Root of The Lung

The root of lung is a short broad pedicle joining the medial outermost layer of the lung with all the mediastinum. It is composed of structures entering and leaving the lung at hilum.

The hilum is the area on the mediastinal outermost layer of the lung via which structures goes into or leave the lung.

The root of lung is encompassed by a tubular sheath originated from the mediastinal pleura.

Parts

The root of lung is made from the following structures:

• Main bronchus in the left lung, and eparterial and hyparterial bronchi in the right lung.
• Pulmonary artery.
• Pulmonary veins (2 in number).
• Bronchial arteries (1 on the right side and 2 on the left side).
• Bronchial veins.
• Lymphatics of the lung.
• Anterior and posterior pulmonary plexuses of the nerves.

The root of lung is located opposite the bodies of T5, T6, and T7 vertebrae.

Arrangement of Structures In The Root Of The Lung At The Hilum

The arrangement of structures in the roots of the lungs is as follows:

1. From before backwards (it’s more or less quite similar on 2 sides):

• Pulmonary vein (superior).
• Pulmonary artery.
• Bronchus (left main bronchus on the left side, and eparterial, and hyparterial bronchus on the right side).

Mnemonic: VAB (Vein, Artery, and Bronchus).

2. From above downwards (it differs on 2 sides):.

The difference in the arrangement of structures from above downwards on the 2 sides is because right main bronchus before going into the lung at hilum splits into 2 lobar bronchi, the upper lobar bronchus enters above the pulmonary artery (eparterial bronchus) and lower lobar bronchus enters below the pulmonary artery (hyparterial bronchus).

Connections of The Root of The Lung

Anterior:

• Phrenic nerve.
• Anterior pulmonary plexus.
• Superior vena cava (on right side only).

Posterior:

• Vagus nerve.
• Posterior pulmonary plexus.
• Descending thoracic aorta (on left side only).

Superior:

• Arch of azygos vein (on right side only).
• Arch of aorta (on left side only).

Inferior:

• Pulmonary ligament.

Clinical Significance

Hilar Darkness In Chest Radiograph

In X-ray chest posteroanterior (PA) perspective, the root of every lung throws a radiopaque darkness referred to as hilar shadow in the medial 1-third of the lung field. The shadow is actually thrown by pulmonary vessels when viewed end on. The enlargements of bronchopulmonary lymph nodes (hilar lymph nodes) raise the density of the hilar shadows.