Arteries and veins of the thoracic wall

Posterior intercostal arteries

The posterior intercostal arteries are the arteries that supply the back muscles, vertebral column, spinal cord segments, and the overlying skin in the related area and are found within the intercostal space. There are eleven posterior intercostal arteries and they arise from two sources:

• The first and second posterior intercostal arteries originate from the superior (supreme) intercostal artery, a branch of the costocervical trunk
• Third to eleventh posterior intercostal arteries arise directly from the posterior surface of the thoracic aorta. 

With the thoracic aorta being displaced towards the left of the vertebral column, the right posterior intercostal arteries must travel the longer course to enter the right intercostal space. Along their course, the right posterior intercostal arteries pass behind the thoracic duct, azygous vein, esophagus, right lung and associated pleura. Both the left and right posterior intercostal arteries are related to the sympathetic trunk and ganglia on the same side. 

The posterior intercostal artery travels in the costal groove between the intercostal vein and nerve. The vessel first passes in the endothoracic fascia before passing between the internal and innermost intercostal muscles. 

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The posterior intercostal artery gives off two main branches along its course; the dorsal (posterior) and the collateral branch. The first is the dorsal (posterior) branch which emerges close to the origin of the parent artery. It follows the posterior ramus of the spinal nerve and gives off spinal, medial, and lateral musculocutaneous branches.

The second branch of the posterior intercostal artery emerges at the costal angle of the rib and is known as the small collateral branch. The posterior and collateral intercostal arteries anastomose with the anterior intercostal arteries of the same side at the anterior axillary line. The only exception to this rule is in the tenth and eleventh intercostal spaces. Here, there are no anterior intercostal arteries and the space is completely supplied by the posterior intercostal artery.

Subcostal artery

The subcostal arteries are direct branches of the thoracic aorta. They are analogous to the posterior intercostal artery, so if there was a 12th intercostal space, they would be the 12th intercostal arteries. Like its preceding counterparts, each subcostal artery gives the anterior and posterior branches that travel along with the subcostal space below the twelfth rib. On the left side, the artery passes behind the accessory hemiazygous vein, while on the right side it passes in front of the twelfth thoracic vertebra and behind the thoracic duct and azygous vein. On either side, the arteries are posteriorly related to the sympathetic trunk, diaphragm, and adjacent pleura. 

The subcostal artery accompanies the subcostal vein and nerve in the subcostal space. The trio enters the abdomen as it passes by the lateral (external) arcuate ligament of the diaphragm, quadratus lumborum and the kidney. The subcostal artery penetrates the aponeurosis of transversus abdominis and anastomoses with the ipsilateral superior epigastric, caudal posterior intercostal, and upper lumbar arteries. 

The subcostal artery helps the lower posterior intercostal artery to supply the musculocutaneous structures of the anterolateral abdominal wall at the level of the twelfth rib.

Internal thoracic artery

The internal thoracic arteries were once referred to as the internal mammary arteries because they indirectly supply the breasts. They stem from the subclavian artery and contribute to supplying the intercostal muscles, skin and parietal pleura associated with the first six intercostal spaces. 

The arteries enter the thoracic cavity as they descend behind the clavicle and first costal cartilage. Upon entering the chest cavity, the vessel is crossed anteriorly by the phrenic nerve. It continues posteriorly to the first six costal cartilages, lateral to the sternum, and deep to the pectoralis major. The vessel passes behind the external intercostal membrane and internal intercostal muscles up to the level of the second or third costal cartilage. Beyond this level, it passes in front of the transversus thoracis muscles. The artery is accompanied by venae comitantes that coalesce at the third costal cartilage to form a single internal intercostal vein that travels medial to the artery. There is also a lymphatic chain that accompanies the vessels along their course.

The internal thoracic artery gives off five sets of branches:

• The upper six anterior intercostal arteries (discussed below)
• Musculophrenic arteries, which give off the 7th to 9th intercostal arteries 
• Superior epigastric arteries that continue inferiorly where and anastomose with the inferior epigastric arteries at the level of the umbilicus to supply the abdominal wall
• Sternal branches that supply the posterior surface of the sternum and transversus thoracis muscle. They anastomose with the pericardiophrenic, bronchial, and posterior intercostal arteries to form the subpleural plexus of the mediastinum. 
• Perforating branches that travel alongside the anterior cutaneous branches of the intercostal nerve to supply pectoralis major and the overlying skin

There is a neat little mnemonic that can help you remember these branches! Just learn ' Mummy Swallowed Ancient Papyrus Scrolls', which stands for:

• Musculophrenic a.
• Superior epigastric a.
• Anterior intercostal a.
• Perforating a.
• Sternal a.

Anterior intercostal artery

The anterior intercostal arteries, like their posterior counterpart, are also paired (left and right) structures. There are nine pairs of the anterior intercostal arteries, occupying the upper nine intercostal spaces. They arise just lateral to the retrosternal area from two sources:

• The upper six vessels arise from the internal thoracic  
• The 7th - 9th vessels arise from the musculophrenic arteries. 

They subsequently ramify into upper and lower branches that occupy the costal groove and the upper edge of the lower rib, respectively. The upper branch travels in a neurovascular bundle with the anterior intercostal vein and nerve in the costal groove, while the lower branch travels parallel to the upper edge of the lower rib.

The first and second anterior intercostal arteries are found in the endothoracic fascia, traveling between the internal intercostal muscles and parietal pleura. The third to sixth anterior intercostal arteries are separated from the parietal pleura by the fibers of the transverse thoracic muscle. The seventh to ninth arteries initially pass between the parietal pleural and the internal intercostal muscles, then deeper to pass between the internal and innermost intercostal muscles.

Each anterior intercostal artery will anastomose with the posterior intercostal and small collateral arteries at the level of the anterior axillary line. The anterior intercostal artery provides musculocutaneous supply to the intercostal muscles and overlying skin of its intercostal space.

Intercostal veins

There are eleven pairs of posterior intercostal veins, nine pairs of anterior intercostal and one pair of subcostal veins supplying the thoracic wall. Each posterior intercostal vein forms an anastomosis with the ipsilateral anterior intercostal veins. 

The intercostal veins have different sites of drainage: 

• The first posterior intercostal vein drains directly into the brachiocephalic vein. 
• The second and third posterior intercostal veins anastomose to form the left and righ superior intercostal veins. The left superior intercostal vein drains into the left brachiocephalic vein, while the right counterpart drains to the superior vena cava via the azygous vein. 
• The remaining fourth to eleventh posterior intercostal veins drain to either the azygous or hemizygous veins.
• The first six pairs of anterior intercostal veins drain directly to the internal thoracic vein. The seventh to ninth anterior intercostal veins will first drain to the musculophrenic vein, which then drains to the internal thoracic vein. 

Learn more about the intercostal veins here:

Internal thoracic vein

The internal thoracic veins accompany the internal thoracic arteries. They unite at about the third costal cartilage to form a single internal intercostal vein that is medial to the accompanying artery.Like most veins in the body, the internal thoracic vein has several valves along its length to promote the unidirectional flow of blood. It receives segmental tributaries at each intercostal level (similar to the points at which the corresponding arteries emerge). The pericardiophrenic vein also drains deoxygenated blood by way of the internal thoracic veins. The internal thoracic vein eventually drains directly into the ipsilateral brachiocephalic vein.