What is the purpose of the precardial sac that surrounds the heart in animals?
The pericardium, also known as the pericardial sac, is a double-walled sac of fibrous tissues surrounding the heart and the roots of the great blood vessels.
The pericardium has outer and inner coats. The outer coat (fibrous layer) is tough and thickened, loosely cloaks the heart, and is attached to the central part of the diaphragm and the back of the sternum. The inner coat (serous layer) is double with one layer closely adherent to the heart while the other lines the inner surface of the outer coat with the intervening space being filled with fluid.
This small amount of fluid, the pericardial fluid, acts as a lubricant to allow normal heart movement within the chest. The pericardial fluid helps to reduce friction when the heart is beating.