pericardium /per′ikär″dē·əm/ pl. pericardia [Gk, peri + kardia, heart] , a fibroserous sac that surrounds the heart and the roots of the great vessels. It consists of the serous pericardium and the fibrous pericardium. The serous pericardium consists of the parietal layer, which lines the inside of the fibrous pericardium, and the visceral layer, which adheres to the surface of the heart. Between the two layers is the pericardial space, which contains a few drops of pericardial fluid to lubricate opposing surfaces of the space and allow the heart to move easily during contraction. Injury or disease may cause fluid to accumulate in the space, causing a wide separation between the heart and the outer pericardium. The fibrous pericardium, which constitutes the outermost sac and is composed of tough, white fibrous tissue lined by the parietal layer of the serous pericardium, fits loosely around the heart and attaches to large blood vessels emerging from the top of the heart but not to the heart itself. It is relatively inelastic and protects the heart and the serous membranes. If pericardial fluid or pus accumulates in the pericardial space, the fibrous pericardium cannot stretch, causing a rapid increase of pressure around the heart. −pericardial, adj.
