Pericardial veins

The pericardial veins are small veins located around the pericardium, the double-walled sac that encloses the heart. These veins drain the blood from the pericardium and surrounding tissues. They run along the outer layer of the pericardium and typically follow the course of the pericardiophrenic arteries, which supply blood to the pericardium. The pericardial veins collect venous blood from the pericardial tissue and drain into the larger venous structures of the thorax, such as the azygos vein, internal thoracic vein, or brachiocephalic vein. These veins are part of the venous system that supports the circulation of the tissues surrounding the heart.

Location

The pericardial veins are located around the pericardium, the fibrous and serous membrane that surrounds the heart. These veins run along the outer fibrous layer of the pericardium, in close association with the pericardiophrenic arteries. The veins are situated within the thoracic cavity, specifically between the pericardium and adjacent structures such as the lungs, diaphragm, and mediastinum.

Anatomy

Origin

The pericardial veins originate from the small venous plexuses and capillaries within the fibrous and serous layers of the pericardium. These plexuses drain deoxygenated blood from the pericardial tissue, which surrounds the heart, and converge to form larger pericardial veins that ultimately connect to major veins in the thorax.

Course

The pericardial veins follow the course of the pericardiophrenic arteries, which supply blood to the pericardium.^[8] These veins run parallel to the arteries and travel within the fibrous pericardium as they ascend through the mediastinum. As they course through the thoracic cavity, the pericardial veins collect blood from the pericardium and adjacent structures. As the veins travel toward the larger venous system, they typically pass alongside the diaphragm and the heart. Their location within the thoracic cavity means they lie between the heart and the lungs, and they are closely associated with the surrounding pleura and diaphragm.

Tributaries

The pericardial veins collect blood from several smaller tributaries:

• Pericardial Capillaries: Small capillaries within the pericardium that drain the outer and inner layers of the pericardium.
• Pericardiophrenic Veins: These veins accompany the pericardiophrenic arteries and serve as a primary drainage route for the pericardial veins. The pericardiophrenic veins collect blood from the pericardium and diaphragm and drain into the internal thoracic veins.

In some cases, small tributaries may also collect blood from the surrounding thoracic structures, including the diaphragm and pleura, as part of the overall venous drainage system.

Termination

The pericardial veins ultimately drain into several larger veins in the thoracic cavity, depending on their specific location and course:

• Azygos Vein: Some of the pericardial veins drain into the azygos vein, which is a major vein responsible for collecting blood from the posterior thoracic wall and upper abdomen. The azygos vein continues superiorly and drains into the superior vena cava.
• Internal Thoracic Vein: The pericardial veins may also drain into the internal thoracic vein, which runs vertically along the inner surface of the anterior chest wall and eventually empties into the brachiocephalic vein.
• Brachiocephalic Vein: On both the left and right sides, some pericardial veins may terminate directly in the brachiocephalic veins, which return blood to the superior vena cava.

Relations to Neighboring Structures

• Pericardium: The pericardial veins are embedded in the outer fibrous layer of the pericardium, which encloses the heart and provides a protective sac around it. They are closely associated with the pericardiophrenic arteries, which supply the pericardium with oxygenated blood.
• Diaphragm: As the pericardial veins descend toward the thoracic cavity, they are closely related to the diaphragm, particularly the regions where the pericardium and diaphragm attach.
• Lungs and Pleura: The pericardial veins are positioned near the lungs, and their course through the mediastinum places them adjacent to the pleura, the membrane covering the lungs.^[1] This proximity means that the pericardial veins may receive venous drainage from nearby pleural tissues as well.
• Azygos and Internal Thoracic Veins: The pericardial veins eventually drain into either the azygos vein or the internal thoracic veins, both of which are located within the thoracic cavity. The azygos vein lies along the posterior thoracic wall, while the internal thoracic veins are positioned more anteriorly, along the sternum.

Variations

The anatomy of the pericardial veins can vary between individuals. The number, size, and exact course of the pericardial veins may differ slightly. In some cases, there may be additional venous connections with nearby thoracic veins, such as the accessory hemiazygos vein or small intercostal veins.^[2] Similarly, the termination points of the pericardial veins may show variation, with some individuals having more prominent drainage into the brachiocephalic vein, while others may show more extensive connections with the azygos system. These anatomical variations do not typically affect overall function but are important for understanding the venous drainage of the pericardium during surgical procedures or medical imaging.

Function

Venous Drainage of the Pericardium

The primary function of the pericardial veins is to drain deoxygenated blood from the pericardium, the protective sac surrounding the heart.^[3] The pericardium is composed of two layers: the fibrous pericardium, which is the outer layer, and the serous pericardium, which lines the inside. These veins collect blood from both layers, ensuring proper circulation of the pericardial tissues, and prevent the accumulation of blood in the pericardium, which could otherwise lead to congestion or fluid buildup.

