The term pericardium is derived from the Greek prefix peri- (“around”) and kardia (“heart”), implying a structure that envelops or encloses the heart. Morphologically, the pericardium is a fluid-filled sac-like structure that surrounds the heart.
The pericardium is a double-walled structure, made up of an outer fibrous layer and an inner serous layer. The fibrous layer is a single, connective tissue layer, made up of collagen (type I and type III mainly) and elastin fibers; it is elastic and yet non-distensible. This layer of the pericardium is held in position cranially by its membranous folds interdigitating with the tunica adventitia of the great vessels; caudally by ligaments connecting to the central tendon of the diaphragm; ventrally to the sternum by superior and inferior pericardiosternal ligaments; and dorsally by ligaments fastening it to the esophagus and spine.
The serous layer is made up of an outer parietal layer and inner visceral layer, each made up of a single layer of epithelial cells. The parietal layer lines the inner regions of the fibrous pericardium, while the visceral layer lines the outer layer of the heart and therefore it is called epicardium.
A void space found among the two linings of serous pericardium is called pericardial cavity, which encircles the heart. Between the parietal and visceral layer is the pleural cavity, which holds the pleural fluid. About 20–30 ml in volume, this serous fluid acts as a lubricant and minimizes friction between the epicardium and parietal layer as the heart muscles expand and contract with the beating of the heart. Thus, the pericardial cavity allows the heart movement to be flexible.
The pericardial cavity surrounds the heart totally except at the inlet and outlet of the cardiac vessels, where they form two significant tubes. One of the tubes serves as an interconnection to the inferior and superior vena cava and the pulmonary veins, whereas the other connects the aorta and the pulmonary trunk.
Blood Supply and Innervation of the Pericardium
Blood supply to the pericardium occurs mainly through the pericardiophrenic artery, although a few minor contributions also occur via the musculophrenic artery, esophageal artery, bronchial artery, and superior phrenic artery. The coronary artery is also involved, but it supplies blood only to the visceral layer. Draining of blood from the pericardium involves the pericardiophrenic veins as well as the azygos venous system.
The innervations of the outer fibrous and parietal pericardium are due to the phrenic nerve (C3-C5), and it also provides motor and sensory innervation to the diaphragm. This nerve originates from the neck and travels down through the thoracic cavity. The phrenic nerve is a general source of referred pain, for example, shoulder pain experienced as a result of pericarditis. And the visceral pericardium is innervated by the vagus and sympathetic nerves.
In medical terminology, a sinus is a passage or channel. There are two such sinuses in the pericardial cavity: the transverse pericardial sinus and the oblique pericardial sinus. Both these are formed during embryonic folding of the heart tube during ontogeny.
The reflections (folds) of the serosal layer result in the formation of a set of complex tubes, one enclosing the aorta and the pulmonary trunk and the other enclosing the superior and inferior vena cave and the pulmonary veins. This results in a transverse sinus across the pericardium that separates the arterial blood vessels and the venous blood vessels. The index finger can pass though the transverse sinus, which helps surgeons to identify the blood vessels of the heart during coronary artery bypass surgery.
Oblique sinus is formed by reflection of the pericardial membranes onto the pulmonary veins. It is bounded by the inferior vena cava and by the right pulmonary veins on one side and the left pulmonary veins on the other.
Functions of the Pericardium
The pericardium is known to have some specific functions:
- Tethering: pericardium holds the heart in place within the cardiac cavity
- Limiting: it prevents overfilling of the ventricles and a consequent overexpansion of the heart
- Lubrication: it lubricates the moving surfaces of the heart
- Protective: Serves as a physical barrier between the muscular heart and the surrounding organs (e.g. lung) that may be prone to infection
The pericardium is also able to transmit changes in intra-thoracic changes in pressure that occur with changes in respiration, as the pressure in the structure is 2–5 mm Hg lower than the normal pressure within the thoracic cavity. Furthermore, the non-distensible nature of the pericardium provides an overall constraint on the overall heart volume and prevents irregular expansion of the chambers. Because of its enclosing anatomic structure, the pericardium ensures that the left and right ventricles are equally compliant. Moreover, as the overall volume is fixed, expansion of one chamber automatically impedes the expansion of another, ensuring that the heart chambers work rhythmically and in unison.