What is the hilum of the lung?

The hilum of the lungs is a triangular-shaped anatomical area located on the mediastinal surface of each lung, next to the cardiac impression, just above the middle mediastinum. In this area the lung tissue meets the bronchi, veins and other structures that form the pulmonary root. The structures that anchor the lungs to the trachea and the heart are also found in the hilum of the lung.

In general, in human anatomy, the hilum of an organ refers to a depression or fissure in that organ through which other structures such as blood vessels, nerves, lymphatic vessels, etc. enter and exit. It is very common for the root of the organ to be found in the hilum, and for this reason hilum and root are often used interchangeably in many contexts, for example, in the Atlas of Anatomy of the University of Michigan, hilum and pulmonary root are used for refer to the same area.

Location

The body has two lungs, one on each side of the chest cavity. The two lungs are separated from each other by the trachea and the heart. The area in which the trachea and heart are located is known as the middle mediastinum, and the surface of the lungs facing the mediastinum is known as the mediastinal face or surface.

The hilum of the lung is located in the upper half of mediastinal aspect of each lung. The hilum of the right lung lies behind the superior vena cava and behind part of the right atrium and under the arch of the azygos vein. The hilum of the left lung is situated below the aortic arch, in front of the descending aorta. Anterior to each hilum is the phrenic nerve, the pericardiophrenic vessels, and the pulmonary plexus. In the back is the vagus nerve.

Structure and characteristics

The rib cage and the lungs are separated by a membranous lining called the pleura which has two layers, the visceral pleura, closest to the lung tissue, and the parietal pleura. At the hilum of the lung the two pleurae are connected serving as a point of union between the mediastinum and the pleural cavity.

The hilum is formed by a reflection of the pleura that surrounds the corresponding bronchus, the pulmonary vein, the pulmonary artery, bronchial veins and arteries, the nerves of the pulmonary plexus, lymphatic vessels, the pulmonary lymph nodes and loose connective tissue. From the hilum, the visceral pleura descends forming what is known as the pulmonary ligament or triangular ligament of the lung.

The left and right hilum are not symmetrical. In each one we find this order of structures (see attached graphics):

  • Right pulmonary hilum (from top to bottom): eparterial bronchus, pulmonary artery, hyparterial bronchus, and pulmonary veins.
  • Left pulmonary hilum (from top to bottom): pulmonary artery, bronchus, and pulmonary veins.
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