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Respiratory System Anatomy & Physiology: From Nose to Alveoli

The respiratory system conducts air, conditions it, and performs gas exchange. It comprises the conducting zone (no gas exchange) and respiratory zone (gas exchange).

Conducting Zone (Airway)

  1. Nose / Nasal Cavity: Warms, humidifies, filters (vibrissae, mucus, cilia). Olfaction. Turbinates ↑ surface area.
  2. Pharynx (Throat): Nasopharynx (adenoids), oropharynx (palatine tonsils), laryngopharynx. Shared with digestive tract.
  3. Larynx (Voice Box): Cartilages (thyroid, cricoid, arytenoids), vocal folds. Phonation, airway protection (cough reflex, epiglottis).
  4. Trachea (Windpipe): 10-12 cm, 16-20 C-shaped cartilages (prevents collapse), lined by pseudostratified ciliated columnar epithelium (mucociliary escalator).
  5. Bronchi: Right main (wider, shorter, more vertical) → 3 lobar → segmental. Left main → 2 lobar. Cartilage plates replace rings.
  6. Bronchioles: <1 mm, no cartilage, smooth muscle (regulates airflow), ciliated → terminal bronchioles (last conducting).

Respiratory Zone (Gas Exchange)

  • Respiratory bronchioles: Alveolar buds appear; some gas exchange.
  • Alveolar ducts → Alveolar sacs → Alveoli: ~300-500 million alveoli; total surface area ~70-100 m² (tennis court).
  • Alveolar wall: Type I pneumocytes (95% surface, thin for diffusion), Type II pneumocytes (surfactant — ↓ surface tension), alveolar macrophages (dust cells).
  • Capillary network: Dense; RBCs in single file; barrier thickness 0.2-0.5 µm.

Gas Exchange (External Respiration)

  • O₂ diffusion: Alveolar pO₂ ~100 mmHg → capillary pO₂ ~40 mmHg → binds Hb.
  • CO₂ diffusion: Capillary pCO₂ ~45 mmHg → alveolar pCO₂ ~40 mmHg → exhaled.
  • Driving forces: Partial pressure gradients (Fick's law); perfusion-limited for O₂/CO₂.

Ventilation Mechanics

  • Inspiration (active): Diaphragm contracts (↓ pleural pressure), external intercostals lift ribs → ↑ thoracic volume → air flows in.
  • Expiration (passive at rest): Elastic recoil of lungs/chest wall + relaxation of inspiratory muscles.
  • Forced expiration: Internal intercostals, abdominal muscles.

Key Volumes & Capacities

  • Tidal volume (TV): ~500 mL
  • Inspiratory reserve (IRV): ~3000 mL
  • Expiratory reserve (ERV): ~1200 mL
  • Residual volume (RV): ~1200 mL (cannot be exhaled)
  • Vital capacity (VC) = IRV + TV + ERV ~4.8 L
  • Total lung capacity (TLC) = VC + RV ~6 L

Defense Mechanisms

  • Mucociliary escalator (cilia beat 10-20 Hz → mucus to pharynx).
  • Alveolar macrophages (phagocytosis).
  • Secretory IgA (mucosal immunity).
  • Cough reflex (irritant receptors).
  • Surfactant (prevents collapse, innate immune proteins).

Efficient gas exchange requires matching ventilation (V) to perfusion (Q) — V/Q matching. Disease disrupts this balance (shunt, dead space).

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