The pericardial sac is a double walled layer of tissue which surrounds the heart and the large blood vessels which supply it. It acts essentially like a protective envelope for the heart, permitting normal heart movement within the confines of the sac while insulating the heart and great blood vessels. The outer layer of the sac is attached to the diaphragm and the sternum to anchor it in place.
This anatomical structure is also known as the pericardium. The outer layer, called the parietal pericardium, is very tough and fibrous, and makes a loose envelope. The inner layer, the visceral pericardium, is more flexible and hugs the heart closely. The pocket between the layers is filled with pericardial fluid, which acts as a lubricant, allowing the heart and visceral pericardium to move around inside the outer layer of the pericardial sac with each heartbeat.
Anatomical drawings of the heart often depict it without the pericardial sac, so that people can see the details of the heart and the vessels which supply it with blood. In surgery, it is necessary to cut through the sac to access the heart, a delicate procedure which is usually performed by a specialist who has received extensive training in heart surgery to learn how to perform such procedures safely and quickly.
Two different medical issues can arise with the pericardial sac. The first is pericardial effusion. This happens when there is too much fluid between the layers. This compresses the heart, and can lead to cardiac tamponade, in which the chambers of the heart cannot fill enough because of the pressure. The treatment for this is to drain off the excess fluid before the patient develops severe complications such as stroke. Fans of medical dramas may have seen dramatic scenes in which needles are plunged into the pericardial sac to drain the fluid buildup.
Another issue which can arise is pericarditis, an inflammation of the pericardial sac which can be associated with viral, bacterial, or fungal infections. It is necessary to treat the underlying infection to resolve the inflammation. People with compromised immune systems are an increased risk of developing this condition because their bodies cannot fight off the infection as effectively. Medications can be administered to treat the infection, and the patient can also be monitored for signs of complications which might suggest that the heart is being compromised by the inflammation.