What anatomical structure surrounds the heart and prevents overfilling?

The anatomical structure that surrounds the heart and prevents overfilling is the pericardial sac. This fibrous and serous membrane is designed to provide a protective layer around the heart. The outer fibrous layer helps anchor the heart within the mediastinum and limits its movement while the inner serous layer provides lubrication, allowing the heart to move smoothly as it beats. This arrangement also plays a crucial role in limiting the amount of fluid that can accumulate around the heart, thus preventing overfilling or excessive stretching of the heart chambers.

The myocardium refers to the muscle tissue of the heart responsible for pumping blood, while the endocardium is the inner lining of the heart chambers. The epicardium, which covers the outer surface of the heart, is part of the pericardium but does not play a direct role in preventing overfilling. Each of these other structures contributes to the overall function and protection of the heart but does not specifically limit its volume as the pericardial sac does.