Topographic anatomy of respiratory system with diagrams in 9 pages in text format only

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TOPOGRAPHIC ANATOMY OF THE RESPIRATORY SYSTEM

A Comprehensive 9-Page Reference

PAGE 1 — OVERVIEW & BOUNDARIES OF THE RESPIRATORY SYSTEM

Introduction

The respiratory system extends from the external nares (nostrils) to the microscopic alveoli. Topographic anatomy describes each component's precise position relative to external landmarks, adjacent structures, bones, and viscera — knowledge essential for clinical examination, imaging interpretation, surgical planning, and airway management.

The respiratory system is divided into two major functional regions:

Upper respiratory tract: Nose → Nasal cavity → Pharynx → Larynx → Upper trachea
Lower respiratory tract: Lower trachea → Bronchi → Bronchioles → Alveoli

The entire system is housed within the head, neck, and thorax. The thorax itself is defined as the space bounded:

Superiorly by the thoracic inlet
Inferiorly by the diaphragm
Laterally and posteriorly by the ribs, sternum, and vertebrae

ASCII Diagram 1 — Sagittal Overview of the Respiratory Tract

         NASAL CAVITY
        /  (above hard palate)
NARES --
        \
         ORAL CAVITY
              |
          NASOPHARYNX
              |
          OROPHARYNX  ─── Waldeyer's Ring (tonsils)
              |
          HYPOPHARYNX / LARYNGOPHARYNX
              |
           LARYNX  (C3–C6)
              |
           TRACHEA  (C6 → T4/T5)
              |
           CARINA  (T4/T5, angle of Louis)
            /   \
    L. MAIN Bx   R. MAIN Bx
        |              |
   Lobar Bx        Lobar Bx
        |              |
  Segmental Bx   Segmental Bx
        |              |
   Bronchioles     Bronchioles
        |              |
    Alveolar ducts → ALVEOLI

Key vertebral landmarks (anterior equivalent in brackets):

Structure	Vertebral Level	Surface Landmark
Cricoid cartilage	C6	Lower neck, palpable
Tracheal start	C6	Just below cricoid
Thoracic inlet	T1	Suprasternal notch
Carina	T4–T5	Angle of Louis (sternomanubrial joint)
Lung apex	2–3 cm above clavicle	Above clavicle
Diaphragm (full expiration)	T8–T9 (right), T9 (left)	Varies with respiration

Sources: Sabiston Textbook of Surgery; Morgan & Mikhail's Clinical Anesthesiology, 7e

PAGE 2 — THE NOSE AND NASAL CAVITY

External Nose

The external nose projects from the face and is supported by:

Bony framework: Nasal bones (upper third), frontal process of maxilla
Cartilaginous framework: Upper lateral (triangular) cartilages, lower lateral (alar) cartilages, and the septal cartilage
Skin: Thin over the dorsum, thicker and sebaceous at the tip (ala)

The tip (apex) lies at the level of the columella; the root (radix) joins the glabella.

Nasal Cavity

Each nasal cavity extends from the vestibule (just inside the nostril) to the choana posteriorly, where it opens into the nasopharynx. The cavities are separated by the nasal septum (composed of the perpendicular plate of the ethmoid, the vomer, and the septal cartilage).

The lateral wall of each nasal cavity bears three turbinates (conchae):

Turbinate	Position	Function
Superior	Posterior upper wall	Olfactory epithelium
Middle	Central	Drainage of frontal, maxillary, anterior ethmoid sinuses
Inferior	Lowest, largest	Air warming, humidifying; drainage of nasolacrimal duct

The meatus beneath each turbinate receives specific drainage:

Superior meatus: Posterior ethmoid sinuses
Middle meatus: Frontal sinus (frontonasal recess), anterior ethmoid, maxillary sinus (ostium)
Inferior meatus: Nasolacrimal duct

Blood Supply of the Nasal Cavity (Kiesselbach's / Little's Area)

The anterior nasal septum (Kiesselbach's area) is the most common site of epistaxis, supplied by anastomoses of:

Anterior ethmoidal artery (ophthalmic → ICA)
Sphenopalatine artery (maxillary → ECA)
Greater palatine artery (descending palatine → ECA)
Superior labial artery (facial → ECA)

