Topics

Topic

design image
Left lung

The left lung is one of the two primary organs of respiration and is slightly smaller than the right lung due to the position of the heart. It is divided into two lobes and characterized by the presence of the cardiac notch and lingula, unique anatomical features not found on the right side. The left lung is responsible for gas exchange and contributes to ventilation, oxygenation, and metabolic functions such as filtering small emboli and participating in immunological processes.

It occupies the left pleural cavity and conforms to the shape of the thoracic cage while being molded around the heart, aortic arch, and esophagus.

Synonyms

  • Pulmo sinister

  • Left pulmonary organ

Lobes and Bronchopulmonary Segments

  • Superior lobe:

    • Apicoposterior segment

    • Anterior segment

    • Lingular segments: Superior lingular, Inferior lingular

  • Inferior lobe:

    • Superior (apical) segment

    • Anteromedial basal segment

    • Lateral basal segment

    • Posterior basal segment

Location and Structure

  • Occupies the left thoracic cavity.

  • Smaller and narrower than right lung due to cardiac impression.

  • Two lobes separated by the oblique fissure.

  • Cardiac notch: Indentation of the anteromedial surface.

  • Lingula: Tongue-like projection of the superior lobe functioning as a right middle-lobe analog.

  • Hilum: Contains left main bronchus, pulmonary artery branches, pulmonary veins, and lymphatic structures.

  • Pleural covering: Enclosed by visceral and parietal pleura.

Relations

  • Medially: Heart, pericardium, esophagus, descending thoracic aorta

  • Superiorly: Apex beneath the left subclavian artery

  • Anteriorly: Costal cartilages and sternum

  • Laterally: Ribs and intercostal muscles

  • Posteriorly: Vertebral column, sympathetic chain

  • Inferiorly: Diaphragm

Attachments

  • Root of lung via pulmonary ligament, bronchi, pulmonary vessels

  • Visceral pleura adheres directly to lung surface

  • Parietal pleura connects lung region to thoracic walls

Function

  • Gas exchange: oxygen uptake and carbon dioxide elimination

  • Ventilation and regulation of intrathoracic pressure

  • Immune defense via alveolar macrophages and lymphatics

  • Metabolic functions including ACE production and filtration of microemboli

  • Supports phonation by regulating airflow

Clinical Significance

  • The left lung’s smaller size and cardiac notch influence surgical approaches and imaging interpretation

  • Lingula may mimic a separate lobe on imaging

  • Bronchopulmonary segments important for segmentectomy and disease localization
    (Note: Pathology is excluded as requested.)

MRI Appearance

T1-weighted images:

  • Normal lung parenchyma: Very low signal due to low proton density and air content

  • Pulmonary vessels: Intermediate-to-bright signal (blood)

  • Heart and mediastinum: Intermediate soft-tissue signal

  • Pleura: Thin low-signal line

T2-weighted images:

  • Lung parenchyma: Predominantly low signal with fine reticular appearance

  • Vascular structures: High signal from flowing blood

  • Major fissures: Low-signal curvilinear lines

  • Pleural surfaces: Smooth, low-signal contour

STIR:

  • Lung parenchyma: Very low to intermediate signal

  • Pleura and fissures: Suppressed low signal

  • Thoracic wall and diaphragm: Higher soft-tissue signal relative to lung

T1 Fat-Saturated Post-Contrast:

  • Normal lung: No significant parenchymal enhancement

  • Pulmonary vessels: Bright homogeneous enhancement

  • Pleura: Thin, uniform enhancement pattern may be faintly visible

  • Mediastinal structures: Enhanced based on vascularity

CT Appearance

Non-Contrast CT:

  • Lung parenchyma: Low-density, air-filled with fine vascular markings

  • Vessels: Soft-tissue attenuation branching from the hilum

  • Fissures: Thin, smooth lines separating lobes

  • Bronchi: Air-filled with thin soft-tissue walls

  • Pleura: Thin, barely perceptible line

Post-Contrast CT:

  • Pulmonary vessels: Bright, well-opacified arteries and veins showing clear branching pattern

  • Bronchial walls: Soft-tissue attenuation without enhancement of lumen

  • Pleura: Very subtle enhancement may be visible

  • Mediastinal boundaries: Sharp enhancement of adjacent vascular structures improves delineation of lung margins

MRI image

Left lung MRI coronal image

CT image

Left lung