describe the distribution in the gas exchange system of cartilage, ciliated epithelium, goblet cells, squamous epithelium of alveoli, smooth muscle and capillaries

Gas‑Exchange System (Cambridge IGCSE/A‑Level Biology 9700 – Topic 9)

Learning Objective

Describe the distribution of the following structures within the human gas‑exchange system and explain why each is found where it is:

  • Cartilage (hyaline)

  • Ciliated epithelium (pseudostratified columnar)

  • Goblet cells

  • Squamous epithelium of the alveoli (type I pneumocytes)

  • Smooth muscle

  • Capillaries

1. Anatomical Hierarchy – Conducting vs. Respiratory Zones

Conducting zone (air‑conduction only)

  • Nasal cavity → Pharynx → Larynx → Trachea → Main (primary) bronchi → Lobar bronchi → Segmental bronchi → Bronchioles → Terminal bronchioles

Respiratory zone (site of gas exchange)

  • Respiratory bronchioles → Alveolar ducts → Alveoli

Key transition points

  • Bronchioles (terminal): no cartilage, thin smooth‑muscle layer, epithelium changes from pseudostratified columnar to simple cuboidal; goblet cells become scarce, club (Clara) cells appear.

  • Respiratory bronchioles: first site where alveolar‑type epithelium (type I & II pneumocytes) is seen on the wall.

  • Alveolar ducts: short, thin‑walled ducts that connect respiratory bronchioles to clusters of alveoli; walls are composed mainly of type I pneumocytes and a scant elastic interstitium, surrounded by a dense capillary network.

2. Structures – Location, Function & Microscopic Identification

StructurePrimary Location(s)Functional SignificanceHow to recognise in a slide / micrograph
Cartilage (hyaline)Trachea (C‑shaped rings) and main & lobar bronchi (plate‑like plates)Provides rigidity; prevents collapse of large airways during the negative pressure of inspiration and the positive pressure of forced expiration.Pink‑staining extracellular matrix with lacunae containing chondrocytes; in transverse tracheal sections the C‑shaped rings appear as dark pink arcs surrounding the lumen.
Ciliated epithelium (pseudostratified columnar)Entire conducting zone – nasal cavity, trachea, bronchi, bronchioles (up to terminal bronchioles)Moves mucus (and trapped particles) toward the pharynx – the mucociliary escalator.Columnar cells with basal nuclei; apical surface shows motile cilia (≈5–7 µm) that appear as fine hair‑like projections in SEM or as a brush‑border in light microscopy.
Goblet cellsInterspersed among the ciliated epithelium of the nasal cavity, trachea and larger bronchi.Secrete mucus that traps dust, microbes and other inhaled particles.Round cells with a clear, mucin‑filled lumen; PAS or alcian‑blue stains give a pink/red or blue colour; often described as “cup‑shaped” cells.
Club (Clara) cellsBronchioles (especially terminal and respiratory bronchioles)Secrete a protective, non‑ciliated secretory fluid and act as progenitor cells for bronchiolar epithelium.Cuboidal cells with a granular cytoplasm; lack cilia; visible as dark, dome‑shaped cells in histological sections.
Squamous epithelium of alveoli (type I pneumocytes)Alveolar walls (respiratory zone)Forms an ultra‑thin diffusion barrier (≈0.1 µm) for rapid O₂ ↔ CO₂ exchange.Extremely flattened cells covering ~95 % of alveolar surface; nucleus lies at the basal side; appear as a thin rim in light microscopy, as a delicate membrane in EM.
Type II pneumocytesAlveolar walls, interspersed among type I cellsProduce pulmonary surfactant (reduces surface tension) and act as stem cells for type I regeneration.Cuboidal cells with abundant lamellar bodies (dense, stacked membranes) visible in EM; stain more eosinophilic than type I cells in light microscopy.
Smooth muscleBronchi, bronchioles and small arterioles within the lung parenchyma.Regulates airway calibre (bronchoconstriction/dilation) and pulmonary vascular resistance; essential for ventilation‑perfusion matching.Spindle‑shaped cells with centrally placed nuclei; no striations; appear as a thin pink layer surrounding the airway or vessel in histological sections.
CapillariesSurround each alveolus; dense network throughout the respiratory zone.Provide a massive surface area (~70 m²) and a very short diffusion distance for O₂ uptake and CO₂ release.Thin‑walled vessels (single endothelial layer); lumen often contains red blood cells; in cross‑section appear as narrow dark circles adjacent to the alveolar wall.