ASCII Diagram 2 — Lateral Wall of the Nasal Cavity

  FRONTAL SINUS ─── drains into middle meatus (frontonasal recess)
         |
  ┌──────────────────────────────────────┐
  │  SUPERIOR TURBINATE  ─ olfactory    │
  │  ─────────────────────────────────  │
  │  MIDDLE TURBINATE    ─ sinuses      │
  │  ─────────────────────────────────  │
  │  INFERIOR TURBINATE  ─ nasolacrimal │
  └──────────────────────────────────────┘
         │
    CHOANA → NASOPHARYNX
         │
    Eustachian tube orifice (lateral wall)
    Pharyngeal tonsil (adenoid) — posterior wall

PAGE 3 — THE PHARYNX

Divisions and Topography

The pharynx is a musculomembranous tube approximately 12–14 cm in length, extending from the base of the skull to the level of C6, where it becomes the esophagus. It is divided into three parts:

1. Nasopharynx

Extent: From the base of the skull (superior) to the soft palate (inferior)
Vertebral levels: C1–C2
Important structures:
- Pharyngeal tonsil (adenoids) on the posterior wall
- Eustachian (pharyngotympanic) tube orifices on the lateral walls at the level of the inferior turbinate
- Torus tubarius (cartilaginous elevation around Eustachian tube opening)
- Pharyngeal recess (fossa of Rosenmüller) — posterior to the torus tubarius; common site of nasopharyngeal carcinoma

2. Oropharynx

Extent: From the soft palate (superior) to the tip of the epiglottis (inferior); anterior boundary is the anterior tonsillar pillar
Vertebral levels: C2–C3
Important structures:
- Palatine tonsils (between anterior and posterior tonsillar pillars)
- Base of tongue (lingual tonsil)
- Soft palate and uvula
- Posterior oropharyngeal wall
- Vallecula: depression between the lingual surface of the epiglottis and the base of tongue — key landmark for Macintosh laryngoscope blade placement

3. Hypopharynx (Laryngopharynx)

Extent: From the tip of the epiglottis to the lower border of the cricoid cartilage (C6), where it transitions to the esophagus
Vertebral levels: C3–C6
Important structures:
- Pyriform sinuses (fossae): bilateral recesses lateral to the larynx, anterior and medial walls of which are the aryepiglottic folds; frequent site of foreign body lodgment and hypopharyngeal carcinoma
- Postcricoid area: posterior to the cricoid; marks the entry of the esophagus
- Posterior hypopharyngeal wall: continues from the oropharyngeal posterior wall

ASCII Diagram 3 — Pharynx (Posterior View / Sagittal)

  BASE OF SKULL
       |
  ─────────────────────────────
  NASOPHARYNX  (C1–C2)
  • Adenoids (posterior)
  • Eustachian tube orifice
  ─────────────────────────────
  Soft palate
  ─────────────────────────────
  OROPHARYNX  (C2–C3)
  • Palatine tonsils (lateral)
  • Vallecula (anterior)
  • Base of tongue (anterior)
  ─────────────────────────────
  Epiglottis tip
  ─────────────────────────────
  HYPOPHARYNX  (C3–C6)
  • Pyriform sinuses (bilateral)
  • Postcricoid area
  ─────────────────────────────
  Cricoid cartilage (C6)
       |
  ESOPHAGUS / TRACHEA

Source: Fishman's Pulmonary Diseases and Disorders, 2-Volume Set

PAGE 4 — THE LARYNX

Topography and Bony Landmarks

The larynx occupies the anterior midline of the neck at C3–C6. It is continuous superiorly with the hypopharynx and inferiorly with the trachea at the inferior border of the cricoid cartilage. In males it is larger and more prominent (Adam's apple).

