Lymphatic Drainage of the Lung and Applied Anatomy

Introduction

Anatomy of the Pulmonary Lymphatic System

Two Plexuses (Dual Lymphatic Drainage)

• Located in the connective tissue of the visceral pleura (beneath the mesothelium)
• Drains the surface of the lung
• Vessels travel along the pleural surface toward the hilum
• Collecting lymphatics in the lung periphery drain toward the visceral pleura but do NOT connect to the pleural space

• Drains the lung parenchyma (interstitium, bronchovascular bundles, interlobular septa)
• Follows air passages (bronchi) and pulmonary blood vessels centrally toward the hilum
• Lymph nodes are found along the course of larger lymphatic vessels
• The vast majority of intralobular lymphatics are associated with blood vessels or located in bronchovascular bundles
• There are very few lymphatics alongside alveoli in the normal lung

Microstructure of Lymphatic Vessels

• Initial lymphatic capillaries: small, thin-walled, blind-ended structures with discontinuous "button" junctions between adjacent endothelial cells - these flap-like openings allow fluid and cells to enter
• Collecting lymphatics: larger, contain valves and zipper-like continuous junctions; in the lung they have only sporadic smooth muscle cell coverage (unlike elsewhere in the body), so they cannot contract robustly - lung lymph flow is driven primarily by extrinsic forces (changes in thoracic pressure with ventilation)

Regional Lymph Nodes (TNM Classification)

N1 Nodes (Pulmonary / Ipsilateral Hilar)

• The interlobar nodes in the depths of the interlobar fissure form a "sump" that receives drainage from ALL lobes of the corresponding lung
• On the right: nodes lie around the bronchus intermedius (bounded above by the right upper lobe bronchus, below by the middle and superior segmental bronchi)
• On the left: nodes lie in the angle between the lingular and lower lobe bronchi
• These nodes are in close proximity to pulmonary arterial branches - must be carefully dissected during lobectomy

N2 Nodes (Ipsilateral Mediastinal)

N3 Nodes (Contralateral / Supraclavicular)

• Contralateral mediastinal or hilar nodes
• Ipsilateral or contralateral scalene/supraclavicular nodes

Final Efferent Drainage

• Right lung lymph drains via the right lymphatic trunk into the right venous angle (junction of right internal jugular and subclavian veins)
• Left lung lymph drains into the thoracic duct, which empties into the left venous angle

Direction of Lymphatic Flow (Clinically Important Asymmetry)

Applied Anatomy

1. Lung Cancer Staging (N-Descriptor)

• N0: No regional node involvement
• N1: Metastasis to ipsilateral intrapulmonary, lobar, interlobar, or hilar nodes (stations 10-14)
• N2: Metastasis to ipsilateral mediastinal or subcarinal nodes (stations 1-9)
• N3: Contralateral mediastinal/hilar nodes; ipsilateral or contralateral scalene/supraclavicular nodes

2. Pulmonary Edema

• At baseline (normal slow entry rate), lymphatics handle this drainage adequately
• In pulmonary edema (raised hydrostatic pressure - e.g., heart failure, ARDS), lymphatic flow increases substantially
• When lymphatic capacity is overwhelmed or impaired, interstitial and then alveolar edema follows
• The pulmonary lymphatics are most important in drainage during injury and inflammation, not at steady state

3. Pleural Effusion

• Parietal pleural lymphatics drain the pleural fluid (not the visceral pleural lymphatics, which do not connect to the pleural space)
• At baseline, limited impairment of pleural lymphatic drainage has little consequence
• When fluid entry into the pleural space increases (e.g., heart failure, malignancy, infection), any limitation in lymphatic drainage contributes directly to pleural effusion formation
• Malignant pleural effusion: Lymph node obstruction by metastatic tumor blocks efferent lymphatic channels, raising pressure and promoting fluid accumulation

4. Chylothorax

• Results from disruption or obstruction of the thoracic duct or its major tributaries
• Causes: iatrogenic injury (esophagectomy, thoracic surgery), blunt trauma, malignancy (especially lymphoma), granulomatous disease (sarcoidosis), lymphangioleiomyomatosis
• Presents as milky pleural effusion on resumption of oral intake
• Diagnosis: pleural fluid triglycerides >110 mg/dL, lymphocyte predominance
• Management: chest drain for lung re-expansion, nil by mouth, somatostatin analogs; if high output persists (>50 mL/day), surgical ligation of the thoracic duct

5. Primary Tuberculosis (Ghon Complex)

• Why hilar lymphadenopathy is a hallmark of primary TB
• How mediastinal compression (of airways, veins, esophagus) can occur from massive adenopathy
• Why the infection can spread via lymphatics to other organs (miliary TB) when lymph node barriers are breached

6. Immune Surveillance and Antigen Trafficking

• The lung is constantly exposed to environmental antigens and pathogens
• Antigen-presenting cells (dendritic cells, macrophages) sample inhaled particles and migrate via the lymphatics to mediastinal lymph nodes, where adaptive immune responses are initiated
• This migration is mediated by CCL21 (on lymphatic endothelium) and its receptor CCR7 (on T cells and dendritic cells)
• Migration from airways to mediastinal nodes takes ~1-2 days
• This lymphatic route is the basis of T-cell sensitization in tuberculosis, sarcoidosis, and hypersensitivity pneumonitis

7. Sarcoidosis

• Characterized by non-caseating granuloma formation in hilar and mediastinal lymph nodes
• Bilateral hilar lymphadenopathy (BHL) - the classic radiological finding - reflects sarcoid involvement of the lymph nodes that are the primary drainage sites for lung lymph
• Increased FDG uptake on PET in mediastinal and hilar lymph nodes is characteristic

8. Lymphangitis Carcinomatosa

• Malignant cells spread via lymphatics into the peribronchial and perivascular lymphatics throughout the lung
• Produces characteristic radiological pattern of bilateral linear ("Kerley B") lines and ground-glass opacification
• Can occur with any adenocarcinoma but is classic with breast, stomach, and lung primaries
• Severely impairs lymphatic drainage capacity, contributing to interstitial edema and breathlessness

9. Surgical Relevance (Pneumonectomy / Lobectomy)

• Complete mediastinal lymph node dissection (MLND) is performed at lung resection for cancer
• The Lymphatic Sump of Borrie must be dissected to expose lobar and segmental pulmonary artery branches
• Cross-drainage from the left lower lobe to right-sided mediastinal nodes means left lower lobe tumors can be understaged if only ipsilateral mediastinal sampling is performed

Summary Table: Key Lymph Node Groups

Sources

• Schwartz's Principles of Surgery, 11th ed. (Lymphatic Drainage section, pp. 694-695)
• Murray & Nadel's Textbook of Respiratory Medicine (Chapter 7: Pulmonary Lymphatics)
• Harrison's Principles of Internal Medicine, 22nd ed. (Anatomic Staging of Lung Cancer)
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology (Lymphatic Vessels of the Lung, p. 1817)
• Schwartz's Principles of Surgery (Chylothorax management, p. 3613)