Anatomy of the Lower Respiratory Tract

Learning Outcomes

• State major anatomical parts of lower respiratory system.
• Relate function of each part to gross & microscopic structure.
• Identify & describe muscles of respiration and their actions during each phase.

Functional Divisions of the Respiratory System

• Conducting division
  • Nose ➔ major bronchioles (nostrils through terminal bronchioles).
  • Functions: airflow only; no gas exchange.
• Respiratory division
  • Alveoli + all microscopic regions where gas exchange occurs.
• Anatomical delimitation
  • Upper tract: nose ➔ larynx (head & neck).
  • Lower tract: trachea ➔ lungs (thorax).

Trachea (Windpipe)

• Rigid tube, length \approx 12\,\text{cm}, diameter \approx 2.5\,\text{cm}.
• Position: anterior to oesophagus; extends from larynx to sternal angle \text{(T4)} where it bifurcates.
• Support
  • 16–20 C-shaped hyaline cartilage rings; opening faces posteriorly.
  • Trachealis muscle spans gap ➔ allows oesophageal expansion & adjusts airflow (contracts ⬇ lumen, relaxes ⬆ lumen).
• Histology
  • Mucosa: pseudostratified ciliated columnar epithelium (goblet, ciliated, stem cells).
  • Mucociliary escalator: mucus traps debris; cilia beat upward toward pharynx for swallowing.
  • Middle layer: connective tissue containing mucous & serous glands, lymphoid nodules, cartilage.
  • Adventitia: outer fibrous CT continuous with mediastinal structures.
• Carina: internal ridge at bifurcation directing airflow into R & L main bronchi; sensitive cough reflex area.

Main (Primary) Bronchi

• Right main bronchus
  • Length 2–3\,\text{cm}, wider, more vertical ➔ aspirated objects lodge here more often.
• Left main bronchus
  • Length \approx 5\,\text{cm}, narrower, more horizontal.
• Both supported by C-shaped cartilage rings; enter lungs at hilum.

The Bronchial Tree

• Hierarchy: Main (primary) ➔ Lobar (secondary) ➔ Segmental (tertiary) ➔ smaller bronchi ➔ bronchioles ➔ terminal bronchioles ➔ respiratory bronchioles ➔ alveolar ducts ➔ alveolar sacs ➔ alveoli.
• About 65\,000 terminal bronchioles per lung.

Lobar (Secondary) Bronchi

• Right lung: 3 lobar bronchi (superior, middle, inferior).
• Left lung: 2 lobar bronchi (superior, inferior).
• Supported by crescent-shaped cartilage plates.

Segmental (Tertiary) Bronchi

• Right lung: 10 segments; Left lung: 8 segments.
• Each supplies a bronchopulmonary segment = functionally independent lung unit (clinical relevance: segmental resections).

Histological Features (All Bronchi)

• Lining: pseudostratified ciliated columnar epithelium; cell height ↓ distally.
• Lamina propria rich in mucous glands & BALT ➔ immune defense.
• Abundant elastic CT throughout tree ➔ recoil for expiration.
• Muscularis mucosae (smooth muscle): regulates airway diameter.
• Blood supply
  • Pulmonary arteries course with bronchi to alveoli (deoxygenated blood for gas exchange).
  • Bronchial arteries from aorta nourish bronchial walls (systemic, oxygenated).

Bronchioles

• Diameter < 1\,\text{mm}; no cartilage.
• Thick smooth-muscle layer; ciliated cuboidal epithelium.
• Each bronchiole ventilates a pulmonary lobule.
• Terminal bronchioles (50–80/lobule)
  • Diameter ≤ 0.5\,\text{mm}.
  • No goblet cells; still possess cilia ➔ mucus clearance via escalator.
  • Branch into 2+ respiratory bronchioles.

Respiratory Bronchioles & Alveolar Ducts

• Respiratory bronchioles: first airways with alveoli budding from walls ➔ begin gas-exchange region.
• Divide into 2–10 alveolar ducts ending in alveolar sacs (grape-like clusters around an atrium).