Landmark	Vertebral Level
Tip of epiglottis	C3
Hyoid bone	C3
Thyroid notch	C4
Thyroid cartilage	C4–C5
Cricothyroid membrane	C5 (lower thyroid to cricoid)
Cricoid cartilage	C6
Trachea begins	C6

Cartilaginous Skeleton

The larynx is supported by nine cartilages, three unpaired and three paired:

Unpaired: Thyroid, Cricoid (only complete ring), Epiglottis
Paired: Arytenoid, Corniculate (Santorini), Cuneiform (Wrisberg)

Subdivisions (Endoscopic Relevance)

Region	Boundaries	Contents
Supraglottis	Tip of epiglottis → superior surface of vocal folds	Epiglottis, aryepiglottic folds, false vocal cords (vestibular folds), arytenoids
Glottis	True vocal cords + rima glottidis	True vocal cords (vocalis/thyroarytenoid muscle + vocal ligament)
Subglottis	Inferior surface of true cords → lower border of cricoid	Subglottic mucosa; transitions to trachea

The true vocal cords consist of the vocal ligament (free edge of the conus elasticus) covered by stratified squamous epithelium. The glottic aperture (rima glottidis) is the narrowest part of the adult larynx; in children the subglottis (at the cricoid) is the narrowest point.

Cricothyroid Membrane — Clinical Key

The cricothyroid membrane connects the thyroid cartilage (superior) to the cricoid cartilage (inferior). It is the preferred site for emergency surgical airway access (cricothyrotomy) because it is:

Avascular in its midline (central third)
Easily palpable
Directly beneath skin (no major intervening structure)

ASCII Diagram 4 — Larynx (Anterior & Endoscopic Views)

  ANTERIOR VIEW:                ENDOSCOPIC VIEW (from above):
                                
  Hyoid bone ─────── H           Epiglottis (anterior)
       |                              |
  Thyrohyoid mb.                    AE fold    AE fold
       |                           /               \
  Thyroid cart. ──── T        False cord         False cord
       |                              \           /
  Cricothyroid mb. ─ CTM          True cord ─── True cord
       |                              |           |
  Cricoid cart. ──── C           GLOTTIS (rima glottidis)
       |                              |
  TRACHEA                        Subglottis ↓ Trachea

  H = Hyoid; T = Thyroid cartilage; CTM = Cricothyroid membrane; C = Cricoid
  AE fold = Aryepiglottic fold

Source: Fishman's Pulmonary Diseases and Disorders; Morgan & Mikhail's Clinical Anesthesiology, 7e

PAGE 5 — THE TRACHEA

Topography

The trachea is a semi-flexible midline tube extending from the lower border of the cricoid cartilage at C6 to the carina at T4–T5 (corresponding anteriorly to the angle of Louis / sternomanubrial joint).

Dimensions:

Length: 10–13 cm in adults (Morgan & Mikhail: 11–13 cm; Sabiston: 10–13 cm)
Diameter: 1.5–2.0 cm

The trachea is divided into:

Cervical trachea: C6–T1 (approximately 5–6 rings); accessible surgically anterior to the neck
Thoracic trachea: T1–T4/T5; passes behind the manubrium and great vessels

Wall Structure

The tracheal wall is composed of:

15–20 incomplete C-shaped hyaline cartilage rings: form the anterior and lateral walls, maintain patency
Membranous (posterior) wall: trachealis smooth muscle + fibroelastic connective tissue; lies directly anterior to the esophagus
The cricoid cartilage is the only complete ring, marking the laryngo-tracheal junction

Relations

Surface	Structures
Anterior (cervical)	Skin, fascia, isthmus of thyroid gland (rings 2–4), strap muscles
Anterior (thoracic)	Manubrium, great vessels (innominate artery, left CCA, aortic arch)
Posterior	Esophagus (throughout), recurrent laryngeal nerves in tracheoesophageal groove
Right lateral	Azygos vein arch, right vagus nerve, right lung
Left lateral	Aortic arch, left recurrent laryngeal nerve (loops under it), left lung
Superior	Larynx (cricoid)

The Carina

The bifurcation of the trachea into the two main bronchi is called the carina
Located at T4–T5 posteriorly, behind the sternomanubrial joint (angle of Louis) anteriorly
Carinal angle is normally less than 70° (widening suggests left atrial enlargement on CXR)

ASCII Diagram 5 — Trachea and Relations (Cross-section at Thoracic Level)

