Pericardium

Fibrous pericardium

The fibrous layer of the pericardium is a dense irregular connective tissue sac. Its base lays over the diaphragm and is attached to the central tendon of the diaphragm by the pericardiacophrenic ligament. The apex of the fibrous pericardium is continuous with the tunica adventitia (perivascular connective tissue) of the great vessels that leave the heart. The posterior border is related by loose connective tissue to the structures of the posterior mediastinum, whereas the anterior border lays directly on the posterior surface of the sternum to which it is attached by the sternopericardial ligaments.

These attachments of the pericardium to surrounding structures fixes the position of the heart, prevents its excessive distension, and provides protection.

Serous pericardium

The thin serous pericardium is a serous membrane, or serosa. Like all serous membranes, it consists of two layers:

• The outer parietal layer that lays directly on the cavity wall, that is, onto the inner surface of the fibrous pericardium
• The inner visceral layer that directly covers the organs in the cavity, that is, the heart. It is also called the epicardium as it is the outer covering of the heart. 

Between the layers is the pericardial cavity, a potential space filled with 15 - 50 milliliters of serous pericardial fluid. Pericardial fluid is secreted by pericardial cells, and it is this fluid that reduces friction between the parietal and visceral layer during heart contractions. 

The layers are separated, but at two places the parietal layer of the serous pericardium reflects toward the visceral and fuses with it, forming dead ends (cul-de-sacs) in order to keep the pericardial cavity closed. These two reflections happen at the following places:

• Superiorly, surrounding the arteries (aorta and pulmonary trunk)
• Posteriorly, surrounding the veins (superior vena cava, inferior vena cava, pulmonary veins)

The cul-de-sac formed around the posterior reflection that is J-shaped and located behind the left atrium is called the oblique pericardial sinus. The passage between the anterior and posterior reflections is the transverse pericardial sinus, which lies superior to the left atrium. This is not to be confused with the transverse sinus which is a dural venous sinus. Note that the serous pericardium is a serous membrane that histologically consists of a layer of squamous mesothelium supported by a loose connective tissue through which neurovasculature can reach it.

Arterial supply

The arterial supply of the pericardium comes predominantly from the pericardiacophrenic artery (a branch of the internal thoracic artery). This artery is located within the fibrous pericardium on its passage through the thoracic cavity. Additionally, the musculophrenic artery (a terminal branch of the internal thoracic), bronchial, esophageal, and superior phrenic arteries (branches of the thoracic aorta) contribute to vascularisation of the pericardium. Note that coronary arteries also take part in arterial supply, but only to the visceral layer of the serous pericardium.

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Venous drainage

Deoxygenated blood is drained via various tributaries into the azygos venous system and the pericardiacophrenic veins, which are tributaries of the brachiocephalic vein.

Innervation

When it comes to innervation, the pericardium has three main sources:

• Phrenic nerves (C3-C5) provide mostly somatic afferent (pain, temperature sensory) innervation and pass directly through the fibrous pericardium. Pain within the diaphragm usually projects to dermatomes C3-C5, so it is felt within the ipsilateral supraclavicular region.
• The sympathetic trunk which provides postganglionic vasomotor fibers
• Vagus nerve (CN X) where the function is uncertain