The pericardium is the membrane that encloses the heart and the roots of the major heart vessels, consisting of an outer fibrous layer (fibrous pericardium) and an inner double serous membrane layer (serous pericardium).
The fibrous pericardium consists of thick fibrous connective tissue and it defines the borders of the middle mediastinum. On the other hand, the serous pericardium is physically in a much closer relation with the heart. Between its two layers is a small amount of serous pericardial fluid that lubricates the layers and prevents friction during heart contractions, which is, along with mechanical protection, the basic function of the pericardium.
|Function||Protects the heart, prevents friction during heart contractions|
Fibrous - outer layer of dense irregular connective tissue
Serous - inner layer of mesothelium, consists of two layers:
- parietal (lays under the fibrous pericardium)
- visceral (epicardium) (lays over the heart)
|Cavity||Pericardial cavity (between visceral and serous sheets) contains 15 - 50 mL of serous pericardial fluid|
Superior - continuous with the tunica adventitia of the great heart vessels
Inferior - attaches to the central tendon of the diaphragm
Anterior - attaches to the posterior surface of the sternum
Posterior - related to the content of the posterior mediastinum
Arterial supply - pericardiacophrenic (main), musculophrenic, bronchial, esophageal, superior phrenic arteries
Venous drainage - azygos system, pericardiacophrenic veins
Innervation - phrenic nerves (C3-C5) (sensory), sympathetic trunk (vasomotor), vagus nerve (CN X)
|Clinical relations||Pericardial effusion, pericarditis|
This article will discuss the anatomy of the pericardium and most common pathological conditions related to it.
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.
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.
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.
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
The pericardium is very rarely affected by an isolated pathological process, but rather it is affected by processes primarily occurring within nearby structures. In clinical practice, you will mostly deal with pericardial effusions and inflammation (pericarditis).
The amount of serous pericardial fluid between the serous layers has a physiological range 15 - 50 milliliters. In some cases, the amount of pericardial fluid can exceed this range. This is mostly due to pericardial inflammation and is called pericardial effusion. Depending on the cause, the content of the excessive fluid is different. So, if the fluid persists in its serous characteristics, the cause is usually congestive heart failure, or any other condition that may cause hemodynamic instability of the organism, such as hypoalbuminemia. On the other hand, if the pericardial discharge contains blood, it usually indicates that there is an ongoing heart malignancy (which is very rare - the rarest malignancy of the humans), aortic dissection or even a rupture of the myocardium. Finally, if the pericardial fluid contains lymph, the cause of the effusion is usually mediastinal lymphatic obstruction.
The consequences of pericardial effusion depend on the accumulation rate of the excessive fluid. If it is fast, like it is in case of aortic dissection or myocardial rupture, even 250 milliliters of fluid can impose a large and acute pressure on the heart and cause heart failure. This type of heart failure caused by acute effusion is called cardiac tamponade (heart compression). On the other hand, if the accumulation of the fluid is slow, even up to 1000 milliliters of accumulating effusion can be tolerated well. In this case, you will have enough time to conduct cardiocentesis, a life saving and symptom relieving procedure where you aspirate the pericardial content with a thin needle.
Inflammation of the pericardium is called pericarditis. It is usually caused by the spreading of an ongoing viral infection within myocardium. It typically causes pericardial effusion which may end with heart failure if not treated well. If the pericarditis takes a chronic course, it may result with fibrosis of the pericardium or even with calcification of it. In these cases, the elasticity of the pericardium is significantly decreased as the pericardium thickens in a way that it seriously hinders heart contractions and limits them. This form of disease, where the heart is practically in a cage, is called constrictive pericarditis.
Read another interesting clinical case about the pericardium: Penetration of the Pericardium