
Visceral layer of serous pericardium:
This layer is closely applied to the heart and is often called the epicardium. There exists a slit like space between the parietal and visceral layers of serous pericardium. This space is known as pericardial cavity and it contains a small amount of tissue fluid called pericardial fluid. Pericardial fluid acts as a lubricant to facilitate the movements of heart.

Pericardial sinuses:

On the posterior surface of the heart, the reflection of the serous pericardium around the large veins form a recess called oblique sinus. There is another sinus on the posterior surface of the heart, which is called transverse sinus. Transverse sinus is in the form of a short passage that lies between the reflection of serous pericardium around the aorta and reflection around large veins. The pericardial sinuses form as a consequence of the way the heart bends during development. They don’t have any clinical significance.

Nerve supply of the pericardium:

Fibrous pericardium and the parietal layer of serous pericardium are supplied by phrenic nerves. The visceral layer of serous pericardium has different innervations than the parietal layer. It is innervated by branches of sympathetic trunks and vagus nerves.