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Key concept
Air enters through the nose or mouth, passes through the trachea (reinforced with C-shaped cartilage rings), splits into two bronchi (one to each lung), which branch into smaller bronchioles, ending at clusters of alveoli where gas exchange occurs.
Labelling diagrams of the respiratory system is common on Paper 2. Beyond naming the structures, you need to know the function of cartilage rings (keeping the airway open) and the mucus-cilia defence system that lines the trachea and bronchi.
The pathway of air
Inhaled air follows this route: nose (or mouth) → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. The trachea is a single tube that sits in front of the oesophagus and is kept open by C-shaped rings of cartilage. These rings prevent the trachea from collapsing during inhalation when pressure inside drops.
The trachea splits into two bronchi (singular: bronchus), one entering each lung. Each bronchus branches repeatedly into smaller and smaller bronchioles. The bronchioles end in clusters of tiny air sacs called alveoli, where gas exchange takes place.
Defence in the airways
The lining of the trachea and bronchi contains goblet cells that secrete mucus. This sticky mucus traps dust, bacteria and other particles inhaled with the air. Ciliated epithelial cells have tiny hair-like projections (cilia) that beat rhythmically, sweeping the mucus and trapped particles upward toward the throat. There it is swallowed and the pathogens are destroyed by stomach acid.
Smoking paralyses and eventually destroys these cilia. Without the ciliary escalator, mucus and pathogens accumulate in the lungs, leading to a persistent cough (smoker's cough) and increased risk of lung infections.
Structure and function summary
Structure
Function
Nose
Warms, moistens and filters incoming air; nasal hairs trap large particles
Trachea
Conducts air to the lungs; cartilage rings keep it open
Bronchi
Branch from trachea into each lung; continue to conduct air
Bronchioles
Smaller branches that direct air to alveoli; smooth muscle can constrict to control airflow
Alveoli
Tiny air sacs where gas exchange with blood occurs
Diaphragm
Sheet of muscle below the lungs; its contraction and relaxation drives ventilation
Try a focused question (2 marks)
State the function of the cartilage rings in the trachea.
Show the answer and marking guidance
The C-shaped cartilage rings support the trachea and keep the airway open.
They prevent the trachea from collapsing when air pressure inside decreases during inhalation.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Calling the trachea the food pipe: the oesophagus is the food pipe; the trachea is the windpipe.
Saying cartilage rings form complete circles: they are C-shaped, open at the back to allow the oesophagus to expand when food passes.
Confusing bronchi with bronchioles: bronchi are larger and have cartilage; bronchioles are smaller and have smooth muscle.
Forgetting to mention goblet cells when describing mucus production.