What membrane surrounds the heart?

The membrane that surrounds the heart is the pericardium, also called the pericardial sac.

It is a double-walled sac containing the heart and the roots of the great vessels. Its tough outer layer is the fibrous pericardium. Inside that is the serous pericardium, which has parietal and visceral layers. The visceral layer lies directly on the heart and is also called the epicardium.

A small amount of fluid occupies the pericardial cavity between the serous layers. This fluid reduces friction as the heart beats, allowing the organ to move smoothly. The pericardium also helps anchor the heart in the mediastinum, limits sudden over-expansion, and provides a protective boundary against nearby inflammation or infection.

The word comes from Ancient Greek: peri means “around,” and kardia means “heart.” Common mix-ups involve the endocardium, which lines the inside chambers of the heart, and the myocardium, which is the muscular pumping layer. The pleura surrounds the lungs, while the peritoneum lines the abdominal cavity. Inflammation of the pericardium is called pericarditis; excess fluid in its cavity is a pericardial effusion.