Cambridge International AS & A Level Biology – Topic 9: The Human Gas‑Exchange System
1. Overview of the Respiratory Pathway
Air enters the body through the nose or mouth, passes down the trachea, divides into the right and left primary bronchi, branches repeatedly into secondary and tertiary bronchi, then into bronchioles, and finally reaches the alveolar sacs where gas exchange occurs across a dense capillary network. The structure of each component is closely matched to its function – a key requirement of the Cambridge 9700 syllabus.
2. Structural Description (Histology, Function & Required Diagrams)
| Structure | Histological Features (syllabus wording) | Specific Function(s) | Diagram(s) to Sketch |
|---|
| Lungs (right & left) | - Encased in visceral pleura (inner) and parietal pleura (outer)
- Right lung – 3 lobes; left lung – 2 lobes (upper & lower)
- Contain the entire bronchial tree and alveolar tissue
| - Provide the space for the alveolar‑capillary membrane where O₂ enters blood and CO₂ is removed.
- Elastic recoil of lung tissue assists both inhalation and exhalation.
| Plan‑view of the two lungs showing lobes; transverse section through a lung lobe showing pleura and bronchial tree. |
| Trachea | - C‑shaped hyaline cartilage rings** (≈ 2 cm apart)
- Inner lining – pseudostratified ciliated columnar epithelium with abundant goblet cells
- Submucosa contains seromucous glands
| - Rigid tube – cartilage prevents collapse during breathing.
- Cilia + mucus form the mucociliary escalator, moving trapped particles upward (clearance).
| Plan‑view showing C‑shaped rings; transverse section labelling cartilage, epithelium, goblet cells & glands. |
| Primary Bronchi (right & left) | - Irregular cartilage plates (larger than in smaller bronchi)
- Pseudostratified ciliated columnar epithelium with goblet cells
- Submucosal seromucous glands (prominent)
| - Distribute inhaled air to each lung lobe.
- Cartilage maintains airway patency.
- Cilia & mucus continue to clear debris.
| Transverse section of a primary bronchus – label cartilage plates, epithelium, goblet cells, glands. |
| Secondary (lobar) & Tertiary (segmental) Bronchi | - Cartilage plates become smaller and more irregular with each division
- Pseudostratified ciliated columnar epithelium with goblet cells (decreasing in number distally)
- Seromucous glands present but reduced in size
| - Further subdivision of airflow to each lung segment.
- Cartilage still provides support; cilia continue mucociliary clearance.
| Transverse section of a secondary bronchus – show reduced cartilage and glands. |
| Bronchioles (terminal & respiratory) | - No cartilage.
- Wall contains a layer of smooth muscle (circular & longitudinal fibres).
- Lining epithelium:
- Terminal bronchioles – simple cuboidal epithelium.
- Respiratory bronchioles – simple squamous epithelium (few cuboidal cells).
- Few or no goblet cells; mucus produced by Clara (club) cells.
| - Regulate airway resistance via bronchoconstriction & bronchodilation (smooth‑muscle tone).
- Provide a conduit to the alveolar ducts and sacs.
| Plan‑view of the bronchial tree showing transition from bronchi to bronchioles; transverse section of a terminal bronchiole (label smooth muscle & cuboidal epithelium). |
| Alveoli (alveolar sacs) | - Wall formed by a single layer of flat type I pneumocytes (squamous epithelili)
- Interspersed type II pneumocytes that secrete pulmonary surfactant
- Elastic fibres in the interstitium
- Surrounded by a dense network of pulmonary capillaries (single endothelial cell layer)
| - Provide a huge surface area (~ 70 m²) for diffusion of O₂ and CO₂.
- Thin diffusion barrier (~ 0.5 µm) minimises distance for gas exchange.
- Surfactant reduces surface tension, preventing alveolar collapse (atelectasis).
| Transverse section of an alveolus – label type I & II cells, surfactant layer, elastic fibres and adjacent capillary. |
| Capillary Network (pulmonary) | - Capillary wall – single layer of endothelial cells with a thin basal lamina
- Extensive anastomoses give uniform perfusion of all alveoli
- Ventilation‑perfusion matching ensures each alveolus receives an appropriate blood supply
| - Rapid diffusion of O₂ into blood and CO₂ out of blood.
