Faces midline; divided into vertebral part (small, posterior) + mediastinal part (large, anterior).

Vertebral Part

• Related to vertebral column, posterior intercostal vessels, greater/lesser splanchnic nerves

Mediastinal Part

• Presents the hilum; related to heart, great vessels, nerves

Structures Forming Left Mediastinal Surface

• Left ventricle + aorta = main structures
• Aorta: ascends → arches over root of left lung → descends behind as descending thoracic aorta
• 3 great vessels from arch: brachiocephalic trunk, left common carotid, left subclavian
• Esophagus shifts left behind heart, crosses descending aorta
• 4 neural structures: left phrenic, left vagus, left recurrent laryngeal, left sympathetic chain

Relations Table

Right lung mediastinal surface differs: right atrium, SVC/IVC, azygos vein, right brachiocephalic vein, esophagus + trachea, only 3 neural structures

Neural Relations

Left phrenic nerve — crosses aortic side → in front of lung root → superficial to left ventricle → diaphragm

Left vagus nerve — held away from trachea by aortic arch; gives left recurrent laryngeal nerve (hooks under arch → tracheoesophageal groove); below arch, runs behind lung root → posterior pulmonary + esophageal branches

Left sympathetic chain — paravertebral gutter; splanchnic nerves pierce diaphragmatic crura

Cardiac Notch and Lingula

• Anterior border: wide cardiac notch — heart/pericardium uncovered by lung → superficial cardiac dullness
• Below notch: lingula — tongue-shaped projection; left lung’s equivalent of right middle lobe

Root of Left Lung — Order (Anteroposterior)

Differs from right: right principal bronchus divides before hilum → upper lobar bronchus above pulmonary artery (eparterial), lower below (hyparterial). Left principal bronchus entirely hyparterial (no pre-hilar division).

Applied Anatomy

• Cardiac notch / superficial cardiac dullness — uncovered pericardium → percussion dullness; used to estimate cardiac borders
• Lingula — surgical landmark; site of localized bronchiectasis/aspiration; may need lingulectomy

The medial surface of the lung faces the midline and is divided into two parts:

1. A small posterior vertebral part
2. A large anterior mediastinal part

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Vertebral Part

The vertebral part is related to:

• The vertebral column
• Posterior intercostal vessels
• Greater and lesser splanchnic nerves

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Mediastinal Part

The mediastinal part presents the hilum of the lung and is related to mediastinal structures — the heart, great blood vessels, and nerves. The relations of the mediastinal surface differ between the right and left lungs because the structures forming the right and left surfaces of the mediastinum differ.

Structures Forming the Left Surface of the Mediastinum

1. The left ventricle and aorta are the main structures forming the left surface of the mediastinum
2. The aorta ascends initially, arches over the root of the left lung, and then descends behind the lung root (as the descending thoracic aorta)
3. Three great vessels — brachiocephalic trunk, left common carotid artery, and left subclavian artery — arise from the arch of the aorta and ascend to reach the root of the neck
4. The esophagus, as it descends through the thorax, shifts to the left behind the heart and gently crosses the line of the descending aorta
5. Three neural structures: (a) left phrenic nerve, (b) left vagus nerve, (c) left sympathetic chain — note that the left side has four neural structures in total when the left recurrent laryngeal nerve is included separately (see table below)

Relations of the Mediastinal Surface of the Left Lung

For comparison, the right lung’s mediastinal surface is related instead to the right atrium, SVC/IVC, azygos vein, right brachiocephalic vein, esophagus and trachea, and only 3 neural structures (right phrenic, right vagus, right sympathetic chain).

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Neural Relations in Detail

Left Phrenic Nerve

The left phrenic nerve crosses the aortic (left) side, passes in front of the lung root, and runs down superficial to the left ventricle to reach the diaphragm.

Left Vagus Nerve

The left vagus nerve is held away from the trachea by the aortic arch. Here it gives off the left recurrent laryngeal nerve, which hooks under the aortic arch and ascends into the tracheoesophageal groove. Below the aortic arch, the left vagus nerve runs behind the lung root and breaks up into posterior pulmonary and esophageal branches.

Left Sympathetic Chain

The position of the sympathetic trunk and splanchnic nerves on the left side is similar to that on the right side — running in the paravertebral gutter, with splanchnic nerves leaving its lower half to pierce the crura of the diaphragm.

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Cardiac Notch and Lingula

The anterior border of the left lung presents a wide cardiac notch, occupied by the heart and pericardium. In this region the heart and pericardium are uncovered by lung tissue, producing an area of superficial cardiac dullness on percussion.

Below the cardiac notch, the anterior border presents a tongue-shaped projection called the lingula — the left lung’s morphological equivalent of the right middle lobe.

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Hilum and Root of the Left Lung

The root of the lung is a short, broad pedicle connecting the medial surface of the lung to the mediastinum. It consists of the structures entering and leaving the lung at the hilum.

Arrangement of Structures in the Root — Left Side (Anteroposterior order)

N.B. The arrangement differs from the right side because the right principal bronchus divides into lobar bronchi before entering the hilum — the upper lobar bronchus passes above the pulmonary artery (eparterial bronchus), and the lower lobar bronchus passes below it (hyparterial bronchus). On the left side, the principal bronchus is entirely hyparterial (passes below the pulmonary artery), since it does not divide before the hilum.

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Applied Anatomy

Cardiac Notch and Area of Superficial Cardiac Dullness

Because the cardiac notch leaves the pericardium and heart uncovered by the left lung, this area produces a zone of dullness on percussion distinct from the resonant note of the surrounding lung — clinically used to estimate cardiac borders.

Lingula as a Surgical Landmark

The lingula, as the morphological counterpart of the right middle lobe, is occasionally the site of localized bronchiectasis or aspiration pathology and may require lingulectomy in selected cases.

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