                    ANTERIOR
          ┌───────────────────────────┐
          │   Manubrium / great vessels│
          └──────────┬────────────────┘
                     │
        ┌────────────▼──────────────┐
        │    TRACHEA                │
        │  [──────────────]         │  ← cartilage rings (anterior)
        │  [  lumen ~1.8cm]         │
        │  [______________]         │  ← membranous wall (posterior)
        └────────────┬──────────────┘
                     │
          ┌──────────▼───────────────┐
          │    ESOPHAGUS             │
          └──────────────────────────┘
  L. recurrent          R. recurrent
  laryngeal nerve       laryngeal nerve
  (in groove)           (in groove)
  
           LEFT                   RIGHT
  Aortic arch          Azygos vein (arch)
  L. vagus             R. vagus

Source: Sabiston Textbook of Surgery; Morgan & Mikhail's Clinical Anesthesiology, 7e

PAGE 6 — THE BRONCHI AND BRONCHOPULMONARY SEGMENTS

Main (Primary) Bronchi

At the carina (T4/T5), the trachea divides into the right and left main (primary) bronchi. They differ significantly:

Feature	Right Main Bronchus	Left Main Bronchus
Angle from trachea	~25° (less acute, more vertical)	~45° (more horizontal)
Length	~2.5 cm (short)	~5 cm (longer)
Diameter	Larger	Smaller
Clinical implication	Foreign bodies preferentially enter right side; inadvertent intubation of right bronchus common	Protected by greater angle; longer allows more room before obstruction

Consequence: Foreign bodies and malpositioned endotracheal tubes most commonly enter the right main bronchus due to its larger diameter and less acute angle.

Lobar (Secondary) Bronchi

Each main bronchus divides into lobar bronchi:

Right lung: 3 lobar bronchi → upper, middle, lower lobes
- Right upper lobe bronchus arises 1–2.5 cm from the carina
Left lung: 2 lobar bronchi → upper and lower lobes (no middle lobe; lingula is part of upper lobe)

Bronchopulmonary Segments (Tertiary Bronchi)

Each lobar bronchus divides into segmental (tertiary) bronchi, each supplying an independent bronchopulmonary segment — the smallest resectable unit of lung.

Lobe	Segments	Number
Right Upper	Apical (S1), Posterior (S2), Anterior (S3)	3
Right Middle	Lateral (S4), Medial (S5)	2
Right Lower	Superior (S6), Medial basal (S7), Anterior basal (S8), Lateral basal (S9), Posterior basal (S10)	5
Left Upper	Apicoposterior (S1+2), Anterior (S3), Superior lingular (S4), Inferior lingular (S5)	4
Left Lower	Superior (S6), Anteromedial basal (S7+8), Lateral basal (S9), Posterior basal (S10)	4

Total: 10 segments per lung (18 segments total; left has some fused = typically 8–10 named segments)

ASCII Diagram 6 — Bronchial Tree (Anterior View, Simplified)

                     TRACHEA
                        |
            ────────────|────────────
            |           CARINA      |
     R. Main Bx                L. Main Bx
     (2.5 cm, 25°)            (5 cm, 45°)
       /    |    \                /      \
  RUL  RML  RLL            LUL          LLL
   |    |    |               |            |
  S1  S4  S6             S1+2          S6
  S2  S5  S7              S3           S7+8
  S3      S8              S4           S9
          S9              S5           S10
          S10

  RUL = Right Upper Lobe    LUL = Left Upper Lobe
  RML = Right Middle Lobe   LLL = Left Lower Lobe
  RLL = Right Lower Lobe
  S = Segment number (Jackson-Huber nomenclature)

Source: Morgan & Mikhail's Clinical Anesthesiology, 7e

PAGE 7 — THE LUNGS

Gross Anatomy and Topography

The lungs are paired, spongy, cone-shaped organs occupying the pleural cavities on either side of the mediastinum. Each lung has:

An apex (summit) projecting 2–3 cm above the clavicle into the root of the neck
A base (diaphragmatic surface) resting on the diaphragm
Costal surface (lateral): relates to the ribs and intercostal spaces
Mediastinal surface (medial): concave, contains the hilum
Hilum: where bronchi, pulmonary vessels, lymphatics, and nerves enter/exit

Lobes and Fissures

Feature	Right Lung	Left Lung
Lobes	3 (Upper, Middle, Lower)	2 (Upper, Lower)
Fissures	2 (Oblique + Horizontal)	1 (Oblique only)
Oblique fissure	Runs from T4/T5 posteriorly → 6th rib costal cartilage anteriorly	Same course
Horizontal (transverse) fissure	Runs at the level of the 4th costal cartilage (anterior) → 5th rib (lateral)	Absent

