Cambridge A-Level Biology – The Gas Exchange System
The Gas Exchange System
The respiratory tract is lined with specialised epithelial structures that protect the delicate alveolar surfaces, keep the airways clear of debris and pathogens, and maintain optimal conditions for gas diffusion.
Key Cellular Components
The lining of the trachea, bronchi and larger bronchioles consists mainly of three types of cells/glands:
Ciliated epithelial cells
Goblet cells (mucus‑secreting epithelial cells)
Mucous glands (submucosal glands)
Ciliated Epithelial Cells
These columnar cells bear numerous motile cilia on their apical surface. Their functions are:
Generate a coordinated, rhythmic beating motion that propels the mucus layer towards the pharynx.
Transport trapped particles, microorganisms and excess secretions out of the lower respiratory tract (the “mucociliary escalator”).
Help maintain a moist surface, which is essential for efficient gas diffusion.
Goblet Cells
Goblet cells are interspersed among the ciliated cells and secrete mucus directly onto the epithelial surface. Their roles include:
Production of a thin, gel‑like mucus that traps inhaled dust, pollen, microbes and other foreign particles.
Providing a protective barrier that prevents desiccation of the underlying epithelium.
Containing antimicrobial peptides (e.g., lysozyme, defensins) that neutralise pathogens.
Mucous Glands (Submucosal Glands)
Located deeper in the submucosa of the trachea and larger bronchi, these glands secrete larger volumes of mucus and serous fluid. Their contributions are:
Supplementing the mucus layer produced by goblet cells, especially during increased respiratory demand (e.g., exercise, infection).
Providing serous fluid rich in enzymes and immunoglobulins that enhance the antimicrobial properties of the mucus.
Regulating the viscosity of the mucus to optimise ciliary transport.
Integrated Function – The Mucociliary Clearance System
The coordinated activity of ciliated epithelial cells, goblet cells and mucous glands forms the mucociliary clearance system, which is essential for respiratory health. The process can be summarised as follows:
Goblet cells and mucous glands secrete mucus that traps inhaled particles.
Cilia beat in a coordinated wave, moving the mucus‑laden debris toward the pharynx.
The mucus is either expectorated or swallowed, removing contaminants from the lower airways.