pericardium

The pericardium is the sac that encloses the heart. Its outer layer is tough fibrous tissue that anchors the heart in the chest and resists overfilling; its inner, serous layer folds into two, one pressed against the heart itself and one lining the fibrous sac, with a thin film of fluid between them. The fluid is why a heart beating tens of millions of times a year does not wear itself out against its own container — the surfaces slide. The sac has a limit, which is the clinical point: fluid that accumulates slowly stretches it, but fluid that arrives fast has nowhere to go, and the pressure inside then squeezes the chambers so that they cannot fill. Because the pericardium is fused to the diaphragm below, the heart moves with breathing, a small detail that shows up on an image.