Surface Projections of Lung Borders

These projections are critical for chest examination:

Border	Right	Left
Apex	2–3 cm above clavicle	2–3 cm above clavicle
Anterior border	Behind sternum → 6th costal cartilage	Behind sternum to 4th costal cartilage, then deviates left (cardiac notch)
Inferior border	6th rib (MCL) → 8th rib (MAL) → 10th rib (paravertebral)	Same (slightly lower on left)
Posterior border	Along vertebral column, T2–T10	Along vertebral column, T2–T10

Mnemonic for inferior lung border: 6–8–10 (6th rib at midclavicular line, 8th at midaxillary line, 10th posteriorly at paravertebral)

Hilum of the Lung

The hilum (root of the lung) lies on the mediastinal surface at the level of T5–T7 vertebrae (corresponds anteriorly to 2nd–4th intercostal spaces).

Structure	Right Hilum	Left Hilum
Pulmonary artery	Superior to bronchus	Superior to bronchus
Main bronchus	Posterior-inferior	Posterior-inferior
Pulmonary veins (2)	Anterior and inferior	Anterior and inferior
Bronchial arteries	Posterior (from aorta)	Posterior (from aorta)
Lymphatics	Hilar/bronchopulmonary nodes	Hilar/bronchopulmonary nodes

ASCII Diagram 7 — Lung Surface Projections (Anterior Chest Wall)

  ANTERIOR CHEST:
                      MANUBRIUM
  ┌──────────────────────────────────────────┐
  │   Right Lung         Left Lung           │
  │   Apex: above clavicle                   │
  │   Ant. border: behind sternum →          │
  │   Right: continues to 6th CC             │
  │   Left: deviates left at 4th CC          │
  │            (CARDIAC NOTCH)               │
  │                                          │
  │   Horizontal fissure:                    │
  │   4th CC → 5th rib (RIGHT ONLY)          │
  │                                          │
  │   Oblique fissure (both lungs):          │
  │   T4/T5 post → 6th rib ant              │
  │                                          │
  │   Inferior border:                       │
  │   MCL: rib 6 | MAL: rib 8 | Post: T10   │
  └──────────────────────────────────────────┘
  MCL = midclavicular line; MAL = midaxillary line

Source: Sabiston Textbook of Surgery; Fishman's Pulmonary Diseases

PAGE 8 — THE PLEURA, MEDIASTINUM, AND DIAPHRAGM

The Pleura

Each lung is enclosed by a double-layered serous membrane — the pleura:

Visceral pleura: Intimately covers all lung surfaces (including fissures); has no pain receptors; supplied by bronchial vessels
Parietal pleura: Lines the inner thoracic wall, diaphragm, and mediastinum; has pain receptors (somatic sensation); supplied by intercostal and phrenic vessels

Pleural cavity: The potential space between the two layers, normally containing 10–20 mL of serous fluid for lubrication.

Pleural recesses (sinuses) — spaces not normally occupied by lung:

Recess	Location	Clinical Significance
Costodiaphragmatic recess	Angle between costal and diaphragmatic pleura	Deepest recess; fluid accumulates here first; reached by needle at posterior chest below 9th rib
Costomediastinal recess	Anterior junction of costal and mediastinal pleura	Behind the sternum (cardiac notch area on left)
Vertebromediastinal recess	Posterior junction (right side)	Small; near azygos vein

Surface projections of the pleura (parietal):

Cupula (dome): Projects 2–3 cm above clavicle (same as lung apex) — at risk during subclavian/IJV cannulation and scalene blocks
Inferior reflection: 2 rib spaces below inferior lung border
- MCL: rib 8 (lung rib 6)
- MAL: rib 10 (lung rib 8)
- Paravertebral: rib 12 / T12 (lung T10)

The Mediastinum

The mediastinum is the central compartment of the thorax, between the two pleural sacs. It contains all thoracic viscera except the lungs.

