The pericardium is a 2-layered membrane that surrounds the heart. The outer membrane is called the fibrous membrane, and the inner layer is called the serous membrane. These membranes also surround some of the blood vessels near the heart, including the aorta, the superior vena cava, and the pulmonary artery.
A small amount of fluid is present between the layers of the pericardium which lubricates the heart, reducing friction and allowing it to beat safely within the chest.
In some cases, such as chest surgery, fluid can build up between the layers of the pericardium resulting in a pericardial effusion. This is a medical emergency if the effusion occurs rapidly, or if a large amount of fluid is present between the layers.