The hilum of the lung is a large triangular depressed area that lies more in the posterior part of the mediastinal surface of each lung.

It is the site where the lung root attaches to the lung. The hilum is the site where various structures enter and leave the lung via the lung root.

The hilum is also a place where the visceral pleura connects with the parietal pleura, which marks the meeting point between the mediastinum and the pleural cavities.

The lung root is a short and broad pedicle formed by a collection of structures that connect the medial surface of the lung to the heart, trachea and surrounding anatomical structures.

The structures that form the root are covered by a sleeve of pleura, a two-layered membranous coating. The visceral pleura covers all surfaces of the lungs, except the hilum.

The structures forming the lung root include:

Principal (main) bronchus
Pulmonary artery
Two pulmonary veins
Bronchial branches from the thoracic aorta
Bronchial veins
Pulmonary autonomic nerve plexus
Lymphatic vessels
Bronchopulmonary lymph nodes
Loose connective tissue

The root of the right lung is located behind the superior vena cava and the right atrium, beneath the azygos vein, while the root of the left lung is situated beneath the aortic arch and anteriorly to the descending thoracic aorta.

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Hilum of lungs