Divisions (traditional four-compartment model):

Compartment	Contents
Superior mediastinum (above T4/sternal angle)	Thymus, great vessels (arch of aorta, brachiocephalic veins, SVC), trachea, esophagus, thoracic duct, phrenic & vagus nerves, left recurrent laryngeal nerve
Anterior mediastinum (anterior to pericardium)	Thymus (inferior part), lymph nodes, internal thoracic vessels, fat
Middle mediastinum	Heart (in pericardium), ascending aorta, pulmonary trunk, main bronchi, phrenic nerve
Posterior mediastinum (posterior to pericardium)	Descending aorta, esophagus, azygos/hemiazygos veins, sympathetic chain, splanchnic nerves, thoracic duct

The Diaphragm

The diaphragm is the musculotendinous dome separating the thorax from the abdomen. It is the principal muscle of respiration.

Origin: Three parts — sternal (xiphoid process), costal (lower 6 costal cartilages), lumbar (crura from L1–L3)

Three major apertures (remember "8–10–12"):

Aperture	Level	Structure Passing Through
Caval hiatus (IVC hiatus)	T8	Inferior vena cava, right phrenic nerve
Esophageal hiatus	T10	Esophagus, vagal trunks, left gastric vessels
Aortic hiatus	T12	Aorta, thoracic duct, azygos vein

ASCII Diagram 8 — Mediastinal Compartments (Lateral View) & Diaphragmatic Apertures

  LATERAL VIEW OF THORAX:
  
  ┌─────────────────────────────────────────┐
  │  SUPERIOR MEDIASTINUM                   │
  │  (above angle of Louis T4)              │
  │  Thymus, great vessels, trachea, esoph  │
  ├──────────────────────────────────────────
  │  ANTERIOR   │  MIDDLE      │ POSTERIOR  │
  │  (pre-peri) │ (pericardium)│(post-peri) │
  │  Thymus     │ Heart        │ Aorta desc │
  │  Lymph      │ Asc. aorta   │ Esophagus  │
  │  nodes      │ Pulm. trunk  │ Azygos v.  │
  │             │ Main bronchi │ Symp.chain │
  └─────────────────────────────────────────┘
  
  DIAPHRAGM:
  
         T8 ── IVC hiatus (IVC + R. phrenic n.)
         T10 ── Esophageal hiatus (esoph + vagi)
         T12 ── Aortic hiatus (aorta + thor. duct)
  
  (mnemonic: I 8 10 eggs AT 12 = IVC-8, Esoph-10, Aorta-12)

Source: Sabiston Textbook of Surgery (Schwartz's Chapter 110)

PAGE 9 — RESPIRATORY MUSCLES, NERVE SUPPLY, AND CLINICAL CORRELATIONS

Muscles of Respiration

The ventilatory pump consists of respiratory muscles acting on the bony thorax. They are organized by priority:

Primary muscles:

Muscle	Origin	Action
Diaphragm	Xiphoid, costal cartilages 7–12, L1–L3 crura	Descends on contraction → increases thoracic vertical diameter (major inspiratory muscle — 60–70% of VE at rest)
External intercostals	Inferior border of rib above	Elevate ribs → increase AP and transverse thoracic diameter
Internal intercostals	Superior border of rib below	Expiration (posterior fibers), stabilization

Secondary (accessory) muscles of inspiration:

Sternocleidomastoid: elevates manubrium and sternum
Scalenes (anterior, middle, posterior): elevate first two ribs; active even in quiet breathing
Pectoralis major and minor (when arms fixed): pull sternum and ribs anteriorly
Serratus anterior: fixes scapula; accessory role in rib elevation
Levatores costarum: elevate ribs

Expiratory muscles (active in forced expiration):

Internal intercostals (interosseous portion)
Abdominals (rectus abdominis, external and internal oblique, transversus abdominis): depress lower ribs, increase intraabdominal pressure to push diaphragm up

Nerve Supply

Structure	Nerve	Level
Diaphragm (motor + sensory)	Phrenic nerve	C3, C4, C5 ("C3, 4, 5 keep the diaphragm alive")
Intercostal muscles	Intercostal nerves	T1–T11
Larynx — supraglottis (sensory)	Internal branch of superior laryngeal nerve (SLN)	Vagus (CN X)
Larynx — cricothyroid (motor)	External branch of SLN	Vagus (CN X)
Larynx — all other muscles + subglottis (motor+sensory)	Recurrent laryngeal nerve (RLN)	Vagus (CN X)
Trachea	RLN + tracheal branches of vagus	CN X
Bronchi	Sympathetic (T1–T4) bronchodilation; Parasympathetic (vagus) bronchoconstriction	CN X
Parietal pleura	Intercostal nerves (costal+peripheral diaphragmatic part); Phrenic (central diaphragmatic + mediastinal part)	T1–T11; C3–C5

