Each small intercostal space contains three arteries, two of which arise anteriorly and one posteriorly.
Posterior intercostal arteries:
The posterior intercostal arteries of the first two spaces branch from the superior thoracic artery (also known as the superior intercostal artery). This artery is a branch of costocervical trunk of the subclavian artery. The posterior intercostal arteries of the lower nine spaces arise directly from the descending thoracic aorta.
Anterior intercostal arteries:
The anterior intercostal arteries of the first six spaces are branches of internal thoracic artery. The anterior intercostal arteries of the remaining lower spaces are branches of the musculophrenic artery that itself is a branch of internal thoracic artery.
Branches of intercostal arteries:
Each intercostal artery gives branches to the intercostal muscles, skin of the intercostal spaces and parietal pleura. In females, in the region of breast, the branches arising towards the superficial structures are significantly large.
All the intercostal arteries are accompanied by corresponding veins. The posterior intercostal veins, which correspond to the posterior intercostal arteries, drain backward into azygous and hemiazygous veins. Veins from the first four or five intercostal spaces drain into hemiazygous while the remaining drain into azygous veins.
The anterior intercostal veins, which accompany the anterior intercostal arteries drain into internal thoracic and musculophrenic veins depending upon the location of the intercostal space.