Cambridge A-Level Biology – The Gas Exchange System
Objective
Describe the functions of cartilage, smooth muscle, elastic fibres and squamous epithelium in the gas exchange system.
Overview of the Gas Exchange System
The gas exchange system of vertebrates consists of a series of specialised structures that transport air from the external environment to the respiratory surface where oxygen (O₂) diffuses into the blood and carbon dioxide (CO₂) diffuses out. Key components include the nasal cavity, trachea, bronchi, bronchioles, and alveoli (or air sacs). Each component contains specific tissues that support its mechanical and functional requirements.
Key Tissues Involved
Cartilage
Smooth muscle
Elastic fibres
Squamous epithelium (simple squamous epithelium)
Functions of Individual Tissues
1. Cartilage
Cartilage is found in the trachea and the larger bronchi as C‑shaped rings (hyaline cartilage) that:
Provide structural support, keeping the airway open during respiration.
Allow flexibility so the airway can bend without collapsing.
Resist compressive forces generated by pressure changes during inhalation and exhalation.
2. Smooth Muscle
Smooth muscle is present in the walls of bronchi and bronchioles. Its functions include:
Regulating airway diameter (bronchoconstriction and bronchodilation) to control airflow resistance.
Facilitating distribution of air to different regions of the lung according to metabolic demand.
Participating in protective reflexes (e.g., narrowing the airway in response to irritants).
3. Elastic Fibres
Elastic fibres are abundant in the walls of larger bronchi and especially in the alveolar walls. They serve to:
Allow the airway and alveolar walls to stretch during inhalation and recoil during exhalation.
Maintain the structural integrity of the lung tissue after repeated cycles of expansion and contraction.
Assist in the passive expulsion of air, reducing the energy cost of breathing.
4. Squamous Epithelium
Simple squamous epithelium lines the alveoli and the capillaries that surround them. Its primary role is to:
Provide an extremely thin diffusion barrier (≈0.5 µm) that permits rapid exchange of O₂ and CO₂ between air and blood.
Minimise the diffusion distance, thereby maximising the rate of gas exchange as described by Fick’s law:
\$\text{Rate of diffusion} = \frac{D \times A \times (P1 - P2)}{T}\$
where \(D\) is the diffusion coefficient, \(A\) the surface area, \(P1 - P2\) the partial pressure difference, and \(T\) the thickness of the membrane.
Facilitate the formation of a continuous, fluid‑filled surface that remains moist, which is essential for gas solubility.
Summary Table
Structure
Key Tissue
Primary Function in Gas Exchange System
Trachea & Large Bronchi
Cartilage (C‑shaped rings)
Maintain airway patency and provide flexibility.
Bronchi & Bronchioles
Smooth Muscle
Control airway diameter and regulate airflow distribution.
Bronchi, Bronchioles, Alveolar Walls
Elastic Fibres
Allow stretch and recoil during breathing cycles.
Alveoli (air sacs) and Capillary Walls
Simple Squamous Epithelium
Provide a thin diffusion barrier for rapid O₂/CO₂ exchange.
Suggested Diagram
Suggested diagram: Cross‑section of a bronchus showing C‑shaped cartilage, smooth muscle layer, elastic fibres, and the transition to alveolar walls lined by simple squamous epithelium.
Key Points to Remember
Cartilage prevents airway collapse while allowing flexibility.
Smooth muscle modulates airway resistance to match ventilation with metabolic needs.
Elastic fibres enable the lung to return to its resting volume after each breath.
Simple squamous epithelium minimises diffusion distance, maximising the efficiency of gas exchange.