The pericardium is a sac that contains the heart and the roots of the great vessels. It normally contains a small amount of pericardial fluid which lubricates the movement of the heart against the pericardial sac.
Pericarditis is inflammation of the pericardium. It can cause fluid to build up in the sac. Excessive amounts of fluid may lead to cardiac tamponade by physically blocking the heart from beating properly or compression of the great vessels of the heart.