The pericardium or pericardial sac, is a double-layered closed sac that surrounds the heart and anchors it within the mediastinum. The two layers of the pericardium are the visceral pericardium and the parietal pericardium. The visceral pericardium covers the outer surface of the heart. The parietal pericardium lines the cavity in which the heart sits.

The human heart is comprised of four chambers: the left and right atria and the left and right ventricles. The atria are the upper chambers and receive blood that is being returned to the heart. The right atrium receives blood that has circulated throughout the body delivering oxygen and nutrients, and the left atrium fills with newly oxygenated blood returning from the lungs when the atria are relaxed. When the atria contract, they push the blood through valves into the relaxed ventricles. When the ventricles contract, the right ventricle pumps blood to the lungs and the left ventricle pumps blood to the body. This continuous cycle of contractions is driven by the heart's electrical system.