The heart has three layers:

  • endocardium, a smooth layer of endothelial cells, lines the interior of the heart and heart valves;
  • myocardium, the middle, muscular layer of the heart wall, is the thickest layer;
  • pericardium, a fibrous and membranous sac, surrounds the heart.

The pericardium is composed of two layers, the visceral pericardium, adhering to the heart, and the parietal pericardium (parietal means wall), lining the outer fibrous coat. The pericardial cavity (between the visceral and the parietal pericardia) normally contains 10 to 15 mL of fluid, which libricates the membranes as the heart beats.

Pericarditis is an inflammation of the sheath surrounding the heart. It may be acute or chronic; it may also be accompanied by an outpouring of fluid or a collection of pus in the pericardial sac.