What is the name of the sac that protects, envelops, and anchors the heart?

The sac that protects, envelops, and anchors the heart is known as the pericardium. This double-walled structure comprises an outer fibrous layer and an inner serous layer, which provides a lubricated environment for the heart to move comfortably as it beats. The function of the pericardium is crucial, as it prevents friction between the heart and the surrounding structures, and also helps to stabilize the heart's position within the thoracic cavity. This protective role is essential for maintaining the heart's function and ensuring it operates effectively within the dynamic environment of the chest.

Understanding the other terms can help clarify the unique role of the pericardium. The peritoneum refers to the lining of the abdominal cavity, the pleura is associated with the lungs and chest cavity, and the term "fibrous capsule" could apply to various structures but does not specifically refer to the heart. Therefore, the pericardium is distinctly identified as the protective sac that surrounds and secures the heart, making it the correct choice in this context.