pericardium

Think of the pericardium as a fitted double-layered bag that wraps snugly around the heart, much like a balloon you have pushed your fist into so that the balloon hugs your hand on both sides. It holds the heart in place inside the chest, lets it slide smoothly as it beats, and shields it from the lungs and other neighbours.

Anatomically there are two main layers. The tough outer fibrous pericardium anchors to the diaphragm below and the great vessels above. Inside it sits the serous pericardium, which itself folds into a parietal layer (lining the fibrous sac) and a visceral layer (the epicardium, fused to the heart's surface). Between these serous layers is a thin film of pericardial fluid, normally only about 15 to 50 millilitres, that acts as lubricant.

Because the fibrous sac does not stretch quickly, the pericardium limits sudden over-filling of the heart and helps the two ventricles influence each other's filling. That same stiffness becomes a problem when fluid builds up fast or the sac thickens, which is the theme of several diseases in this field.