The pericardium is a closed, fluid-filled sac that surrounds the heart and the attached arteries and veins. It serves several functions, including acting to reduce friction between the walls of the heart and the surrounding tissue. Because the heart is in constant motion, it could rub against the thoracic cavity or against other organs, and it could suffer wear from that friction; the pericardium keeps the heart from directly contacting other objects, and the fluid inside allows the heart to expand and contract smoothly. That fluid also acts as a shock absorber to the various impacts of daily life; without the pericardium's absorbent properties, the heart could suffer an impact that stops or interrupts its rhythm. The pericardium also acts to limit the expansion of the heart during its normal beating; it keeps the heart from expanding too far when the heart fills with blood. A side effect of this limitation is that if the amount of pericardial fluid increases too far, the heart's expansion could be limited negatively; this is called pericardial tamponade.