Cambridge International AS & A Level Biology 9700: Gas exchange – Revision Notes

These revision notes cover Topic 9, Gas exchange, in the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: section 9.1 The gas exchange system, outcomes 1 to 7. It is AS Level content, examined on Papers 1 and 2, and Paper 4 can draw on it. For full explanations and worked examples, use the Gas exchange study guide.

Course hub: Cambridge A Level Biology · Checklist: 9700 checklist · Exam-style questions: Gas exchange practice · Check your AS topics: 9700 AS diagnostic

The structures (9.1.1)

Air route: trachea → bronchi (one per lung) → bronchioles (many branches) → alveoli, with a capillary network wrapped around every alveolus. The lungs hold the bronchi, bronchioles, alveoli and their blood supply.

The syllabus limits this outcome to those six structures, so do not spend revision time on the mechanism of breathing for 9.1.

Tissue distribution (9.1.2)

Tissue	Trachea	Bronchus	Bronchiole	Alveolus
Cartilage	C-shaped rings	Irregular plates	–	–
Ciliated epithelium	✓	✓	✓ larger ones	–
Goblet cells	✓	✓	few / none in smallest	–
Mucous glands	✓	✓	–	–
Smooth muscle	✓	✓	✓ large proportion	–
Elastic fibres	✓	✓	✓	✓
Squamous epithelium	–	–	–	✓
Capillaries	✓	✓	✓	✓ dense network

Two trends: cartilage decreases then disappears before the bronchioles; the proportion of smooth muscle rises from trachea to bronchioles.

Recognition cues (9.1.3 and 9.1.4)

You see	It is
Pale glassy matrix, cells in small spaces	Cartilage
Tall cells with a surface fringe	Ciliated epithelium
Cup-shaped pale cells among ciliated cells	Goblet cells
Very thin flattened cells lining air spaces	Squamous epithelium
Bands of spindle-shaped cells around a tube	Smooth muscle
Tiny vessels with red blood cells in single file	Capillaries

Airway	Deciding feature
Trachea	C-shaped cartilage ring; glands; widest
Bronchus	Cartilage in separate plates
Bronchiole	No cartilage; smooth muscle ring; folded lining
Alveoli	Thin-walled air spaces; lace-like pattern

Method in steps: identifying an airway

1. Look for cartilage. A C-shaped ring means trachea; separate plates mean bronchus; none means bronchiole or alveolus.
2. No cartilage? A tube with a ring of smooth muscle and a folded lining is a bronchiole.
3. Thin-walled air spaces with no muscle are alveoli.
4. Confirm with the lining: ciliated epithelium with goblet cells in the airways; flattened squamous cells in the alveoli.
5. In your answer, give the observable feature as the reason, for example “no cartilage visible”, not a fact you cannot see.

Light micrograph or electron micrograph?

• Light micrographs (slides and photomicrographs) show tissue layers clearly: cartilage, glands, smooth muscle and the epithelium with its goblet cells. Use them for plan diagrams and for identifying the airway.
• Electron micrographs show much finer detail: individual cilia on an epithelial cell, mucus granules inside a goblet cell, and the thin squamous alveolar cell lying against the capillary endothelium, with a red blood cell in the capillary.
• Check the magnification or scale bar first: it tells you which kind of image you are looking at and what size of structure to expect.

Plan diagrams: the checklist

For TS of the trachea or bronchus wall:

1. Outline tissue layers only; no individual cells.
2. Single, clear, continuous pencil lines; no shading.
3. Layers in the correct proportions (measure the photomicrograph).
4. Show every layer from lumen to outer edge.
5. Label lines ruled, touching the layer, not crossing.

Keeping the system healthy (9.1.5)

Method in steps

1. Goblet cells and mucous glands secrete mucus.
2. Mucus traps dust, pollen and pathogens.
3. Cilia on ciliated epithelial cells beat and move the mucus up towards the throat.
4. Mucus is swallowed; particles are kept away from the alveoli.

Tissue functions (9.1.6)

Tissue	Function in one line
Cartilage	Supports trachea and bronchi; keeps them open so they do not collapse
Smooth muscle	Contracts to narrow and relaxes to widen bronchi and bronchioles, controlling airflow
Elastic fibres	Stretch as alveoli fill; recoil to help expel air
Squamous epithelium	Very thin; short diffusion distance in the alveolar wall

Structure matches function down the airways

• Trachea and bronchi must stay open against pressure changes, so they have cartilage. They carry the most dust-laden air, so they have the most goblet cells, glands and cilia.
• Bronchioles need an adjustable diameter to control airflow to groups of alveoli, so smooth muscle makes up a large part of the wall and there is no cartilage.
• Alveoli need the shortest possible diffusion distance, so the wall is squamous epithelium with elastic fibres but no muscle, cartilage or cilia.

