What is the outer sac surrounding the heart?

The outer sac surrounding the heart is known as the pericardium. This tough, fibrous membrane serves several important functions: it provides a protective barrier that helps keep the heart in place within the thoracic cavity, prevents overexpansion of the heart during vigorous activity, and supports the heart structurally.

Additionally, the pericardium contains pericardial fluid, which acts as a lubricant to reduce friction as the heart beats, further protecting the heart from damage during its contraction and relaxation cycles. Understanding the role of the pericardium is crucial in cardiology, as conditions affecting this membrane, such as pericarditis, can lead to significant cardiac issues.

In contrast, the myocardium refers to the muscular layer of the heart responsible for contraction, the endocardium lines the interior chambers and valves of the heart, and the epicardium is the inner layer of the pericardium and also considered part of the heart wall. Thus, the distinction of the pericardium as the outer sac highlights its unique protective quality around the heart.