• Second branch of the aortic arch
• Arises behind manubrium sterni and ascends to a point behind the left sternoclavicular joint
• Has a short thoracic course in the superior mediastinum

Chief relations in the thoracic portion
Anterior                                                                                                                                                          

• Left brachiocephalic vein

Posterior

• Trachea
• Recurrent  left laryngeal nerve
• Oesophagus
• Thoracic duct

Left

• Left pleura
• Left phrenic
• Vagus nerves
• Left subclavian artery