Drainage of the Pericardiophrenic Region

The pericardial veins are closely associated with the pericardiophrenic veins, which drain blood from both the pericardium and the diaphragm.^[4] The diaphragm is involved in the mechanics of breathing, and by draining both the pericardium and the diaphragm, the pericardial veins help maintain proper blood circulation in the region. The veins ensure that blood from these areas is efficiently transported away, preventing venous congestion and maintaining healthy tissue function in both the pericardium and the diaphragm.

Contribution to the Thoracic Venous System

The pericardial veins serve as an important component of the thoracic venous system, contributing to the venous return from the pericardium to larger veins such as the azygos vein, internal thoracic vein, and brachiocephalic vein. This connection integrates the venous drainage of the pericardium into the systemic venous circulation, ensuring that blood from the pericardial tissues is returned to the heart. By linking to the larger thoracic veins, the pericardial veins provide an essential pathway for the removal of deoxygenated blood from the tissues surrounding the heart.

Prevention of Pericardial Fluid Accumulation

The efficient drainage provided by the pericardial veins helps to prevent the accumulation of excess fluid within the pericardial sac, which can lead to pericardial effusion or cardiac tamponade if unchecked.^[5] By collecting and transporting blood from the pericardial capillaries and venous plexuses, the pericardial veins ensure that fluid balance is maintained in the pericardial space, reducing the risk of excess fluid compressing the heart and impairing its function.

Removal of Metabolic Waste Products

As part of the body’s venous system, the pericardial veins play a role in the removal of metabolic waste products from the pericardial tissue. The pericardium, like other tissues, undergoes metabolic processes that produce waste products that need to be cleared to maintain healthy tissue function. The pericardial veins transport these waste products away from the pericardium, allowing them to be processed and removed from the body through the systemic venous circulation.

Regulation of Blood Flow in the Pericardium

The pericardial veins assist in regulating blood flow within the pericardium by ensuring that deoxygenated blood is efficiently drained from the tissue. This prevents the buildup of venous pressure within the pericardial tissues, which could affect the overall function of the pericardium in protecting and lubricating the heart. Proper blood flow regulation is crucial for maintaining a healthy pericardium, especially in response to physiological changes such as increased cardiac activity or respiratory movements.

Support for the Pericardium’s Protective Function

By ensuring effective venous drainage, the pericardial veins contribute to the pericardium’s role as a protective structure around the heart. The pericardium helps to anchor the heart within the thoracic cavity, prevents overexpansion of the heart during increased blood volume, and reduces friction during cardiac movements. The pericardial veins support these functions by preventing congestion or fluid buildup within the pericardium, allowing it to function optimally in protecting the heart from mechanical stress and injury.

Maintenance of Fluid Balance in the Thoracic Cavity

The pericardial veins also contribute to the overall fluid balance within the thoracic cavity. By draining blood from the pericardium, these veins help prevent the accumulation of fluid in the thoracic cavity, which could interfere with respiratory function or compress the heart.^[6] The drainage provided by the pericardial veins is part of the broader network of veins in the thorax, including the azygos and internal thoracic veins, which help maintain a balanced fluid environment in and around the lungs and heart.

Collateral Circulation in the Thorax

The pericardial veins can play a role in collateral circulation in the thorax. In cases where the primary venous pathways, such as the azygos vein or brachiocephalic vein, are obstructed or compromised, the pericardial veins can help provide an alternative pathway for venous return from the pericardium. This collateral circulation helps prevent venous congestion in the pericardium and thoracic cavity and ensures that blood flow remains consistent even in the event of vascular blockages or damage to other thoracic veins.

Venous Drainage from the Diaphragm

As part of their role in the pericardiophrenic venous system, the pericardial veins assist in draining blood from the diaphragm, particularly the regions where the diaphragm attaches to the pericardium.^[7] This function is important for maintaining proper circulation in the diaphragm, which is the main muscle involved in respiration. The pericardial veins help ensure that the diaphragm remains well-vascularized and able to function efficiently during breathing.

Clinical Significance

The pericardial veins are clinically significant due to their role in draining blood from the pericardium and nearby thoracic structures. In cases of pericarditis (inflammation of the pericardium) or pericardial effusion (fluid buildup around the heart), the pericardial veins are involved in the management of venous drainage from the inflamed or fluid-filled pericardium. Disruptions to the flow through these veins can lead to complications such as venous congestion or further exacerbation of pericardial conditions. During thoracic surgeries, particularly heart surgeries or procedures involving the pericardium, these veins are at risk of injury, which could lead to excessive bleeding or venous complications. Their relationship to the azygos and internal thoracic veins also makes them important during procedures in the mediastinum or for conditions like cardiac tamponade, where proper venous drainage is critical for reducing pressure on the heart.