The pericardial sac is a double-walled structure that surrounds the heart. It consists of an outer fibrous layer and an inner serous layer, which is further divided into the parietal and visceral pericardium. This sac helps protect the heart and anchors it within the chest cavity. Between the two layers of the serous pericardium is the pericardial cavity, which contains a small amount of fluid. This fluid reduces friction as the heart beats, allowing it to move smoothly within the sac. The pericardial sac plays a crucial role in maintaining heart function and overall cardiovascular health.