Posterior intercostal veins

The posterior intercostal veins are located in the intercostal spaces between the ribs, running along the posterior aspect of the thoracic wall. These veins accompany the posterior intercostal arteries and lie between the innermost and internal intercostal muscles. They collect venous blood from the intercostal muscles, skin, and parietal pleura. The posterior intercostal veins drain into larger veins such as the azygos vein on the right side and the hemiazygos and accessory hemiazygos veins on the left side. These veins are part of the systemic venous circulation, playing a key role in draining the thoracic wall.

Structure and Anatomy

Location

The posterior intercostal veins are located within the intercostal spaces of the thorax, between adjacent ribs. They run along the posterior aspect of the thoracic wall, between the innermost and internal intercostal muscles. These veins course parallel to the posterior intercostal arteries and nerves, situated deeper in the intercostal space, near the bony rib cage. They are present on both the right and left sides of the thorax, draining blood from the back of the chest wall.

Structure

The posterior intercostal veins form a series of segmental veins, with each vein corresponding to a specific intercostal space. These veins are organized in pairs, with one posterior intercostal vein for each intercostal space on either side of the thorax. Each posterior intercostal vein runs laterally and slightly inferiorly along the rib, following the course of the corresponding posterior intercostal artery and nerve. The veins are thin-walled but are capable of draining significant amounts of blood from the intercostal muscles, skin, and surrounding tissues.

Relations

• Anterior relations: Anteriorly, the posterior intercostal veins are related to the internal intercostal muscles, which lie between the ribs. The parietal pleura, which lines the thoracic cavity, lies deeper to the veins.
• Posterior relations: Posterior to the posterior intercostal veins are the vertebrae of the thoracic spine and the spinal nerves that emerge from the intervertebral foramina. The veins are also related to the sympathetic trunk.
• Superior and inferior relations: Superiorly, the posterior intercostal veins run alongside the posterior intercostal arteries and nerves, which are located slightly superior to the veins within the same intercostal space. Inferiorly, the veins are bordered by the rib below the intercostal space they occupy.

Course

The posterior intercostal veins originate from the lateral thoracic wall, collecting blood from the intercostal muscles, skin, and parietal pleura. From there, they run posteriorly and medially along the rib cage, accompanying the posterior intercostal arteries and nerves within the intercostal spaces. The veins travel in a neurovascular bundle, typically situated in the costal groove of the rib (a depression on the underside of the rib). As the veins approach the vertebral column, they drain into larger venous structures on either side of the thorax.

• Right side: The posterior intercostal veins on the right side drain into the azygos vein, which ascends along the right side of the vertebral column and empties into the superior vena cava.
• Left side: The posterior intercostal veins on the left side are divided into upper and lower groups. The upper veins (usually from the 4th to 8th intercostal spaces) drain into the accessory hemiazygos vein, while the lower veins (from the 9th to 12th spaces) drain into the hemiazygos vein. Both the accessory hemiazygos and hemiazygos veins cross the midline and drain into the azygos vein.

Tributaries

The posterior intercostal veins receive blood from several smaller tributaries as they course along the thoracic wall, including:

• Muscular tributaries: These veins drain the intercostal muscles, which are responsible for aiding in respiration by moving the ribs during breathing.
• Cutaneous tributaries: The posterior intercostal veins also collect blood from the skin and subcutaneous tissue overlying the thorax, specifically from the posterior thoracic wall.
• Vertebral tributaries: Small veins that drain the muscles and ligaments of the posterior thoracic spine, including parts of the vertebral bodies and the vertebral venous plexus, contribute to the posterior intercostal veins.

Termination

The posterior intercostal veins terminate by draining into larger veins that return blood to the central venous system:

• Right side: The right posterior intercostal veins drain into the azygos vein, which then empties into the superior vena cava. The azygos vein serves as the major drainage system for the posterior thoracic wall on the right side.
• Left side: The left posterior intercostal veins drain into the hemiazygos vein (for lower intercostal spaces) and the accessory hemiazygos vein (for upper intercostal spaces). These veins cross over to the right side of the thorax and join the azygos vein, which subsequently drains into the superior vena cava.

Communication with the Anterior Intercostal Veins

The posterior intercostal veins communicate with the anterior intercostal veins, which drain the anterior portion of the intercostal spaces. These anterior veins empty into the internal thoracic veins, which run along the inner surface of the anterior thoracic wall. The posterior and anterior intercostal veins are connected via small venous anastomoses that allow blood to flow between the anterior and posterior systems, ensuring balanced drainage of the intercostal spaces.

Variations

The anatomy of the posterior intercostal veins can exhibit anatomical variations in their course and drainage patterns. For example, the exact point at which the left posterior intercostal veins join the hemiazygos or accessory hemiazygos veins may vary slightly between individuals. In some cases, the pattern of venous drainage on the left side may involve a direct connection between certain intercostal veins and the azygos vein, bypassing the hemiazygos or accessory hemiazygos system. Additionally, the size and prominence of individual posterior intercostal veins can vary, depending on factors such as the volume of blood being drained from the thoracic wall.

Function

Venous Drainage of the Thoracic Wall

The primary function of the posterior intercostal veins is to drain venous blood from the posterior thoracic wall, which includes the intercostal muscles, skin, and the parietal pleura (the outer lining of the lungs and thoracic cavity). These veins collect deoxygenated blood from these structures and transport it toward larger venous systems for return to the heart. The intercostal muscles play a significant role in breathing by helping to expand and contract the ribcage, and the posterior intercostal veins ensure that blood is efficiently drained from these active muscles during respiration.

