When there is excess fluid between the heart and the sac surrounding the heart, it is known as the pericardium. Most conditions are not harmful, but they sometimes can make the heart work poorly.
The pericardium is a tough and layered sac. When your heart beats, it slides easily within it. Normally, there are 2 to 3 tablespoons of clear, yellow pericardial fluid between the two layers of the sac. That fluid helps your heart move easier within the sac.
But if you have a pericardial effusion, much more fluid will sit there. Small ones may contain 100 milliliters of fluid. Very large ones may have more than 2 liters.