The pericardium is a double-walled sac that surrounds the heart, providing protection and support. It consists of two layers: the outer fibrous layer and the inner serous layer, which is further divided into the parietal and visceral layers. This structure helps to anchor the heart in place and prevents it from over-expanding during vigorous activities. The pericardium also contains a small amount of fluid, known as pericardial fluid, which reduces friction between the heart and surrounding tissues as the heart beats. This fluid allows for smooth movement and helps maintain the heart's position within the chest cavity.