Pericardium

The pericardium is a double-walled sac that encloses and protects the heart. It consists of two main layers: the fibrous pericardium, which is the tough outer layer, and the serous pericardium, which is further divided into the parietal layer and the visceral layer (epicardium). The space between the parietal and visceral layers, known as the pericardial cavity, contains a small amount of lubricating fluid to reduce friction during heart movements.

Location

The pericardium is located in the mediastinum, the central compartment of the thoracic cavity. It surrounds the heart and the roots of the major blood vessels, including the aorta, pulmonary trunk, and superior and inferior vena cavae. The fibrous pericardium is attached to the diaphragm inferiorly and is anchored to the sternum anteriorly. This position helps stabilize the heart within the thoracic cavity, ensuring that it remains in place as it beats.

Structure and Anatomy

Overview

The pericardium is a fibroserous sac that encloses the heart and the roots of the great vessels. It has two major components: the fibrous pericardium and the serous pericardium, which work together to encase and protect the heart. Each of these components has its own structure, location, and anatomical relationships, making the pericardium a complex and important part of the cardiovascular system.

Fibrous Pericardium

The fibrous pericardium is the outermost layer of the pericardium. It is a tough, inelastic, and dense connective tissue structure that serves as a protective covering. The fibrous pericardium plays a key role in maintaining the position of the heart within the thoracic cavity by anchoring the heart to nearby structures.

Location and Relations:

• Inferiorly: The fibrous pericardium is firmly attached to the central tendon of the diaphragm, anchoring the heart to the diaphragm. This attachment helps synchronize the movement of the heart with the diaphragm during breathing.
• Anteriorly: It is loosely attached to the sternum by the sternopericardial ligaments, which stabilize the heart’s position within the mediastinum.
• Laterally: It is in close relation to the pleura of the lungs on both sides, although it is separated from the lungs by the pleural cavities.
• Posteriorly: The fibrous pericardium is adjacent to structures like the esophagus, descending thoracic aorta, and vertebral bodies.
• Superiorly: It blends with the outer layers of the great vessels that enter and leave the heart, including the ascending aorta, pulmonary trunk, and superior vena cava.

Serous Pericardium

The serous pericardium is a thinner, more delicate double-layered membrane located beneath the fibrous pericardium. It is divided into two layers:

• Parietal Layer:The parietal layer lines the internal surface of the fibrous pericardium. It is a smooth, glistening layer that is in direct contact with the fibrous pericardium but remains separate in structure and function. It provides a smooth surface that allows the heart to move easily within the pericardial sac.
• Visceral Layer (Epicardium):The visceral layer is the innermost layer of the serous pericardium and is also referred to as the epicardium. It lies directly on the surface of the heart, adhering tightly to the myocardium. The visceral layer covers the heart itself and extends over the roots of the great vessels. It is continuous with the parietal layer at the base of the heart, where the great vessels enter and exit.

Pericardial Cavity

The space between the parietal and visceral layers of the serous pericardium is called the pericardial cavity. This potential space normally contains a small amount of pericardial fluid (approximately 15-50 mL), which acts as a lubricant, reducing friction as the heart beats and allowing the heart to move smoothly within the pericardium. The presence of this fluid is essential to prevent adhesions and friction between the layers during the continuous movement of the heart.

Anatomical Relationships

• Anteriorly: The pericardium lies directly behind the sternum and costal cartilages. The fibrous pericardium is in contact with the anterior chest wall, which makes it relatively accessible during certain surgical procedures such as pericardiocentesis.
• Laterally: The lateral surfaces of the pericardium are related to the pleurae of the lungs. The phrenic nerves and accompanying pericardiophrenic vessels run between the fibrous pericardium and the pleura on each side, supplying both the pericardium and diaphragm.
• Posteriorly: The pericardium is adjacent to the esophagus, descending thoracic aorta, and the vertebral column. The close relationship to the esophagus can lead to certain clinical implications, such as pericardial effusion compressing the esophagus, leading to difficulty swallowing.

