Human heart has three layers, the outer one being pericardium. Pericardial sac is made of two thin layers of tissue that enclose our heart. Between the two layers is a small amount of fluid. This fluid keeps the layers from rubbing against each other and causing friction.
Cardiac tamponade or pericardial tamponade is a life threatening situation, where fluid (mostly blood) accumulates in the space between the pericardial sacs, faster than the pericardial sac can stretch. The outer pericardium is made of fibrous tissue which does not easily stretch,