Clinical note on RLN: The left RLN loops under the aortic arch (left of the trachea in the aorto-pulmonary window), while the right RLN loops under the right subclavian artery — making the left RLN more susceptible to damage from aortic aneurysm, lung cancer, or mediastinal lymphadenopathy.

Lymphatic Drainage of the Lung

Lymph drains centrally via two routes:

Superficial (subpleural) network: Drains toward the hilum
Deep (peribronchial/perivascular) network: Follows bronchi and vessels to hilar nodes

Lymph node stations (important in lung cancer staging):

N1 nodes: Intrapulmonary → Peribronchial → Hilar (ipsilateral)
N2 nodes: Ipsilateral mediastinal (subcarinal, paratracheal, aortopulmonary)
N3 nodes: Contralateral mediastinal/hilar, scalene, supraclavicular

ASCII Diagram 9 — Nerve Supply Summary (Diagram)

  BRAINSTEM (pons + medulla)
         |  (automatic breathing control)
  CEREBRAL CORTEX
         |  (voluntary breathing)
         |
  SPINAL CORD
   ├── C3/C4/C5 → PHRENIC NERVE → DIAPHRAGM
   ├── T1–T11 → INTERCOSTAL NERVES → Intercostals
   └── T1–T4 → Sympathetic → Bronchodilation
         |
  VAGUS NERVE (CN X)
   ├── Superior Laryngeal N.
   │    ├── Internal branch → Supraglottic sensory
   │    └── External branch → Cricothyroid muscle
   ├── Recurrent Laryngeal N. (RIGHT: loops R. subclavian)
   │                          (LEFT: loops aortic arch)
   │    └── All intrinsic laryngeal muscles (except cricothyroid)
   │        + Subglottic sensation + Tracheal innervation
   └── Bronchial branches → Bronchoconstriction

  FEEDBACK:
   ├── Peripheral chemoreceptors: carotid body (IX) + aortic body (X)
   ├── Central chemoreceptors: medullary — respond to CO2/pH
   ├── Pulmonary stretch receptors (Hering-Breuer reflex) — via vagus
   └── J-receptors (juxtacapillary) — respond to edema, emboli

Sources: Murray & Nadel's Textbook of Respiratory Medicine; Fishman's Pulmonary Diseases; Morgan & Mikhail's Clinical Anesthesiology, 7e; Sabiston Textbook of Surgery

SUMMARY TABLE — Key Topographic Levels at a Glance

Structure	Vertebral Level	Surface/Bony Landmark
Nasal cavity	Skull base → C1	Above hard palate
Nasopharynx	C1–C2	Soft palate inferior
Oropharynx	C2–C3	Soft palate → epiglottis
Hypopharynx	C3–C6	Epiglottis → cricoid
Larynx	C3–C6	Thyroid notch (C4), cricoid (C6)
Trachea (start)	C6	Below cricoid
Thoracic inlet	T1	Suprasternal notch
Carina	T4–T5	Angle of Louis (sternomanubrial joint)
Lung apex	2–3 cm above clavicle	Above midclavicle
Lung inferior border (MCL)	6th rib	Midclavicular
Lung inferior border (MAL)	8th rib	Midaxillary
Lung inferior border (post.)	T10	Paravertebral
Pleural reflection (post.)	T12	12th rib
IVC hiatus (diaphragm)	T8	—
Esophageal hiatus	T10	—
Aortic hiatus	T12	—
Phrenic nerve origin	C3–C5	Cervical spinal cord

References: Murray & Nadel's Textbook of Respiratory Medicine, 2-Volume (9780323655873) | Fishman's Pulmonary Diseases and Disorders, 2-Volume Set (9781260473940) | Morgan & Mikhail's Clinical Anesthesiology, 7e (9781260473797) | Sabiston Textbook of Surgery, 21e (9780443124341)

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