Lymphatic Drainage of the Lung and Applied Anatomy

1. Introduction

2. Structural Anatomy of Pulmonary Lymphatics

A. Two Plexuses

• Located beneath the visceral pleura
• Forms a continuous network over the surface of the lung
• Drains toward the hilum along the pleural surface

• Runs alongside bronchi, bronchioles, and pulmonary arteries and veins within the bronchovascular bundles
• Begins as blind-ended initial lymphatic capillaries at the level of the respiratory bronchioles
• Drains centrally toward the hilum

"Within the human lung, the vast majority of lymphatic vessels are located either in the interlobular septa or subpleural space rather than within the lobule. Among intralobular lymphatics, nearly all are associated with a blood vessel or in a bronchovascular bundle." - Murray & Nadel, p. 175

B. Vessel Structure: Initial vs. Collecting Lymphatics

3. Lymph Node Stations - The Naruke Map

N1 Nodes (Intrapulmonary/Hilar - within pleural sac, stations 10+)

1. Intrapulmonary (segmental) nodes - at bifurcations of segmental bronchi and pulmonary artery
2. Lobar nodes - along upper, middle, and lower lobe bronchi
3. Interlobar nodes - in the angles formed by main bronchi bifurcating into lobar bronchi
4. Hilar nodes - along the main bronchi

"The interlobar lymph nodes lie in the depths of the interlobar fissure on each side and constitute a lymphatic sump for each lung, referred to as the lymphatic sump of Borrie; all of the pulmonary lobes of the corresponding lung drain into this group of nodes." - Schwartz's/Fischer's, p. 695

• Right side: The sump lies around the bronchus intermedius (bounded above by the right upper lobe bronchus, below by the middle lobe and superior segmental bronchi)
• Left side: Confined to the interlobar fissure, in the angle between the lingular and lower lobe bronchi

N2 Nodes (Mediastinal - true mediastinal, stations 1-9)

1. Anterior mediastinal nodes - along upper pericardium, phrenic nerves, ligamentum arteriosum, left innominate vein
2. Posterior mediastinal nodes - paraesophageal nodes within the inferior pulmonary ligament and between esophagus and trachea near the arch of the azygos vein
3. Tracheobronchial nodes - three subgroups near the tracheal bifurcation:
   • Subcarinal nodes (in the obtuse angle between trachea and each main stem bronchus)
   • Nodes anterior to the lower trachea
4. Paratracheal nodes - in the superior mediastinum alongside the trachea; on the right they form a chain with tracheobronchial nodes inferiorly and scalene (deep cervical) nodes above

"The mediastinal lymphatic system is quite complex and variable. Mediastinal lymph nodes are interconnected; thus, involvement of one group of lymph nodes in a pathologic process frequently leads to involvement of other groups." - Fishman's, p. 1431

4. Flow Pathway - From Alveoli to Systemic Veins

Alveolar/interstitial fluid
        ↓
Initial lymphatic capillaries (peribronchovascular, interlobular septa, subpleural)
        ↓
Collecting lymphatics (with valves) coursing in septal structures, pleura, peribronchial and perivascular sheaths
        ↓
Lymph nodes intercalated along the course (segmental → lobar → interlobar → hilar)
        ↓
Mediastinal lymph nodes (subcarinal → paratracheal)
        ↓
Right mediastinal lymphatic duct → Right subclavian vein
Left mediastinal lymphatics + Thoracic duct → Left subclavian vein

5. Laterality of Drainage - Clinically Critical

"In contrast, in the left lung, particularly the left lower lobe, lymphatic drainage occurs with equal frequency to ipsilateral and contralateral superior mediastinal nodes." - Schwartz's Surgery, p. 695

6. Functions of Pulmonary Lymphatics

A. Fluid Homeostasis and Edema Prevention

• Pulmonary lymph flow increases in settings of injury and inflammation
• Impairment of lymphatic flow predisposes to pulmonary edema particularly during injury (not necessarily at baseline)
• This is analogous to parietal pleural lymphatics: impaired drainage contributes to pleural effusions when fluid entry increases

