Pericardium. Coloured transmission electron micrograph (TEM) of a section through the serous pericardium. The pericardium is a double-walled sac surrounding the heart. The serous pericardium is the wall adjacent to the heart (bottom right). It is split into two layers, the epicardium (just above the heart) and the parietal layer (across top). Between the two layers is the pericardial cavity, which contains serous fluid. Both layers consist of flattened squamous epithelial cells, whose cell nuclei are light brown. The serous pericardium lubricates the heart, preventing friction.