Also known as the pericardium, the pericardial sac is a conical pouch of fibrous tissue that encloses the heart and the big main blood vessels that supply the heart. This conical pouch of fibrous tissue mainly acts a protective and insulating coat for the heart and the large blood vessels while allowing normal movements of the heart within the boundaries of the sac. Other functions include cardiac lubrication and defense against infections. The outer pericardial sac layer is connected to the sternum and the diaphragm so that it stays in its position.
The meaning of the term ‘pericardium’ is ‘around the heart.’ The inner layer of the pericardial sac that closely surrounds the heart is known as the epicardium or visceral pericardium, while its outer layer is referred to as the parietal pericardium.
Structure of the pericardial sac
The pericardial sac is a strong fibro-serous punch made up to two layers that surrounds the heart. The areas between the 2 layers is called the pericardial cavity and consists of serous fluid that guards the cardiac system from any form of external shock or trauma. The outermost layer of the pericardial sac is called the fibrous pericardium while the inner layer is known as serous pericardium.Both the layers help lubricate the heart and thus prevent any friction or resistance during cardiac activity.
- Serous pericardium: It is divided into 2 different layers, i.e., the visceral pericardium which forms a section of the epicardium, and the parietal pericardium which is attached to the fibrous pericardium and inseparable from it.Both the layers help lubricate the heart and thus prevent any friction or resistance during cardiac activity.
- The visceral layer continues till the start of the large blood vessels that supply the heart and joins into the serous pericardium’s parietal layer. Such attachment occurs in 2 regions, i.e., in the area where the inferior vena cava, superior vena cava, and pulmonary veins gain entry into the cardiac system.
- The contact point of the visceral layer of the pericardial sac with the heart, and not the large blood vessels, is referred to as the epicardium. It is defined as the layer that is just on the outer periphery of the myocardium or the cardiac proper muscle. The epicardium mostly consists of connective tissue and helps protect the heart. During contraction of the ventricles, the depolarization wave moves to the epicardial from the endocardial surface.
- The space between the visceral and parietal pericardial layers is known as the pericardial cavity. It is full of lubricating serous liquid called pericardial fluid.
- Fibrous pericardium: It is the most superficial or thinnest layer of the pericardial sac and is made of loose and dense connective tissue. This tissue helps protect the cardiac system, prevents the heart from filling with excess blood, and anchors it to the enclosing walls. The tissue runs continuously with the outside adventitial layer of the nearby large blood vessels.
Functions of the pericardial sac
Some of the major functions of the pericardial sac are listed below:
- It protects the heart from infections that migrate from other organs like the lungs
- It anchors the heart to the mediastinum and thus restricts its movements.
- The pericardial sac lubricates the cardiac system
- It prevents increased cardiac dilation during acute volume surplus or overload
The adjoining parts
- The pericardial sac is deep to the front chest wall and sternum
- It encloses the bases of the aorta and the pulmonary artery as well as the heart
- Towards the right of the pericardial sac is the pathway of the right phrenic nerve
- The dorsal area of the pericardial sac gets blood from the pericardial arteries
- The left phrenic nerve runs above the sac of the left ventricle.
Conditions of the pericardial sac
The pericardial sac may suffer from two major health problems, i.e., pericardial effusion and pericarditis.
- Pericardial effusion: It occurs due to the occurrence of excessive fluid between the layers. Increased fluid exerts pressure on the heart and can trigger cardiac tamponade, wherein the cardiac chambers cannot sufficient fill up due to the compression.Pericardial effusion is treated by draining the excess fluid from between the layers before the onset of serious complications like stroke, etc. Needles are usually inserted into the pericardial sac so as to drain the accumulated fluid.
- Pericarditis: It is a condition marked by pericardial sac inflammation that occurs due to infections by bacteria, virus, or fungus, etc. The inflammation can only be alleviated by treating the underlying causative infection. The bodies of people with weakened immune system do not have normal capacity to efficiently fight infections. Hence they are more vulnerable to developing pericarditis. The infections are often treated with medications. The patient is also kept under monitor to check for the development of any health complications that are suggestive of cardiac weakening due to the inflammation.
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