Hilum of the Lung (Root of the Lung)

What Is a Lung Hilum?

A lung hilum is the opening on the inner surface of each lung where the airway, blood vessels, nerves and lymph vessels pass between the lung and the center of the chest. Each of the two lungs has one hilum, and it is the only place where the lung is attached to anything; everywhere else the lung moves freely inside its pleural sac.

The word hilum, plural hila, is used in anatomy for the notch or depression where vessels and ducts enter an organ; the kidneys and spleen have hila too. In the lung, the hilum is a roughly wedge-shaped or racquet-shaped area on the mediastinal surface, outlined by the line where the pleura covering the lung turns back to line the chest.

Explained simply

Think of each lung as a large, soft house with only one doorway, on the side facing the heart. Every pipe and cable the house needs, the air pipe, the blood pipes in and out, the wires and the drains, comes in through that single doorway. The doorway is the hilum, and the bundle of pipes and cables running through it is the root of the lung.

Where Is the Hilum of the Lung?

The hilum of the lung is on the medial surface of each lung, the side that faces the mediastinum, a little above the middle of that surface and slightly toward the back. The root of the lung lies roughly opposite the bodies of the fifth to seventh thoracic vertebrae.

On the right lung, the hilum sits just behind the shallow cardiac impression made by the right atrium and beneath the groove for the arch of the azygos vein. On the left lung, it lies behind and above the deeper impression made by the heart, which continues forward as the cardiac notch, and beneath the groove for the arch of the aorta. Each hilum connects the lung to the structures in the middle of the thoracic cavity.

Relations of the root of the lung

• In front: the phrenic nerve on both sides; the superior vena cava and right atrium on the right.
• Behind: the vagus nerve on both sides; the descending aorta on the left.
• Above: the arch of the azygos vein on the right and the arch of the aorta on the left.
• Below: the pulmonary ligament, a fold of pleura hanging down from the root.

Hilum of Lung Anatomy: What Passes Through It?

Structures passing through the hilum of each lung include one main bronchus, one pulmonary artery, two pulmonary veins, the bronchial arteries and veins, the pulmonary nerve plexus, lymph vessels and the bronchopulmonary (hilar) lymph nodes. Together these form the root of the lung.

• Main bronchus. Carries air from the trachea into the lung, where it begins the branching of the bronchial tree. On the right, the upper lobe bronchus often branches off before the hilum, so two bronchi may be seen. See the bronchi page.
• Pulmonary artery. Brings oxygen-poor blood from the right ventricle into the lung for gas exchange.
• Pulmonary veins. Usually two on each side, a superior and an inferior, carrying oxygen-rich blood back to the left atrium.
• Bronchial arteries and veins. Small vessels that supply the walls of the airways and lung tissue.
• Pulmonary plexus. A network of autonomic nerve fibers from the vagus nerve and the sympathetic trunks that controls airway width, glands and blood vessels.
• Lymph vessels and hilar lymph nodes. Drain lymph from the lung toward the tracheobronchial nodes around the carina.

How the structures are arranged

In both hila, the pulmonary veins lie in front and lowest, the pulmonary artery lies in the middle and the bronchus lies at the back. The main difference between the two sides is the position of the artery relative to the bronchus. In the left hilum, the pulmonary artery is the highest structure, lying above the bronchus. In the right hilum, the upper lobe bronchus is the highest structure, sitting above the pulmonary artery, which is why it is called the eparterial bronchus. Students often remember this as RALS: Right, Anterior; Left, Superior, describing where the artery lies relative to the bronchus.

Hilum of Lung vs Root of Lung: What Is the Difference?

The difference between the hilum and the root of the lung is that the hilum is the place, an area on the lung surface, while the root is the content, the bundle of structures that passes through that area. The two terms are often used as if they meant the same thing, which is why this page covers both.

The root of the lung runs a short distance from the hilum to the mediastinum and is wrapped in a sleeve of pleura. At the hilum, the visceral pleura covering the lung folds back on itself and becomes continuous with the parietal pleura lining the mediastinum, enclosing the root like a cuff. Below the root, this sleeve hangs down as a double fold called the pulmonary ligament, which gives the vessels room to expand and lets the root move as the lung inflates. The pleural cavity page shows how the two pleural layers form one closed sac.

A useful way to keep the two apart: you can see the hilum on a lung removed from the body, as a cut area with vessel and airway openings, but the root only exists while the lung is still attached to the heart and trachea.

What Is the Function of the Hilum of the Lung?

The function of the hilum of the lung is to act as the single gateway through which air, blood, nerve signals and lymph travel between each lung and the rest of the body, while also anchoring the lung to the mediastinum. Without it, the lung could not connect to the airways and the heart.

• Air in and out: every breath passes through the main bronchus at the hilum on its pathway to the alveoli.
• Blood for gas exchange: the pulmonary artery and veins at the hilum form the link between the lung and the pulmonary circulation.
• Nourishment of the lung: the bronchial arteries carry oxygen-rich blood to the airway walls.
• Nerve control: nerves of the pulmonary plexus pass through the hilum to the airways and vessels.
• Immune filtering: lymph from the lung passes through hilar lymph nodes, which trap microbes and particles.
• Anchoring: the root holds each lung in position while the rest of its surface slides freely during breathing.

What Does the Hilum of the Lung Look Like on an X-Ray?

On a chest X-ray, each hilum appears as a branching, slightly denser shadow beside the heart, formed mainly by the pulmonary arteries and veins. The air-filled bronchi and normal-sized lymph nodes add little to the shadow.

In most people the left hilum sits slightly higher than the right, because the left pulmonary artery arches over the left main bronchus while the right pulmonary artery runs lower, in front of the right bronchus. Radiologists compare the size, shape, density and height of the two hila on every chest film. A hilum that looks larger or denser than expected can reflect enlarged lymph nodes or enlarged pulmonary arteries, and a hilum pulled up or down can point to shrinkage or collapse of part of a lung. A CT scan shows the separate structures of the root in much more detail.

Medical students also study the hilum on anatomy models and preserved specimens, where its arrangement of veins in front, artery in the middle and bronchus behind is easy to trace.

Common Conditions That Affect the Hilum of the Lung

Most conditions involving the hilum cause the hilar lymph nodes or the pulmonary vessels to enlarge, or involve a growth near the main bronchus. Our respiratory diseases section has more background, and anyone with symptoms should see a doctor.

• Hilar lymph node enlargement (hilar lymphadenopathy): swelling of the nodes at one or both hila, seen in infections such as tuberculosis, in sarcoidosis, in lymphoma and when cancer spreads to the nodes.
• Lung cancer: tumors starting in or near the main bronchi, often called central tumors, can make one hilum look larger.
• Pulmonary hypertension: high pressure in the pulmonary arteries can widen them, enlarging both hilar shadows.
• Pulmonary embolism: a large clot may lodge in a pulmonary artery near the hilum.
• Lung collapse: loss of air in a lobe, for example from a blocked bronchus, can pull the hilum up or down on an X-ray.