The pericardial sac, also known as the pericardium, is a double-walled fibroserous membrane that encloses the heart and the roots of the great vessels, providing protection and reducing friction between the heart and surrounding structures. It consists of two layers: the fibrous pericardium, which is the tough outer layer, and the serous pericardium, which is further divided into the parietal and visceral layers, with the pericardial cavity in between containing lubricating serous fluid.