The heart is enclosed in which type of membrane sac?

The heart is enclosed in the pericardium, which is a protective membrane sac that surrounds the heart and attaches to the surrounding structures. This fibrous and serous sac provides a lubricated environment that reduces friction during heartbeats and also helps to anchor the heart in place within the thoracic cavity. The pericardium consists of two layers: the outer fibrous pericardium and the inner serous pericardium, which further divides into parietal and visceral layers. This structure is vital for maintaining the proper functioning of the heart and protecting it from infection and inflammation.

The epicardium is actually part of the heart wall itself, specifically the outer layer, and is synonymous with the visceral layer of the serous pericardium. The endocardium lines the inner chambers of the heart and is not a protective outer layer. The pleura refers to the membrane surrounding the lungs, which is unrelated to the heart. Thus, the pericardium is the correct answer as it uniquely encompasses the heart, ensuring its protection and proper functioning.