Posterior intercostal aa.

The posterior intercostal arteries are arteries that supply blood to the posterior parts of the intercostal spaces. There are eleven paired posterior intercostal arteries, one on each side.

• The 1st and 2nd posterior intercostal arteries arise from the supreme intercostal arteries, branches of the costocervical trunks of the subclavian arteries.

• The lower nine posterior intercostal arteries are the aortic intercostals, so called because they arise from the back of the thoracic aorta.

The right aortic intercostals are longer than the left because of the position of the aorta on the left side of the vertebral column; they pass across the bodies of the vertebrae behind the esophagus, thoracic duct, and azygos vein, and are covered by the right lung and pleura.

The left aortic intercostals run posteriorly on the sides of the vertebrae and are covered by the left lung and pleura. The upper two left posterior intercostal arteries are crossed by the left superior intercostal vein. The lower aortic intercostal arteries are crossed by the hemiazygos vein.

Opposite the heads of the ribs, the sympathetic trunk passes inferiorly anterior to the posterior intercostal (aortic) arteries. The thoracic splanchnic nerves also descend anterior to the lower posterior intercostal arteries.