Pericardium is defined as the double layered membrane which covers the heart externally.
The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of serous membrane (serous pericardium).
The pericardium acts as mechanical protection for the heart and big vessels, and a lubrication to reduce friction between the heart and the surrounding structures. A very important role in all aspects of pericardial functions is played by mesothelial cells.
The Pericardium. The pericardium is the fibrous sac that surrounds the heart. It can be divided into three layers, the fibrous pericardium, the parietal pericardium, and the visceral pericardium. The parietal and visceral pericardia together form the serous pericardium.