- Thin barrier and large contact area with alveoli maximise exchange efficiency.
| Plan‑view of the pulmonary capillary plexus around an alveolar sac; transverse section showing capillary wall adjacent to type I cell. |
3. Key Diagrams to Master (Plan & Transverse Views)
- Figure 1: Plan‑view of the trachea – C‑shaped cartilage rings and lumen.
- Figure 2: Transverse section of the trachea – cartilage, pseudostratified epithelium, goblet cells, seromucous glands.
- Figure 3: Transverse section of a primary bronchus – irregular cartilage plates, ciliated epithelium, goblet cells.
- Figure 4: Transverse section of a secondary bronchus – reduced cartilage, fewer glands.
- Figure 5: Transverse section of a terminal bronchiole – smooth‑muscle layer, simple cuboidal epithelium.
- Figure 6: Transverse section of a respiratory bronchiole – simple squamous epithelium, occasional alveolar outpouchings.
- Figure 7: Transverse section of an alveolus – type I & II cells, surfactant, elastic fibres, surrounding capillary.
- Figure 8: Plan‑view of the pulmonary capillary network around an alveolar sac.
All sketches should be labelled clearly with the histological features listed in the table. Both a plan view (looking down the airway) and a transverse view (cross‑section) are required for each structure in the exam.
4. Gas‑Exchange Process – Diffusion
Oxygen diffuses from the alveolar air into pulmonary capillary blood; carbon dioxide diffuses in the opposite direction, driven by concentration gradients.
Fick’s First Law of Diffusion
\( J = -D \dfrac{\Delta C}{\Delta x} \)
- J – diffusion flux (amount per unit area per unit time)
- D – diffusion coefficient (depends on the gas)
- ΔC – concentration difference between alveolar air and blood
- Δx – thickness of the diffusion barrier (≈ 0.5 µm)
Factors that increase the rate of diffusion (syllabus list)
- Large surface area – millions of alveoli give ≈ 70 m² of exchange surface.
- Thin diffusion barrier – type I pneumocyte + capillary endothelium (≈ 0.5 µm).
- Steep concentration gradient – high PO₂ in alveoli, low PO₂ in deoxygenated blood (and vice‑versa for CO₂).
- High diffusion coefficient – O₂ and CO₂ are small, non‑polar molecules.
5. Integration of Structure and Function
- The hierarchical branching from trachea → bronchi → bronchioles → alveoli ensures that fresh air reaches every alveolus with minimal resistance.
- Cartilage in the trachea and bronchi provides rigidity; smooth muscle in bronchioles fine‑tunes airway calibre (bronchoconstriction/bronchodilation).
- Cilia and mucus trap and remove particles, protecting the delicate alveolar surface.
- Surfactant, secreted by type II cells, lowers surface tension, keeping alveoli open and maintaining a thin diffusion distance.
- Elastic fibres in the lung parenchyma give the lungs their recoil, assisting both inhalation and exhalation.
- Ventilation‑perfusion matching, achieved by the extensive capillary network, guarantees that well‑ventilated alveoli receive an adequate blood supply.
6. Exam‑Style Checklist (Summary Points)
- Identify each structure (lungs, trachea, primary/secondary/tertiary bronchi, bronchioles, alveoli, capillaries) and list the required histological features: cartilage, ciliated epithelium, goblet cells, seromucous glands, smooth muscle, type I & II pneumocytes, endothelial lining.
- Link every histological feature to its specific function (e.g., C‑shaped cartilage → prevents airway collapse; smooth muscle → regulates airway resistance).
- State and briefly explain the four factors that influence the rate of diffusion.
- Be able to draw and label both a plan view and a transverse section for each of the following:
- Trachea
- Primary bronchus
- Secondary bronchus
- Terminal bronchiole
- Alveolus with surrounding capillary
- Recall Fick’s law and use it qualitatively to predict how changes in surface area, barrier thickness, concentration gradient or diffusion coefficient affect gas exchange.
7. Suggested Further Reading
- Cambridge International AS & A Level Biology (9700) – Topic 9 Gas‑exchange system.
- Human Physiology (e.g., Guyton & Hall) – Chapter on the respiratory system, focusing on histology.
- BBC Bitesize – Respiratory System (concise diagrams and summaries).
- “Respiratory Physiology” – sections on diffusion, surfactant and ventilation‑perfusion matching.