Explain how the structure of the human trachea and bronchial tree facilitates the movement of air to and from the alveoli whilst maintaining airway patency.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
C‑shaped cartilage rings in the trachea give rigid support, keeping the lumen open and preventing collapse. The open part of the C faces the oesophagus, so the oesophagus can expand during swallowing without blocking the airway. The bronchi then bifurcate repeatedly into smaller bronchioles, which increases the total cross‑sectional area and reduces resistance, allowing air to reach all lung regions. The smooth muscle in bronchiolar walls can contract or relax, changing airway diameter and resistance to regulate airflow to different lung areas.
Examiner tips
- Use the exact terms: C‑shaped rings, oesophagus, cross‑sectional area, smooth muscle.
- Show the causal link: support → patency; branching → reduced resistance; muscle → regulation.
- Keep each point brief and directly tied to a mark scheme point.
Common mistakes
- Confusing the C‑shaped rings with complete rings; forgetting the oesophageal opening.
- Not mentioning the increase in cross‑sectional area or the role of smooth muscle in bronchioles.
Mark scheme (4 marks)
- C-shaped / incomplete rings of cartilage in the trachea prevent collapse of the airway by providing rigid structural support, maintaining an open lumen.
- The gap / opening in the C-shaped rings faces the oesophagus, allowing the oesophagus to expand during swallowing without obstruction.
- Progressive branching / repeated bifurcation of bronchi into bronchioles greatly increases total cross-sectional area, reducing airflow resistance and allowing air to reach all regions of the lungs.
- Smooth muscle in the walls of bronchioles can contract or relax to adjust airway diameter / resistance, regulating airflow to different parts of the lung.
Key terms in this question
airway patency · bronchial tree
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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