Left lung

Pulmo sinister

Also known as: lung, lingula, cardiac notch, left upper lobe, left lower lobe

The smaller lung (≈45% of lung volume), divided by a single oblique fissure into superior and inferior lobes, with a cardiac notch and lingula accommodating the heart.

Show on body map Respiratory Tract

Overview

Lobes / fissures2 lobes / 1 fissure (oblique)

Bronchopulmonary segments8–10 (commonly 8 with fused segments)

Mass (adult male)≈565 g

The left lung is longer and narrower than the right because the heart projects to the left and the left dome of the diaphragm lies lower. Its oblique fissure separates the superior lobe from the inferior lobe. The anterior border of the superior lobe is indented by the cardiac notch (at the level of the 4th–6th costal cartilages), below which a thin, tongue-like projection, the lingula, curves around the heart; the lingula is the counterpart of the right middle lobe and has its own lingular bronchus.

The mediastinal surface carries a deep cardiac impression (left ventricle), a prominent groove for the arch of the aorta above the hilum and for the descending aorta behind it, grooves for the left subclavian and left common carotid arteries near the apex, and an oesophageal impression near the base.

Location: Left half of the thoracic cavity within the left pleural sac; apex in the root of the neck; base on the left dome of the diaphragm (over the spleen, stomach and left lobe of liver).

Parts & subdivisions

Superior (upper) lobe
Apicoposterior, anterior, superior lingular and inferior lingular segments.
Lingula
Inferior tongue-like part of the superior lobe below the cardiac notch; homologue of the right middle lobe.
Inferior (lower) lobe
Superior, anteromedial basal, lateral basal and posterior basal segments.
Cardiac notch
Deep indentation of the anterior border exposing the pericardium (area of superficial cardiac dullness).
Oblique fissure
The only fissure; same course as on the right.
Hilum
Pulmonary artery most superior, bronchus posterior and below it, pulmonary veins anterior and inferior.

Anatomy

Innervation: Pulmonary plexus (left vagus and sympathetic trunk T1–T5), as for the right lung.
Arterial supply: Pulmonary: left pulmonary artery (arches over the left main bronchus); two left pulmonary veins to the left atrium. Bronchial: usually two left bronchial arteries directly from the descending thoracic aorta.
Venous drainage: Pulmonary veins → left atrium; bronchial veins → accessory hemiazygos or left superior intercostal vein.
Lymphatic drainage: Bronchopulmonary → tracheobronchial → paratracheal nodes → left bronchomediastinal trunk/thoracic duct. Part of the left lower lobe drains to the right superior tracheobronchial nodes, so left-sided cancers may involve contralateral nodes.
Relations: Medially: heart and pericardium (left ventricle), arch and descending aorta, left subclavian artery, oesophagus; inferiorly: left dome of the diaphragm over the fundus of the stomach, spleen and left colic flexure; apex: left subclavian vessels and brachial plexus.

Function

Gas exchange (≈45% of total).
Metabolic and filtering roles of the pulmonary circulation.

Clinical significance

Cardiac notch and pericardiocentesis

Because the lung is deficient over the heart at the cardiac notch, the pericardium can be approached at the left 5th–6th intercostal space near the sternum (or, more often, subxiphoid) without traversing lung.

Lingular pneumonia

Consolidation of the lingula obscures the left heart border on a PA radiograph (silhouette sign), whereas left lower lobe consolidation obscures the left hemidiaphragm.

Test yourself

A smoker has ptosis, miosis and facial anhidrosis on the right, with wasting of the small muscles of the right hand. An apical lung tumour has most likely invaded the:

Cervical sympathetic chain and lower trunk of the brachial plexus
Right phrenic nerve and superior vena cava
Right recurrent laryngeal nerve
Right vagus nerve and oesophagus

Answer: Cervical sympathetic chain and lower trunk of the brachial plexus

A Pancoast (superior sulcus) tumour at the lung apex invades the stellate ganglion/cervical sympathetic chain (Horner's syndrome: ptosis, miosis, anhidrosis) and the C8–T1 roots/lower trunk of the brachial plexus (intrinsic hand muscle weakness, ulnar-side pain).