The intercostal arteries pass together with the intercostal nerves and intercostal veins in the intercostal spaces. They go between the intercostal veins above and the intercostal nerves below. The intercostal arteries can be subdivided into two groups - anterior intercostal arteries and posterior intercostal arteries.

Note: The subcostal artery is a paired blood vessel that travels below the last rib. It is not classified as the intercostal artery as it does not go between two ribs. The subcostal arteries are the lowest parietal branches of the descending thoracic aorta and are analogous to the posterior intercostal arteries. They mainly supply the flat abdominal muscles, such as the external oblique muscles , internal oblique muscles and transversus abdominis muscles.

Anterior intercostal arteries

Each person has nine pairs of anterior intercostal arteries. The left intercostal arteries and right intercostal arteries arise lateral to the sternum:

The upper six anterior intercostal arteries originate from the internal thoracic artery.
The seventh through ninth anterior intercostal arteries are branches of the musculophrenic artery.
The lowest two intercostal spaces do not have anterior intercostal arteries.

Soon after the anterior intercostal arteries arise, they divide into two branches - superior (upper) and inferior (lower). The superior branches run within the subcostal grooves of the ribs above and anastomose with the anterior branches of the posterior intercostal arteries, while the inferior branches pass along the superior borders of the ribs below and anastomose with the collateral branches of the anterior branches of the posterior intercostal arteries.

Overall, the anterior intercostal arteries supply the muscles and skin of the anterior aspects of the intercostal spaces.

Note: The internal thoracic artery arises from the subclavian artery. From its origin site, it travels downward within the thoracic cage along the side of the sternum - approximately 1 centimeter (0.4 inches) laterally from it. It is accompanied by a pair of internal thoracic veins. The internal thoracic artery terminates by dividing into the superior epigastric artery and musculophrenic artery.

Posterior intercostal arteries

Each person has eleven pairs of posterior intercostal arteries:

The first two posterior ingtercostal arteries originate from the superior (supreme) intercostal artery, which is a branch of the costocervical trunk.
The third to eleventh posterior intercostal arteries arise from the posterior surface of the descending thoracic aorta.

Each posterior intercostal artery has two main branches - posterior and anterior. The posterior branches arise close to the origin of the posterior intercostal arteries. They supply the posterior aspects and deep muscles of the back and course together with the posterior branches of the intercostal nerves. The anterior branches pass along the costal grooves, where they are accompanied by the intercostal veins and nerves. The anterior branches further split into even smaller branches. Close to the costal angle, they give off small collateral branches. The anterior branches of the intercostal arteries provide supply to the lateral aspects of the intercostal spaces.

Overall, the posterior intercostal arteries provide arterial blood supply to the spine and its content, thoracic walls, parietal pleura and the skin and muscles of the back. The right posterior intercostal arteries are longer than the left because the descending thoracic aorta is positioned on the left side of the spine.

Note: The anterior and posterior intercostal arteries form anastomoses and connect with each other approximately at the anterior axillary line.

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Intercostal arteries