3. Plan‑Diagram Checklist (Transverse Sections)

When drawing a cross‑section of each region, label the features listed below. Use the same colour‑coding as your textbook for consistency.

  • Trachea: C‑shaped cartilage rings, ciliated pseudostratified columnar epithelium, goblet cells, submucosal seromucous glands, outer smooth‑muscle layer, adventitia.

  • Main / Lobar Bronchus: Plate‑like cartilage, ciliated epithelium with goblet cells, inner smooth‑muscle layer, mucous glands (in larger bronchi), adventitia.

  • Bronchiole (terminal): No cartilage; simple cuboidal epithelium (ciliated in terminal bronchioles, non‑ciliated in respiratory bronchioles), sparse goblet cells, club cells, smooth‑muscle sheath, surrounding capillaries.

  • Alveolar duct: Thin wall composed mainly of type I pneumocytes, occasional type II cells, elastic fibres, and a dense capillary sheath.

  • Alveolus: Type I pneumocytes (ultra‑thin squamous cells), type II pneumocytes (surfactant‑filled lamellar bodies), basal lamina, elastic interstitium, capillary network.

4. Integration of Structures in the Gas‑Exchange Process

  1. Air entry & filtration – Nasal hairs and mucus (from goblet cells) trap particles; ciliated epithelium transports the mucus toward the pharynx (mucociliary clearance).

  2. Airway support – Hyaline cartilage rings/plates keep the trachea and larger bronchi open, preventing collapse during the negative pressure of inspiration.

  3. Airway calibre control – Autonomic signals cause smooth‑muscle contraction (bronchoconstriction) or relaxation (bronchodilation), altering airway resistance and influencing regional ventilation.

  4. Transition to gas exchange – In bronchioles cartilage disappears, goblet cells become rare, and club cells appear, preparing the airway for diffusion.

  5. Alveolar diffusion surface – Type I pneumocytes provide a minimal diffusion barrier; surfactant from type II cells reduces surface tension, keeping the thin squamous layer stable during breathing cycles.

  6. Capillary network – Thin‑walled pulmonary capillaries lie directly against the alveolar wall, maximising surface area and minimising diffusion distance.

  7. Diffusion of gases – Governed by Fick’s law:

    J = -D ΔC / Δx

    where J = flux of O₂ or CO₂, D = diffusion coefficient, ΔC = concentration gradient, and Δx = diffusion distance (kept ~0.1 µm by the type I epithelium).

5. Summary Checklist (AO1 + AO2)

  1. Identify hyaline cartilage in the trachea (C‑shaped) and bronchi (plate‑like).

  2. Locate ciliated pseudostratified columnar epithelium and goblet cells throughout the conducting zone; note their reduction in bronchioles.

  3. Recognise club (Clara) cells in terminal and respiratory bronchioles.

  4. Describe type I pneumocytes (ultra‑thin squamous cells) lining alveoli and the supporting role of type II cells (surfactant). Include a brief description of alveolar ducts.

  5. Identify smooth‑muscle layers in bronchi, bronchioles and pulmonary arterioles and explain their regulatory function.

  6. Explain the dense capillary network that envelops each alveolus and its importance for rapid gas diffusion.

  7. Distinguish clearly between the conducting zone (air‑conduction) and the respiratory zone (site of gas exchange).

  8. When presented with a microscope or electron‑micrograph, use the “How to recognise” cues in the table to label each structure accurately.

Suggested diagram: transverse view of the respiratory tract showing (from top to bottom) cartilage rings, ciliated epithelium with goblet cells, club cells, smooth muscle, type I & II pneumocytes, surfactant, and the surrounding capillary network.