Edexcel IGCSE Biology · Spec 2.46
Bronchioles and Pleural Membranes
Covers trace the route of air to the alveoli, State the roles of bronchioles and pleural membranes and Relate thorax structures to ventilation.
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Bronchioles and Pleural Membranes
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Explained
The route air takes, and how the chest moves it
Air enters through the nose and mouth, passes down the trachea, and reaches the lungs. The trachea divides into two bronchi, one to each lung, and each bronchus branches repeatedly into smaller and smaller bronchioles.
At the end of the finest bronchioles are the alveoli, tiny air sacs where gas exchange actually happens. The order to learn is trachea, bronchus, bronchiole, alveolus, and the branching structure is often called the bronchial tree for obvious reasons.
Keeping the airway open and clean
The trachea and bronchi contain rings of cartilage, which hold the airway open so it does not collapse when the pressure inside falls during breathing in.
Their lining has goblet cells that make mucus, which traps dust and bacteria, and ciliated cells whose cilia sweep that mucus up towards the throat to be swallowed. This is the system that tar destroys in a smoker.
Bronchioles have no cartilage. They have muscle in their walls instead, which can narrow them, and that narrowing is what happens in an asthma attack.
The pleural membranes
Two pleural membranes surround each lung, one attached to the lung and one lining the inside of the chest wall. Between them is a thin layer of pleural fluid.
The fluid does two things. It lubricates, so the lungs slide against the chest wall without friction as they expand and contract. And it holds the two membranes together, so that when the chest wall moves outwards the lung is pulled outwards with it.
That second point is the one usually missed. The lungs have no muscle of their own and cannot inflate themselves. They move because the chest moves and the pleural fluid links them to it.
Ventilation
Breathing in: the intercostal muscles contract, pulling the ribcage up and out, and the diaphragm contracts and flattens, moving down. The volume of the thorax increases, so the pressure inside falls below atmospheric pressure, and air flows in from high pressure to low pressure.
Breathing out: those muscles relax, the ribcage moves down and in, and the diaphragm returns to its domed shape. The volume decreases, the pressure rises above atmospheric, and air flows out.
Volume changes first, pressure follows, and air moves down the pressure gradient. That order is the answer to every ventilation question.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks how breathing in happens, for three marks, and credits exactly three things: the diaphragm moves down or flattens, the volume of the thorax increases, and the pressure decreases so air flows from high pressure to low pressure.
Its guidance is unusually restrictive. It says to ignore contracts, because that word appeared in the question stem, and to ignore references to the ribs, other muscles or the lungs.
That second instruction surprises people, since the ribs genuinely are involved. The question asked about the diaphragm, so only the diaphragm chain earns credit, and correct extra material about the ribs is simply passed over. Answer the structure named.
The first instruction is the more general lesson, and it appears across these papers. Repeating a word given to you in the question demonstrates nothing, so it earns nothing.
On a neighbouring part, asked why a bronchiole counts as an organ, the mark scheme wants that it contains more than one tissue, accepting a group or collection of tissues. Size and importance have nothing to do with it.
Why the branching matters
Each division makes the tubes narrower but there are many more of them, so the total cross sectional area increases and the air slows down as it goes deeper. Slower moving air spends longer in contact with the alveoli, which gives more time for diffusion.
Spec 2.46
What you need to know
- Trace the route of air to the alveoli
- State the roles of bronchioles and pleural membranes
- Relate thorax structures to ventilation
Active recall
Quick check
Answer each question before opening the answer.
Put bronchi, alveoli and bronchioles in the order reached by inhaled air
Bronchi, bronchioles, alveoli
What is the role of pleural fluid?
It reduces friction as the lungs move against the chest wall
What happens to the diaphragm during inhalation?
It contracts and flattens
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