Conducting Zone (no gas exchange):
• Nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles.
• Lined by 'respiratory epithelium' = pseudostratified ciliated columnar with goblet cells.
• Mucociliary escalator clears debris.
• Trachea has C-shaped hyaline cartilage rings.

Respiratory Zone (gas exchange):
• Respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli.
• Alveolus: thin sac lined by:
– Type I pneumocytes — squamous, gas exchange.
– Type II pneumocytes — cuboidal; secrete surfactant.
– Alveolar macrophages (dust cells).
• Blood-air barrier: alveolar epithelium + basement membrane + capillary endothelium (~0.5 μm).

RESPIRATORY SYSTEM HISTOLOGY

Two functional zones with distinct histology:

I. CONDUCTING ZONE (filters, warms, humidifies; NO gas exchange):

1. Nasal Cavity:
• Vestibule — stratified squamous with hairs (vibrissae) and sebaceous glands.
• Respiratory region — respiratory epithelium + many seromucous glands + rich venous plexuses (warm air).
• Olfactory region — pseudostratified specialised epithelium with olfactory receptor neurons + supporting cells + basal cells + Bowman's glands.

2. Trachea:
• Mucosa: respiratory epithelium (pseudostratified ciliated columnar with goblet cells) on basement membrane → ciliary beating drives mucus toward pharynx ('mucociliary escalator').
• Submucosa: seromucous glands.
• Cartilage: C-shaped hyaline cartilage rings (open dorsally, closed by tracheal muscle); ~50 in ox.
• Adventitia: CT.

3. Bronchi:
• Same epithelium as trachea but cartilage is in plates (no longer C-rings).
• Smooth muscle layer present in walls.
• Lymphoid tissue (BALT) common.

4. Bronchioles (<1 mm):
• NO cartilage, NO submucosal glands.
• Epithelium becomes simple columnar then cuboidal.
• Goblet cells decrease, replaced by club (Clara) cells — dome-shaped, secretory; defence + stem-cell function.
• Smooth muscle layer relatively prominent → bronchospasm in asthma.
• Terminal bronchioles = last purely conducting bronchioles.

II. RESPIRATORY ZONE (gas exchange):

1. Respiratory Bronchioles:
• Wall has scattered alveoli — first sites of gas exchange.
• Cuboidal epithelium with patches of thin alveolar epithelium.

2. Alveolar Ducts + Sacs:
• Heavily alveolated tubes ending in clusters of alveoli.
• Surrounded by smooth muscle bands ('alveolar knobs').

3. Alveolus (Functional unit of gas exchange):
• Thin-walled polyhedral sac (~200 μm).
• Lined by:
– Type I pneumocytes (Type I alveolar cells, ~95% of surface area, ~40% of cells) — extremely flat squamous cells (~0.2 μm thick); gas diffusion.
– Type II pneumocytes (~5% of surface, ~60% of cells) — cuboidal with foamy cytoplasm; lamellar bodies; secrete SURFACTANT (DPPC + proteins) → reduces surface tension → prevents alveolar collapse. Also stem cells for Type I.
– Alveolar Macrophages (Dust cells) — phagocytose dust + bacteria + dead cells; eventually swallowed up the mucociliary escalator.
• Alveolar pores (of Kohn) — connect adjacent alveoli; equalise pressure.

BLOOD-AIR BARRIER:
• Total thickness ~0.5 μm.
• 3 components:
1. Type I pneumocyte cytoplasm.
2. Fused basement membranes (alveolar + capillary).
3. Capillary endothelial cell cytoplasm.
• Surfactant film + tissue fluid layer over the surface.

SPECIES NOTES:
• Ruminant lung = highly LOBULATED with thick interlobular septa → 'cobblestone' pattern at necropsy.
• Horse lung — thin/absent septa.
• Bird lung — flow-through (parabronchi + air capillaries) + 9 air sacs; no alveoli; Cross-current gas exchange supports highly effective avian gas exchange without requiring an unsupported universal ranking.