Ligaments and Attachments

The fibrous pericardium is anchored in place by several key ligaments and attachments that help maintain its position within the thoracic cavity:

• Sternopericardial Ligaments: These ligaments attach the anterior aspect of the fibrous pericardium to the posterior surface of the sternum, helping to stabilize the heart during movement.
• Pericardiophrenic Ligaments: These attach the pericardium to the diaphragm, providing a strong anchor that holds the heart in place. These ligaments prevent excessive movement of the heart during respiration and changes in body position.

Pericardial Reflections

At the points where the great vessels leave and enter the heart, the visceral and parietal layers of the serous pericardium reflect around them. These reflections form two main sinuses:

• Transverse Pericardial Sinus: This passageway is found posterior to the ascending aorta and pulmonary trunk and anterior to the superior vena cava. It is an important anatomical landmark during cardiac surgery, particularly for procedures that involve clamping the great vessels.
• Oblique Pericardial Sinus: This is a blind-ended space located posterior to the heart, formed by the reflection of the serous pericardium around the pulmonary veins and inferior vena cava. It provides some mobility to the heart as it contracts and expands.

Vascular Supply of the Pericardium

The blood supply to the pericardium is provided by several arteries:

• Pericardiophrenic Arteries: These are the most important vessels supplying the fibrous pericardium and the parietal layer of the serous pericardium. They run alongside the phrenic nerves.
• Branches of the Internal Thoracic Arteries: The internal thoracic arteries contribute to the blood supply of the pericardium.
• Other Contributing Arteries: The pericardium also receives blood from the musculophrenic arteries, bronchial arteries, esophageal arteries, and the coronary arteries (for the visceral layer).

Venous Drainage

The venous drainage of the pericardium occurs through:

• Pericardiophrenic Veins: These accompany the pericardiophrenic arteries and drain into the brachiocephalic veins.
• Azygos Venous System: Blood from the pericardium can also drain into the azygos veins, which ultimately drain into the superior vena cava.
• Internal Thoracic Veins: These veins also contribute to the venous drainage of the pericardium.

Innervation of the Pericardium

The pericardium is innervated by several nerves:

• Phrenic Nerves: The phrenic nerve provides the main sensory innervation to the pericardium, especially the fibrous and parietal layers. This innervation is responsible for the sensation of pain, which can be referred to the shoulder in cases of pericardial inflammation due to the shared innervation with the diaphragm.
• Vagus Nerve: The vagus nerve contributes to some parasympathetic innervation of the pericardium, although its role is less significant.
• Sympathetic Trunks: The sympathetic nervous system provides vasomotor innervation to the blood vessels of the pericardium.

Function

Protection of the Heart

One of the primary functions of the pericardium is to protect the heart from physical trauma and injury. The fibrous pericardium, with its tough, inelastic structure, acts as a barrier that shields the heart from external impacts or pressure from surrounding organs. This protective layer prevents the heart from being compressed by structures in the thoracic cavity, such as the lungs or the chest wall, while also guarding against sharp objects or sudden trauma.

Fixation and Positioning of the Heart

The pericardium plays a crucial role in stabilizing the heart’s position within the thoracic cavity. The fibrous pericardium is anchored to surrounding structures, including the diaphragm (via the pericardiophrenic ligaments), the sternum (via the sternopericardial ligaments), and the great vessels (via the adventitial connections). These attachments hold the heart in place, preventing excessive movement during body motion, respiration, or changes in body position. This is important because the heart needs to remain in a stable position to function efficiently.

Prevention of Over-Distension

The inelastic nature of the fibrous pericardium limits the excessive expansion of the heart. During times of increased blood volume or high cardiac output, such as during exercise, the heart needs to expand to accommodate the increased volume of blood. However, the fibrous pericardium restricts over-expansion, preventing the heart chambers from dilating excessively. This regulation of heart size helps maintain optimal functioning, as over-distension could compromise the efficiency of cardiac contractions.

Lubrication and Reduction of Friction

The serous pericardium (specifically the parietal and visceral layers) plays a vital role in reducing friction as the heart beats. The pericardial cavity, which lies between these two layers, contains a small amount of serous fluid (approximately 15-50 mL). This fluid acts as a lubricant, allowing the heart to move smoothly within the pericardial sac during contraction (systole) and relaxation (diastole). Without this lubrication, the constant movement of the heart would cause friction and irritation between the pericardial layers, leading to inflammation or damage.

