The thin, fluid-filled sac that surrounds the heart, including the top of the heart that is connected to major blood vessels, such as the aorta and vena cava. The pericardium is made up of an outer layer of connective tissue that holds the heart in place within the chest, protects it from inflammation, and acts as a barrier against infection. It also keeps the heart from stretching out and filling up with too much blood. The pericardium is also made up of two thin, inner layers of tissue that have a small amount of fluid in between them. The fluid helps keep the tissues from rubbing against each other as the heart moves within the pericardium.