Alveoli

• Number: 1.5\times10^8 (150 million) per lung; surface area \approx70\,\text{m}^2.
• Cell types
  • Type I (squamous) cells ➔ 95 % surface; very thin for rapid diffusion.
  • Type II (great/septal) cells ➔ 5 %; cuboidal; functions:
  • Secrete pulmonary surfactant (phospholipid-protein mix) to reduce surface tension & prevent collapse during exhalation.
  • Repair epithelium after damage.
  • Clinical: deficiency causes Hyaline Membrane Disease (infant respiratory distress syndrome).
  • Alveolar macrophages (dust cells)
  • Most numerous; phagocytose debris & pathogens.
  • ~10^8 die daily, cleared by mucociliary escalator & swallowed.
• Respiratory membrane (barrier for diffusion)
  1. Type I alveolar cell.
  2. Shared basement membrane.
  3. Capillary endothelial cell.
• Fluid balance
  • Alveoli kept dry by capillary absorption & extensive lymphatic drainage.
  • Low pulmonary capillary pressure prevents membrane rupture.

Gross Anatomy of the Lungs

• Shape: conical; base rests on diaphragm; apex extends above clavicle.
• Surfaces
  • Costal (against ribs).
  • Mediastinal (faces heart) ➔ hilum where bronchi, vessels, lymphatics, nerves form root.
• Right Lung
  • Shorter (large liver below).
  • 3 lobes: superior, middle, inferior.
  • Fissures: horizontal & oblique.
• Left Lung
  • Taller, narrower (cardiac impression & notch).
  • 2 lobes: superior, inferior.
  • Single oblique fissure; lingula = tongue-like part of superior lobe.
• Lungs do not entirely fill rib cage; asymmetrical due to heart & liver.

Bronchopulmonary Segments (Key Clinical Map)

• Right (10): Apical (S1), Posterior (S2), Anterior (S3), Lateral (S4), Medial (S5) in middle lobe; Superior (S6), Medial basal (S7/S8), Anterior basal (S8), Lateral basal (S9), Posterior basal (S10) in lower lobe.
• Left (8): Apico-posterior (S1+S2), Anterior (S3), Superior lingular (S4), Inferior lingular (S5), Superior (S6), Antero-medial basal (S7+S8), Lateral basal (S9), Posterior basal (S10).

Pleurae & Pleural Cavity

• Visceral pleura: adheres to lung surface.
• Parietal pleura: lines thoracic wall, diaphragm, mediastinum.
• Pleural cavity: potential space with thin film of fluid.
• Functions
  • Lubrication (↓ friction).
  • Pressure gradient (intrapleural pressure < atmospheric) assists lung inflation.
  • Compartmentalisation limits spread of infection.

Respiratory Muscles

Primary Muscle

• Diaphragm (innervation: phrenic nerve C3–C5, chiefly C5 stated):
  • Contraction ➔ flattens, ⬆ thoracic vertical dimension ➔ inspiration (~2/3 of quiet airflow).
  • Relaxation ➔ domes upward, recoil aids passive expiration.

Intercostal Muscles

• External & internal intercostals (between ribs)
  • Synergists to diaphragm.
  • Stiffen thoracic cage; prevent inward collapse; add ≈1/3 of ventilatory volume.

Scalenes

• Fix/elevate ribs 1–2 during quiet inspiration (synergists).

Accessory Muscles (forced inspiration)

• Erector spinae, sternocleidomastoid, pectoralis major & minor, serratus anterior, scalenes ➔ greatly enlarge thoracic cavity.

Forced Expiration Muscles

• Internal intercostals (interosseous part), rectus abdominis, external abdominal oblique, lumbar & pelvic muscles ➔ increase abdominal & thoracic pressure to expel air.

Respiratory Mechanics

• Quiet inspiration: active; diaphragm + external intercostals contract.
• Quiet expiration: passive; elastic recoil of lungs & cage.
• Forced inspiration/expiration use accessory groups above.

Valsalva Manoeuvre

• Steps: deep breath ➔ close glottis ➔ contract abdominal muscles.
• Raises abdominal pressure to expel contents.
• Applications: childbirth, urination, defaecation, vomiting, heavy lifting.

Intrapulmonary & Pleural Blood Flow

• Pulmonary arteries follow airways to alveolar capillaries.
• Pulmonary veins run intersegmentally toward hilum.
• Bronchial arteries (systemic) supply bronchial walls.

Clinical & Physiological Notes

• Conducting zone cleansing: mucociliary escalator crucial; smoking or Kartagener’s syndrome impairs clearance ➔ infections.
• Right main bronchus orientation explains object aspiration & pneumonia distribution.
• Surfactant deficiency in premature neonates ➔ Hyaline Membrane Disease; treated with exogenous surfactant.
• Bronchopulmonary segment independence allows segmental lung resections without affecting neighbours.
• Pleural pressure gradient essential; pneumothorax abolishes gradient ➔ lung collapse.
• Valsalva manoeuvre alters intrathoracic pressure; affects venous return & can be used diagnostically (e.g., heart murmurs).