Drainage from the Intercostal Muscles

The posterior intercostal veins provide venous drainage from the intercostal muscles, which are located between the ribs and play a key role in the mechanics of breathing. These muscles expand and contract the ribcage to assist with inhalation and exhalation. The posterior intercostal veins collect deoxygenated blood from the intercostal muscles after each contraction and prevent the buildup of metabolic waste products, such as carbon dioxide, that accumulate during muscular activity. This ensures the muscles remain functional and are properly oxygenated after each respiratory cycle.

Venous Return from the Skin and Subcutaneous Tissues

Another important function of the posterior intercostal veins is the venous return from the skin and subcutaneous tissues overlying the thoracic wall. These veins drain blood from the cutaneous (skin) layers of the posterior thoracic wall, as well as from the subcutaneous tissues and fat. The skin and underlying tissues of the thorax receive blood from small capillary networks that eventually drain into the posterior intercostal veins. By collecting this blood, the veins help maintain proper circulation and tissue health in the back of the chest wall.

Drainage from the Parietal Pleura

The posterior intercostal veins are also involved in draining blood from the parietal pleura, the outermost layer of the pleural membrane that lines the thoracic cavity and separates it from the lungs. This pleura is richly supplied with blood vessels, and the posterior intercostal veins help drain venous blood from the pleura, particularly in the posterior regions of the thorax. By ensuring venous drainage from the pleura, the posterior intercostal veins help maintain the health of this membrane, which plays a critical role in lung function and the mechanics of breathing.

Connection to the Azygos and Hemiazygos Systems

The posterior intercostal veins serve as conduits that connect the thoracic wall to the larger venous networks of the azygos and hemiazygos systems. On the right side, the posterior intercostal veins drain into the azygos vein, which ascends along the right side of the vertebral column and empties into the superior vena cava, returning blood to the heart. On the left side, the posterior intercostal veins drain into the hemiazygos and accessory hemiazygos veins, which cross over to the right side and also join the azygos system. This connection ensures that blood from the thoracic wall is efficiently returned to the heart through the systemic circulation.

Venous Drainage from the Vertebral Structures

The posterior intercostal veins also contribute to the venous drainage of the vertebral structures in the thoracic region. They receive small tributaries from the vertebrae, including blood from the vertebral bodies, muscles, and ligaments of the posterior spine. These veins help drain the blood from the spinal region and connect the vertebral venous plexus to the larger azygos and hemiazygos systems, ensuring that venous blood from the vertebral column is efficiently transported away from the spine.

Role in Regulating Venous Pressure in the Thoracic Wall

The posterior intercostal veins play an important role in regulating venous pressure within the thoracic wall. By draining venous blood from both the muscles and tissues of the thoracic wall and transporting it to the azygos system, the posterior intercostal veins help maintain balanced pressure throughout the venous system of the thorax. This regulation of pressure is essential for preventing venous congestion, which can occur when blood flow is obstructed or slowed. The veins’ direct connection to the azygos system allows for smooth venous return from the thorax, even during changes in posture or respiratory activity.

Communication with the Anterior Intercostal Veins

The posterior intercostal veins also communicate with the anterior intercostal veins, which drain the anterior portion of the thoracic wall. Through small venous anastomoses, the posterior and anterior intercostal veins are connected, allowing blood to flow between these two systems. This communication ensures balanced venous drainage from both the anterior and posterior thoracic walls, helping to maintain overall circulation within the thorax and prevent localized areas of venous stasis.

Contribution to Systemic Circulation

The posterior intercostal veins contribute to the systemic venous circulation by draining deoxygenated blood from the thoracic wall and returning it to the heart. By connecting with the azygos and hemiazygos systems, the posterior intercostal veins form an essential part of the thoracic venous network, which ultimately transports blood to the superior vena cava. From there, the blood is returned to the right atrium of the heart, where it is reoxygenated through the pulmonary circulation. The efficient function of the posterior intercostal veins ensures that blood flow from the thoracic wall is seamlessly integrated into the larger systemic circulation.

Support for Venous Return During Respiration

During the respiratory cycle, the thorax undergoes significant movement as the ribcage expands and contracts with inhalation and exhalation. The posterior intercostal veins play a key role in supporting venous return during respiration, as they collect venous blood from the intercostal muscles, which are active during breathing. The veins must adapt to the changes in pressure within the thorax that occur with each breath, ensuring that blood is efficiently drained from the intercostal spaces and transported to the azygos system. This adaptability allows the posterior intercostal veins to maintain continuous venous return, even during the physical activity of breathing.

Clinical Significance

The posterior intercostal veins are clinically significant due to their role in the venous drainage of the thoracic wall and their connection to the azygos and hemiazygos venous systems, which are critical for venous return to the heart. These veins are commonly involved in conditions like pleural effusions, where fluid accumulation in the pleural cavity can compress the veins and hinder venous drainage, potentially leading to venous congestion.

In trauma cases, such as rib fractures, injury to the posterior intercostal veins can cause internal bleeding in the thoracic cavity, leading to complications like hemothorax. The posterior intercostal veins are also a potential route for the spread of metastatic cancer, as they connect the thoracic wall with the larger systemic venous circulation.

During thoracic surgeries, such as procedures involving the azygos vein or rib resection, careful management of the posterior intercostal veins is crucial to prevent excessive bleeding and ensure proper venous drainage from the thoracic wall.