Gas exchange in the alveoli (9.1.7)

• O₂: alveolar air → alveolar wall → capillary wall → plasma → red blood cell (binds haemoglobin).
• CO₂: blood → alveolar air → breathed out.
• By diffusion down partial pressure gradients.

Why it is efficient

• Large surface area (hundreds of millions of alveoli).
• Short diffusion distance: two single layers of flattened cells (alveolar squamous epithelium + capillary endothelium).
• Narrow capillaries: red blood cells pressed against the wall and slowed.
• Gradients kept steep by blood flow and by breathing, which replaces alveolar air.
• Moist surface: gases dissolve before diffusing.

Small worked reminders

• Magnification. A trachea 18 mm in diameter appears 54 mm across in a photograph. Magnification = 54 ÷ 18 = ×3.
• Actual size. An alveolus measures 20 mm across on a micrograph at ×100. 20 ÷ 100 = 0.2 mm = 200 µm.
• Gradient. Subtract the lower partial pressure from the higher one and say which way the gas moves.
• Scale bar. A scale bar 10 mm long labelled 50 µm gives magnification 10 000 µm ÷ 50 µm = ×200. Measure the bar yourself; do not assume its length.

Link to Topic 8

The capillary network around the alveoli is supplied by the pulmonary artery (deoxygenated blood from the right ventricle) and drained by the pulmonary vein (oxygenated blood to the left atrium). Oxygen that crosses the alveolar wall is loaded onto haemoglobin at high pO₂, and carbon dioxide released in the lungs comes from hydrogencarbonate ions and carbaminohaemoglobin. See the Transport in mammals revision notes.

Command words for this topic

• Describe the distribution: say which tissue is present in which structure, and how it changes along the airways.
• Describe the functions: one clear statement of what each tissue does.
• Explain: add the reason, e.g. “cartilage keeps the trachea open so that it does not collapse”.
• Suggest: apply the ideas to an unfamiliar context, such as a disease or an animal you have not studied.

Must-know distinctions

• Mucus traps; cilia move. Cilia do not trap anything.
• Goblet cells (single cells in the epithelium) vs mucous glands (groups of cells deeper in the wall of the trachea and bronchi).
• Smooth muscle contracts; elastic fibres recoil. Elastic fibres are not muscle.
• Squamous epithelium (alveoli, thin) vs ciliated epithelium (airways, taller cells with cilia).
• Bronchus has cartilage; bronchiole does not.

Quick self-test

1. Name the structure in which gas exchange takes place.
2. State where cartilage is found in the gas exchange system and its shape in each place.
3. Which two structures in 9.1 secrete mucus?
4. Explain how ciliated epithelial cells keep the alveoli clean.
5. State one tissue found in a bronchiole but not in an alveolus, other than ciliated epithelium.
6. What is the function of elastic fibres in the alveolar walls?
7. Why is squamous epithelium suited to the alveolar lining?
8. A photograph at ×4 shows a bronchus 32 mm wide. Calculate its actual width.
9. Alveolar pO₂ is 13.0 kPa and blood arriving has a pO₂ of 5.5 kPa. Calculate the gradient.
10. Give two features of a plan diagram that would lose credit.
11. Suggest why bronchioles have a large proportion of smooth muscle but no cartilage.
12. Which blood vessel brings deoxygenated blood to the alveolar capillaries?

Answers

1. The alveoli.
2. Trachea: C-shaped rings. Bronchi: irregular plates. None in bronchioles or alveoli.
3. Goblet cells and mucous glands.
4. Their cilia beat to move mucus, with trapped particles and pathogens, up and away from the alveoli towards the throat.
5. Smooth muscle (goblet cells in larger bronchioles also accepted).
6. They stretch when the alveoli fill with air and recoil to help push air out.
7. The cells are very thin, so the diffusion distance for gases is short.
8. 32 ÷ 4 = 8 mm.
9. 13.0 − 5.5 = 7.5 kPa, from air into blood.
10. Any two: individual cells drawn; shading; sketchy or broken lines; wrong proportions of layers.
11. They are narrow enough to stay open without support, and smooth muscle lets their diameter change to control airflow.
12. The pulmonary artery.

Where marks are usually lost

• Writing that bronchioles have cartilage “rings”.
• Saying the cilia trap dust or pathogens.
• Writing that mucus is moved down into the lungs.
• Describing elastic fibres as contracting.
• Forgetting that the capillary wall is part of the diffusion distance.
• Saying alveoli have “thin walls” without naming squamous epithelium or giving the one-cell-thick detail.
• Not converting mm to µm before a size calculation.
• Drawing individual cells in a plan diagram, or getting the thickness of the cartilage layer out of proportion.
• Confusing goblet cells with mucous glands.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, Cambridge University Press & Assessment – Topic 9, Gas exchange (section 9.1).