Two sacs of mesothelial cells surround the heart, protecting it from friction during its normal activity. They also restrict the heart's motion, so that it does not contact other surfaces that could prove harmful. Finally, these sacs prevent nearby inflammations and infections from spreading to the heart. The deeper of the two is known as the serous pericardium, and it is divided into two layers. These are the parietal and visceral layers of the serous pericardium.
The parietal layer of the serous pericardium lies next to the fibrous pericardium, the outer sac protecting the heart. Although these linings are considered separate anatomical entities, they are actually joined together. The visceral layer is also joined to a surface, in this case, the outside of the heart.
Both layers, the parietal and visceral layers, rejoin one another at several points. These points are where the great vessels, such as the aorta and venae cavae, pass out of the heart. At these places, the serous pericardium joins the outer layers of these veins. This fusion with different membranes allows the pericardium to completely enclose the heart.
The pericardial cavity is a small space that separates the two layers. This cavity in the heart is filled with a serous fluid of proteins and water. Each layer of the serous pericardium produces this fluid, which is called pericardial fluid.
At times, trauma or conditions like tuberculosis may increase the amount of fluid in the pericardial sac. This is known as a pericardial effusion, and can be life-threatening. Depending on the cause, the pericardial effusion may dissipate on its own, but it may require surgical intervention. Normally, there is anywhere from 15 milliliters (ml) to 50 ml of pericardial fluid. An effusion that leads to more than 120 ml of fluid will create excess pressure on the heart.
If the pericardial fluid exerts pressure on the heart, it can restrict its movement. This can cause several negative effects, including low blood pressure and a drop in heart output. How quickly the fluid accumulates can influence the severity of this condition.
In the worst-case scenario, a cardiac tamponade, there is a rapid increase in fluid, coupled with a rapid decrease in cardiac activity. Ultimately, this can cause death. Usually, hemorrhages are the main cause of a cardiac tamponade, but there are other potential causes. These other disease states include tuberculosis, cancer, reactions to drugs, radiation therapy, or infection, or systemic lupus.