The Common Vein

Segment of Lung

• Etymology:
  • The term “segment” originates from the Latin word “segmentum,” meaning “a piece or section.”
• AKA and Abbreviation:
  • No widely used abbreviations or alternative names.
• What is it:
  • A lung segment is a subdivision of a lung lobe, separated by connective tissue septa, and served by its own segmental bronchus and accompanying segmental artery.
  •  
• Principles:
  • Parts:
    • Segmental bronchus (tertiary bronchus).
    • Segmental pulmonary artery.
    • Intersegmental veins, which drain blood to the pulmonary veins.
    • Lymphatic vessels within the connective tissue septa.
  • Capsule:
    • While not encapsulated, lung segments are separated by connective tissue septa that define their boundaries.
  • Size:
    • Varies depending on location; upper lobe segments are smaller, and lower lobe segments are larger.
  • Shape:
    • Typically wedge-shaped, with the apex pointing toward the hilum and the base toward the pleural surface.
  • Position:
    • Organized anatomically within each lobe:
      • Right Lung:
        • Above and Abutting the Minor Fissure:
        • Anterior segments of the upper lobe.
        • Below and Abutting the Minor Fissure: Lateral and medial segments of the middle lobe.
        • Above and Abutting the Major Fissure: Superior segment of the lower lobe.
        • Below the Major Fissure: Medial basal, anterior basal, lateral basal, and posterior basal segments.
      • Left Lung:
        • Above and Abutting the Oblique Fissure: Apicoposterior and anterior segments of the upper lobe.
        • Abutting the Oblique Fissure (Lingula): Superior and inferior lingular segments.
        • Below and Abutting the Oblique Fissure: Superior segment of the lower lobe.
        • Below the Oblique Fissure: Anteromedial basal, lateral basal, and posterior basal segments.
  • Character:
    • Pinkish color and spongy texture in the fresh state.
    • Highly vascularized and aerated.
  • Time:
    • Segmental changes can be acute (e.g., segmental pneumonia) or chronic (e.g., bronchiectasis).
  • Blood Supply:
    • Supplied by a segmental pulmonary artery and bronchial arteries.
  • Venous Drainage:
    • Drained by intersegmental veins into the pulmonary veins.
  • Lymphatic Drainage:
    • Lymphatics drain toward hilar and mediastinal nodes.
  • Nerve Supply:
    • Autonomic innervation via the pulmonary plexus.
    • Parasympathetic fibers from the vagus nerve and sympathetic fibers from thoracic ganglia.
  • Embryology:
    • Develops during the pseudoglandular stage (5-17 weeks of gestation).
  • Histology:
    • Composed of alveoli, bronchioles, and associated blood vessels, supported by a fine connective tissue framework.
  • Physiology and Pathophysiology:
    • Physiologically, segments enable efficient gas exchange.
    • Pathophysiological processes often localize to specific segments, such as infection or obstruction.
• Applied Anatomy to Radiology:
  • CXR:
    • Lung segments are inferred based on their relationship to the horizontal and oblique fissures, as well as adjacent thoracic structures (e.g., heart borders, diaphragm).
    • Segmental pathology may appear as areas of consolidation, atelectasis, or hyperlucency.
  • CT:
    • Provides detailed visualization of segmental anatomy and pathology.
    • Useful for identifying segmental consolidations, nodules, or air trapping.
  • MRI:
    • Limited utility for segmental evaluation but can assess associated mediastinal structures or vascular abnormalities.
• Pathological Implications:
  • Segment-specific diseases include:
    • Segmental Atelectasis: Collapse due to airway obstruction.
    • Segmental Pneumonia: Infection confined to a single segment.
    • Bronchiectasis: Chronic dilation often affecting lobe segments.
    • Pulmonary Embolism: Segmental perfusion defects visualized on CT pulmonary angiography.
• Key Points and Pearls:
  • Understanding segmental anatomy is essential radiological interpretation and for surgical planning (e.g., segmentectomy)
  • Intersegmental veins are important landmarks for defining segmental boundaries during surgery.
  • Accurate localization of pathology aids in differential diagnosis and targeted intervention.
• Parallels with Human Endeavors:
  • Lung segments mirror the modular design of cities, with each segment functioning like a district that operates independently while contributing to the whole.
  • The segmentation reflects principles of compartmentalization and efficiency, seen in engineering, architecture, and urban planning.
  • Cubism and Triangularity: The geometric structure of lung segments resonates with cubist art, where shapes are broken down and reassembled into functional compositions.
  • The Music of Breath: Breath sounds, including harsh tones of cough and subtle whispered pectoriloquy, create a rhythmic interplay of health and disease, echoing the complexity and harmony of music.

A lung segment is a subdivision of a lung lobe, defined by the
segmental bronchi and the corresponding segmental arteries. Each
lung is divided into lobes (three in the right lung and two in the left),
and these lobes are further segmented into bronchopulmonary
segments. There are 10 segments in the right lung and 8-10
segments in the left lung, each with its own airway (bronchus),
blood supply (artery), and lymphatic drainage, separated from
adjacent segments by connective tissue. Understanding lung
segments is crucial in clinical practice, especially in surgical
procedures (like segmentectomy) and bronchoscopy, as well as in
diagnosing and treating localized lung diseases such as pneumonia,
tumors, or bronchiectasis. Imaging studies, such as CT scans, often
reference specific segments to accurately describe the location of
abnormalities, aiding in precise diagnosis and treatment planning.