B. Macromolecule Clearance

C. Immune Cell Trafficking

• At steady state in healthy lungs, antigen-presenting cells (dendritic cells, macrophages) migrate from the airways to mediastinal lymph nodes via lymphatics in 1-2 days
• This migration is driven by the CCL21/CCR7 chemokine axis:
  • CCL21 is expressed on lymphatic endothelium
  • CCR7 is expressed on dendritic cells and T cells
• Additional signals: S1P, ICAM-1, and VCAM-1 on lymphatic endothelium
• Migration ensures antigen presentation for adaptive immune responses or tolerance induction

D. Defense Role (Fishman's perspective)

• Lymph nodes along the bronchi filter foreign particles, microorganisms, and pigment-laden macrophages
• Peripheral nodes are tiny (1-2 mm); hilar nodes reach 5-10 mm near the tracheal bifurcation
• Carbon pigment from inhaled smoke/dust is deposited in the medullary cords, protecting the blood from dissemination of indigestible matter and infectious agents
• Nodes arranged along major bronchi extend to subsegmental bronchi ~5 mm in diameter

7. Applied Anatomy - Clinical Correlations

A. Lung Cancer Staging (Most Important Application)

• The lymphatic sump of Borrie (interlobar nodes) is the first node group to be involved in all lobe cancers - making it the primary target for intraoperative sentinel node assessment
• N2 nodes at station 7 (subcarinal) are accessible by bronchoscopic EBUS (endobronchial ultrasound), and by mediastinoscopy at stations 2, 4, 7

B. Pulmonary Edema

C. Malignant Pleural Effusion

"Tumors can infiltrate this lymphatic system anywhere between the parietal pleura and mediastinal lymph nodes, resulting in the development of a pleural effusion." - Fishman's, p. block16

D. Lymphangitis Carcinomatosa

E. Mediastinal Disease Patterns and Lymph Node Mapping

• Lung carcinoma - ipsilateral hilar + subcarinal + paratracheal nodes predominantly
• Head and neck squamous carcinoma - supraclavicular nodes more often
• Sarcoidosis - bilateral hilar lymphadenopathy (BHL) due to involvement of the hilar nodes
• Tuberculosis/histoplasmosis - right paratracheal involvement common; massive hilar/mediastinal nodes can cause airway compression
• Lymphoma - anterior mediastinal predominance (Hodgkin's)

F. Post-Transplant Implications

1. Impaired clearance of interstitial fluid → early pulmonary edema
2. Impaired immune cell trafficking → altered susceptibility to infection and rejection

G. Lymphangiectasia and Lymphangiomatosis

H. iBALT in Chronic Lung Disease

8. Key Points Summary for MD Exam

1. Two plexuses: superficial (subpleural) and deep (peribronchial/perivascular)
2. No alveolar lymphatics in normal lung - initial lymphatics begin at alveolar ducts/respiratory bronchioles
3. Initial lymphatics have button junctions (discontinuous); collecting lymphatics have zipper junctions (continuous)
4. Pulmonary collecting lymphatics lack adequate smooth muscle - rely on thoracic pressure changes (breathing) for propulsion
5. Lymphatic sump of Borrie = interlobar nodes that receive drainage from ALL lobes of that lung
6. Right lung drains ipsilaterally; left lower lobe drains equally ipsi- and contralaterally
7. Final drainage: right → right subclavian vein; left + thoracic duct → left subclavian vein
8. Kerley B lines = distended interlobular septal lymphatics in pulmonary edema
9. N1 nodes (within pleural sac) vs. N2 nodes (true mediastinal) - Naruke map
10. CCL21/CCR7 axis drives antigen-presenting cell trafficking to mediastinal lymph nodes

• Murray & Nadel's Textbook of Respiratory Medicine, 7e - Chapter 7: Pulmonary Lymphatics (pp. 175-183)
• Fishman's Pulmonary Diseases and Disorders, 5e - Chapter 2: Structural Aspects of the Defense System (p. 70); Chapter 80: Mediastinal Disease - Lymphatics (p. 1431)
• Schwartz's Principles of Surgery, 11e - Chapter 19: Thoracic Surgery - Lymph Node Anatomy (pp. 694-695)