Limiting the Spread of Infection

The pericardium acts as a physical barrier to limit the spread of infection from adjacent structures into the heart. Infections from nearby organs such as the lungs, pleura, or esophagus are less likely to spread to the heart due to the tough fibrous pericardium. This containment function is essential in preventing potentially life-threatening infections, such as pericarditis, from originating in surrounding tissues and spreading into the heart itself.

Equalizing Pressure Changes

The pericardium helps to buffer and equalize sudden changes in pressure around the heart. By surrounding the heart with a flexible, lubricated cavity, the pericardium provides a controlled environment that reduces the impact of sudden external pressure fluctuations on the heart. This is particularly important during respiration, as the diaphragm’s movement can alter thoracic pressure, or during physical activities when there are rapid changes in blood pressure.

Prevention of Adhesions

The serous pericardium prevents adhesions between the heart and surrounding structures, such as the pleura of the lungs, the diaphragm, or the chest wall. The smooth, lubricated surfaces of the visceral and parietal layers of the pericardium ensure that the heart can move freely within its sac. Adhesions could limit the heart’s movement, restrict its ability to contract effectively, or even lead to complications like constrictive pericarditis, where the heart is unable to expand properly.

Separation from Surrounding Structures

The pericardium functions as a physical boundary that separates the heart from other structures within the mediastinum. It provides a distinct anatomical compartment that isolates the heart, ensuring that other organs, such as the lungs or esophagus, do not interfere with the heart’s motion or function. This separation also prevents any mechanical disruption to the heart, particularly from the lungs during respiration or the esophagus during swallowing.

Assistance in Diastolic Filling

During diastole, the heart fills with blood in preparation for the next contraction. The pericardium, particularly the fibrous layer, helps to optimize this filling by providing a structured yet flexible environment. The limitation on over-expansion set by the pericardium ensures that the heart chambers expand just enough to allow adequate filling, but not so much that it affects the efficiency of contraction during systole. By maintaining a balance between expansion and contraction, the pericardium contributes to the heart’s overall function and cardiac output.

Facilitation of Efficient Contraction

The inelastic nature of the fibrous pericardium not only prevents over-expansion but also aids in efficient cardiac contraction. By limiting the outward expansion of the heart during filling, the pericardium helps the heart maintain a more optimal shape and structure. This ensures that when the myocardium contracts, the force generated is focused on ejecting blood into the circulation rather than stretching the walls of the heart. This allows for more efficient pumping and improved cardiac output during each beat.

Coordination with Respiratory Movements

The fibrous pericardium is attached to the diaphragm, allowing it to move in coordination with the respiratory cycle. During inspiration, the diaphragm descends, creating negative pressure within the thoracic cavity, which helps pull blood into the right atrium and ventricle (a phenomenon known as respiratory pump). The connection between the pericardium and the diaphragm facilitates the heart’s movement with respiration, allowing the heart to function more efficiently in response to changes in intrathoracic pressure.

Facilitation of Surgical Access

Though not a functional aspect in everyday physiology, the pericardium’s anatomical positioning and structure make it easier for surgeons to access the heart during open-heart procedures. The presence of the transverse pericardial sinus, for example, is an important landmark during cardiac surgery, allowing surgeons to clamp the great vessels. The fibrous pericardium’s distinct and protective nature allows surgeons to safely open it and expose the heart for various interventions.

Clinical Significance

The pericardium plays a vital role in protecting and supporting the heart, and abnormalities or diseases affecting the pericardium can lead to significant clinical consequences. Pericarditis, an inflammation of the pericardium, can cause chest pain, fever, and fluid accumulation in the pericardial cavity (pericardial effusion). Severe cases can result in cardiac tamponade, a life-threatening condition where excess fluid compresses the heart, impairing its ability to pump blood effectively.

Constrictive pericarditis occurs when the pericardium becomes thickened and scarred, limiting the heart’s ability to expand and fill during diastole, leading to heart failure symptoms. Surgical removal of the pericardium (pericardiectomy) may be necessary in such cases.

The pericardium also plays a role in trauma, where hemopericardium (blood in the pericardial cavity) due to injury or rupture of heart structures can result in tamponade, requiring immediate intervention. Understanding the pericardium’s function and potential pathologies is critical for diagnosing and managing a range of cardiovascular conditions.