describe the functions of cartilage, smooth muscle, elastic fibres and squamous epithelium in the gas exchange system

Objective

Describe the functions of cartilage, smooth muscle, elastic fibres and simple squamous epithelium in the gas‑exchange system and relate each tissue to the relevant respiratory structures, as required by Cambridge International AS & A Level Biology (9700) – Topic 9.1.

1. Overview of the Gas‑Exchange System (in order of airflow)

1. Nasal cavity – lined by pseudostratified ciliated columnar epithelium with goblet cells; filters, warms and humidifies inhaled air.
2. Trachea – a rigid tube 10–12 cm long; wall layers: (i) mucosa (ciliated pseudostratified columnar epithelium + goblet cells), (ii) submucosa, (iii) C‑shaped hyaline cartilage rings, (iv) adventitia. Provides a patent airway.
3. Bronchi (primary and secondary) – divide the trachea into left and right branches; wall contains incomplete C‑shaped cartilage plates, a circular layer of smooth muscle, abundant elastic laminae and the same mucosal epithelium as the trachea.
4. Bronchioles – conducting bronchioles lack cartilage; wall consists of a thin mucosa, a layer of smooth muscle (circular in larger bronchioles, both circular and longitudinal in terminal bronchioles) and a rich elastic fibre network.
5. Alveolar sacs (alveoli) – terminal air‑spaces formed by clusters of thin‑walled sacs; each sac is lined by type I pneumocytes (simple squamous epithelium) and a few type II pneumocytes (secrete surfactant). Surrounded by a dense capillary network.
6. Capillary network – continuous capillaries whose endothelium is also simple squamous; together with the alveolar epithelium they form the respiratory membrane.

2. Histological Distribution of Key Tissues

3. Functions of the Four Core Tissues

3.1 Cartilage (hyaline)

• Maintain airway patency: C‑shaped rings resist the inward collapse that occurs when intrathoracic pressure becomes negative during inspiration.
• Provide flexibility: Incomplete rings in the bronchi allow slight bending and twisting of the airway without loss of lumen diameter.
• Protect against external compression: Rigid framework shields the trachea from neck movements and external forces.

3.2 Smooth Muscle (non‑striated)

• Regulate airway calibre: Contraction → bronchoconstriction; relaxation → bronchodilation, thereby altering airway resistance.
• Match ventilation to perfusion: Adjusts airflow to lung regions with higher metabolic demand (e.g., during exercise).
• Protective reflexes: Rapid constriction in response to irritants, cold air or allergens helps limit the entry of harmful particles.

3.3 Elastic Fibres (elastin)

• Elastic recoil: Stores mechanical energy during lung expansion and releases it during passive exhalation, reducing the work of breathing.
• Durability: Allows repeated stretch‑recoil cycles without permanent deformation, essential for lifelong respiratory function.
• Alveolar stability: Prevents alveolar collapse (atelectasis) at the end of expiration by maintaining a baseline outward tension.

3.4 Simple Squamous Epithelium (type I pneumocytes)

• Minimise diffusion distance: Thickness ≈ 0.5 µm, providing the shortest possible path for O₂ and CO₂ (Fick’s law: Rate ∝ A · ΔP / d).
• Maximise surface area: Hundreds of millions of alveoli give a total gas‑exchange area of ≈ 70 m² in an adult.
• Maintain a moist surface: A thin fluid layer on the epithelium ensures gases remain dissolved, facilitating diffusion.

4. The Respiratory Membrane

The respiratory membrane is the composite barrier through which gases diffuse. It consists of:

• Alveolar type I pneumocyte (simple squamous epithelium)
• Fused basement membranes of the alveolus and capillary (≈ 0.05 µm combined)
• Capillary endothelium (simple squamous)

Overall thickness is about 0.6 µm, and the surface area is ~70 m². According to Fick’s law, the diffusion rate (V) is directly proportional to the surface area (A) and the partial pressure difference (ΔP) and inversely proportional to the diffusion distance (d):

V = (D · A · ΔP) / d, where D is the diffusion coefficient. This relationship explains why any increase in membrane thickness (e.g., pulmonary fibrosis) or decrease in surface area (e.g., emphysema) markedly impairs gas exchange.

5. Quantitative Relationships (AO2)

• Fick’s law of diffusion – as above; useful for calculations of O₂ uptake or CO₂ elimination.
• Poiseuille’s equation for airway resistance – R = 8 η l / π r⁴ (η = viscosity, l = length, r = radius). Highlights the profound effect of smooth‑muscle‑induced changes in radius on airflow.
• Alveolar ventilation equation – VA = (VT – VD) × f (VT = tidal volume, V_D = dead space, f = respiratory rate). Links mechanical changes (e.g., bronchoconstriction) to overall gas exchange efficiency.

6. Mucociliary Escalator and Clinical Relevance

The combination of ciliated pseudostratified columnar epithelium and goblet‑cell mucus forms the mucociliary escalator. Its functions are to:

• Trap inhaled particles and microorganisms in a sticky mucus layer.
• Transport the mucus upward toward the pharynx where it can be swallowed or expectorated.

Clinical note: Impaired ciliary activity (as in smoking, COPD or cystic fibrosis) leads to mucus retention, bacterial colonisation and chronic infection, illustrating the importance of this defence mechanism.

7. Summary Table – Core Tissues and Their Primary Roles

8. Suggested Diagram

9. Key Points to Remember

1. Cartilage prevents airway collapse during the negative pressure of inhalation while allowing the trachea to bend.
2. Smooth muscle fine‑tunes airway resistance; its contraction or relaxation directly influences ventilation distribution (Poiseuille’s equation).
3. Elastic fibres store mechanical energy during inhalation and release it during passive exhalation, reducing the metabolic cost of breathing.
4. Simple squamous epithelium, together with capillary endothelium, forms a ~0.6 µm thick respiratory membrane that maximises diffusion according to Fick’s law.
5. The mucociliary escalator (ciliated epithelium + goblet‑cell mucus) cleans the conducting airways; dysfunction contributes to diseases such as COPD and cystic fibrosis.
6. Quantitative relationships (Fick’s law, Poiseuille’s equation, alveolar ventilation equation) link structural features to functional performance and are often examined in AO2 questions.