The pericardium--the thin membrane that forms a sac around the heart--helps hold the fist-sized muscle in place, and keeps it from overexpanding. You won't even know the pericardium is there unless you get pericarditis: when it becomes inflamed due to anything from infection to injury.

When this condition occurs, the membrane lining becomes thicker, compressing the fluid that lies between the sac and your heart and making it harder for your heart to pump blood.