recognise cartilage, ciliated epithelium, goblet cells, squamous epithelium of alveoli, smooth muscle and capillaries in microscope slides, photomicrographs and electron micrographs

The Gas‑Exchange System (Cambridge A‑Level 9700)

The mammalian respiratory tract is specialised for rapid O₂ uptake and CO₂ removal. Mastery of the microscopic structures—from the trachea to the alveolar capillary network—is a core requirement of the Cambridge syllabus (Topic 9).

Key Structures to Recognise

• Cartilage (C‑shaped rings in trachea & bronchi)
• Ciliated pseudostratified columnar epithelium
• Goblet cells (mucus‑secreting)
• Bronchi & bronchioles (pattern of cartilage and smooth muscle)
• Alveolar sac (type I & type II pneumocytes, thin basement membrane)
• Smooth muscle in bronchi, bronchioles and arterioles
• Capillary network surrounding each alveolus

Why These Structures Matter

• Surface‑area to volume ratio: ≈ 70 m² of alveolar surface maximises diffusion.
• Ventilation‑perfusion (V/Q) matching: Airflow and blood flow are coordinated so every alveolus receives optimal O₂ and CO₂ exchange.
• Partial‑pressure gradients: O₂ diffuses from ≈ 100 mm Hg (alveolar air) to ≈ 40 mm Hg (pulmonary blood); CO₂ diffuses in the opposite direction (≈ 40 mm Hg → 46 mm Hg).
• Diffusion distance: The combined thickness of type I pneumocyte + capillary endothelium is ≤ 0.5 µm, allowing rapid gas transfer.

Microscopic Identification

1. Cartilage (C‑shaped rings)

• Light microscopy (H&E): Dense, basophilic extracellular matrix; chondrocytes reside in lacunae; rings are ≈ 2–3 mm thick in the adult trachea.
• Electron microscopy: Parallel collagen fibrils (~50 nm diameter) embedded in a proteoglycan‑rich ground substance.
• Function: Provides rigidity to keep trachea and larger bronchi patent during inhalation and exhalation.

2. Bronchi and Bronchioles

• Bronchi (light microscopy): C‑shaped cartilage plates, submucosal glands, abundant smooth muscle, ciliated pseudostratified epithelium with goblet cells.
• Bronchioles (light microscopy): No cartilage; epithelium becomes simple cuboidal then simple columnar; smooth muscle layer thickens toward terminal bronchioles; goblet cells become sparse.
• Key point for exam: Presence of cartilage distinguishes bronchi from bronchioles; loss of cartilage is a hallmark of the smaller airways.

3. Ciliated Pseudostratified Columnar Epithelium

• Light microscopy: Tall columnar cells; nuclei at varying depths give a “pseudo‑stratified” look; apical surface bears motile cilia.
• Electron microscopy: Each cilium shows the classic 9 + 2 microtubule axoneme with dynein arms.
• Function: Propels mucus (and trapped particles) toward the pharynx – the mucociliary escalator.

4. Goblet Cells

• Light microscopy: Scattered among ciliated cells; large pale‑staining mucin droplets push the nucleus basally.
• Electron microscopy: Electron‑lucent mucin granules surrounded by a faint “halo”.
• Function: Secrete mucus to humidify inhaled air and trap debris.

5. Alveolar Sac

Type I Pneumocytes (Simple Squamous Epithelium)

• Light microscopy: Extremely thin, flat cells forming a continuous lining of the alveolus.
• Electron microscopy: Cytoplasmic rim ≈ 0.2 µm; abundant surface microvilli increase effective diffusion area.
• Function: Provides the minimal diffusion barrier for O₂ and CO₂.

Type II Pneumocytes

• Light microscopy: Cuboidal cells interspersed among type I cells; contain prominent lamellar bodies.
• Electron microscopy: Lamellar bodies packed with phospholipid surfactant.
• Function: Produce surfactant, lowering surface tension and preventing alveolar collapse during exhalation.

Alveolar Basement Membrane

• Thin (≈ 0.05 µm) shared basal lamina between type I pneumocyte and capillary endothelium; contains collagen IV and laminin.

6. Smooth Muscle (Bronchi, Bronchioles & Arterioles)

• Light microscopy: Spindle‑shaped cells with centrally placed nuclei; arranged in a circumferential layer.
• Electron microscopy: Dense bodies (analogous to Z‑lines), abundant smooth ER, and few mitochondria.
• Function: Regulates airway diameter (bronchoconstriction/dilation) and vascular tone, influencing V/Q matching.

7. Capillaries

• Light microscopy: Thin‑walled, red‑staining vessels tightly apposed to the alveolar wall.
• Electron microscopy: Single layer of flattened endothelial cells; fenestrations ≈ 50 nm; basal lamina present.
• Typical dimensions: Lumen diameter 5–10 µm; wall thickness ≈ 0.3 µm.
• Function: Provides a vast vascular surface for rapid gas diffusion.

Quick Fact Box – Quantitative Highlights

• Alveolar wall (type I + endothelium) thickness: ≈ 0.3 µm total diffusion distance.
• Capillary fenestration diameter: ≈ 50 nm.
• Total alveolar surface area: ≈ 70 m² (≈ 750 times a tennis court).
• Number of alveoli per lung: ≈ 480 million.
• Cartilage ring thickness in trachea: ≈ 2–3 mm.
• Bronchiolar smooth‑muscle thickness (terminal bronchioles): ≈ 0.2 mm.

Linking Structure to Function – The Gas‑Exchange Process

1. Air enters the trachea – kept open by C‑shaped cartilage rings.
2. Warm, humidified air passes through ciliated pseudostratified epithelium; goblet‑cell mucus traps particles.
3. Bronchi retain cartilage; bronchioles lose cartilage and rely on smooth‑muscle tone to regulate airflow.
4. At the alveolar level, type I pneumocytes provide a thin barrier; type II cells secrete surfactant to keep alveoli open.
5. Partial‑pressure gradients drive O₂ into capillary blood and CO₂ into the alveolar air.
6. The large surface area, short diffusion distance, and close capillary apposition enable rapid gas exchange.

Clinical Relevance (Why It Matters)

• Smoking: Damages ciliated epithelium and goblet cells → impaired mucociliary clearance → ↑ infection risk.
• Asthma: Hyper‑responsive smooth muscle → bronchoconstriction → reduced airflow and V/Q mismatch.
• Emphysema: Destruction of alveolar walls and capillary network → ↓ surface area → reduced diffusion capacity.
• Neonatal Respiratory Distress Syndrome: Surfactant deficiency → alveolar collapse → severe hypoxia.

Summary Table

Exam Tips

• State slide orientation (e.g., “transverse section of trachea” or “longitudinal section of a bronchiole”).
• Use cartilage to differentiate bronchi from bronchioles; loss of cartilage is a key marker of the smaller airways.
• Identify cilia + goblet cells as the respiratory epithelium; note the 9 + 2 axoneme in EM images.
• Remember only type I pneumocytes are thin enough for efficient diffusion; type II cells are mentioned for surfactant production.
• When viewing EM, look for hallmark ultrastructural markers:

  • 9 + 2 arrangement in cilia
  • Parallel collagen fibrils in cartilage
  • Dense bodies in smooth muscle
  • Fenestrations in capillary endothelium

• Link every structure to its function in your answer – e.g., “cartilage maintains airway patency, allowing the mucociliary escalator to clear mucus efficiently.”
• Briefly mention the driving force for diffusion (partial‑pressure gradients) and note that the oxygen‑dissociation curve will be examined later.