The heart lies protected in the thoracic cavity within the pericardial sac (Fig 1).

The pericardium has three layers: fibrous, parietal and visceral. The attachments of the fibrous pericardial sac are to the great vessels superiorly, to the posterior aspect of the sternum and the central tendon of the diaphragm.

The function of the pericardium is to stabilize the heart within the thorax, whilst permitting free movement of the heart within the pericardial sac, in particular filling and ejecting during the cardiac cycle.