Microscopic Anatomy of Respiratory System

Upper Airways Histology - Nose & Throat Linings

• Nasal Cavity:
  • Vestibule: Keratinized Stratified Squamous Epithelium (KSSE); vibrissae (hairs).
  • Respiratory Region: Pseudostratified Ciliated Columnar Epithelium (PSCCE) with goblet cells (Respiratory Epithelium). Lamina propria has rich vasculature (Kiesselbach’s plexus) & seromucous glands.
  • Olfactory Region: Specialized PSCCE; olfactory receptor cells (bipolar neurons), sustentacular cells, Bowman's glands (serous).
• Nasopharynx: PSCCE; pharyngeal tonsils (adenoids).
• Oropharynx & Laryngopharynx: Non-Keratinized Stratified Squamous Epithelium (NKSE) - for protection against food abrasion.
• Larynx:
  • Epiglottis: NKSE on lingual & upper posterior surface; PSCCE on laryngeal surface.
  • True Vocal Cords: NKSE for vibration; Reinke's space underneath.
  • False Vocal Cords (Vestibular folds) & most other laryngeal areas: PSCCE.

⭐ True vocal cords are covered by Non-Keratinized Stratified Squamous Epithelium to withstand the mechanical stress of phonation, unlike the surrounding respiratory epithelium (PSCCE).

Trachea & Bronchi - Windpipe Wall Wonders

• Wall Layers (Typical):
  • Mucosa: Pseudostratified ciliated columnar epithelium (PCCE) with goblet cells.
  • Submucosa: Connective tissue with seromucous glands (more in trachea).
  • Cartilaginous Layer: Hyaline cartilage.
  • Adventitia: Outer connective tissue.
• Trachea Specifics:
  • 16-20 C-shaped hyaline cartilage rings, open posteriorly.
  • Trachealis muscle (smooth) spans posterior gap.
• Bronchi Specifics:
  • Irregular hyaline cartilage plates (not C-shaped).
  • Circumferential smooth muscle layer.
• Key Trend (Trachea → Bronchi → Bronchioles): ↓Cartilage, ↓Glands, ↓Goblet cells; ↑Relative smooth muscle.

⭐ Trachea features C-shaped hyaline cartilage rings; bronchi have irregular cartilage plates. Cartilage is absent beyond segmental bronchi (i.e., in bronchioles).

Bronchioles & Terminal Units - Tiny Tube Transitions

• Bronchioles: < 1mm diameter; NO cartilage/glands.
  • Epithelium: Ciliated columnar → cuboidal. Goblet cells ↓, absent terminally.
  • Club (Clara) Cells: Dome-shaped; secrete CCSP (CC16), surfactant components; detoxify; progenitor cells. 📌 Club cells Calmly Coat & Cleanse.
  • Smooth muscle prominent.
• Airway Path & Units:
  • Terminal Bronchioles (TB): Smallest purely conducting. Cuboidal epithelium (Club cells dominant, few cilia).
  • Respiratory Bronchioles (RB): Gas exchange begins; alveoli appear in walls.
  • Acinus: Functional unit distal to TB (RB, alveolar ducts, sacs, alveoli).

  ⭐ Club cells (SCGB1A1+) are key to bronchiolar defense; CC16 is a major protective protein.

Alveoli & Blood-Air Barrier - Air Sac All-Stars

• Alveoli: Terminal air sacs; primary gas exchange sites.
  • Type I Pneumocytes: Squamous cells, form ~95% of alveolar surface; thin cytoplasm ideal for gas diffusion.
  • Type II Pneumocytes: Cuboidal cells, ~5% surface area; secrete surfactant (dipalmitoylphosphatidylcholine - DPPC); act as progenitor cells. 📌 Mnemonic: Type II cells have Two key roles (surfactant + repair).
  • Alveolar Macrophages (Dust Cells): Phagocytose debris & pathogens in alveoli.
  • Pores of Kohn: Inter-alveolar connections, allow collateral ventilation.
• Blood-Air Barrier (Respiratory Membrane): Extremely thin (~0.2-0.6 µm); facilitates efficient gas exchange.
  • Components (from air to blood): Surfactant layer → Type I pneumocyte cytoplasm → Fused basal laminae (of Type I pneumocyte & capillary endothelium) → Capillary endothelial cytoplasm.

  ⭐ Surfactant (mainly DPPC) reduces alveolar surface tension, preventing collapse (atelectasis). Its deficiency is key in Neonatal Respiratory Distress Syndrome (NRDS).

High‑Yield Points - ⚡ Biggest Takeaways

• Respiratory epithelium: Pseudostratified ciliated columnar with goblet cells, lines most conducting airways.
• Trachea: C-shaped hyaline cartilage; Bronchi: Cartilage plates, smooth muscle.
• Bronchioles: No cartilage or glands; prominent smooth muscle; Clara (club) cells appear.
• Alveoli: Lined by Type I pneumocytes (gas exchange) & Type II pneumocytes (surfactant production).
• Surfactant: Secreted by Type II pneumocytes, reduces surface tension, prevents alveolar collapse.
• Clara (club) cells: In bronchioles; detoxify, secrete protective proteins, act as progenitor cells.
• Dust cells: Alveolar macrophages responsible for phagocytosis in the alveoli.