The high levels of urea irritate the serous pericardium, making it secrete a thick pericardial fluid that's full of fibrin strands and white blood cells.
On an echocardiogram, a pericardial effusion makes the heart look like it's dancing within the pericardium, whereas chronic pericarditis shows the stiff serous pericardium restricting the heart's movement.
Meanwhile, the inner serous pericardium consists of an inner visceral layer, or epicardium — which is actually part of the heart wall — and an outer parietal layer.
In chronic pericarditis, immune cells start to recruit fibroblasts which create lots of fibrin - a protein that makes the serous pericardium stiffer - a bit more like the fibrous pericardium.
This is similar to tamponade physiology but happens more gradually and is a result of a change in the composition of the serous pericardium, rather than a fluid collection around the serous pericardium.
The inner layer of the pouch is the serous pericardium that includes the pericardial cavity, and is filled with a small amount of fluid that lets the heart slip around as it beats.
The cells of the serous pericardium secrete and reabsorb the fluid, so usually there's no more than 50 milliliters of fluid in the pericardial cavity at one time - that's about as much as a shot glass.