Cambridge International AS & A Level Biology 9700: Gas exchange – Study Guide

This study guide teaches Topic 9, Gas exchange, from the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers the single section 9.1 The gas exchange system, learning outcomes 1 to 7. This is AS Level content: it is examined on Paper 1 (Multiple Choice) and Paper 2 (AS Level Structured Questions), and Paper 4 can draw on it because A Level questions require knowledge of the AS content. The microscope skills in this topic also connect to the practical papers.

Course hub: Cambridge A Level Biology · Printable checklist: 9700 checklist · Condensed version: gas exchange revision notes · Test yourself: gas exchange practice questions

What this topic covers

Outcome	What you must be able to do
9.1.1	Describe the structure of the lungs, trachea, bronchi, bronchioles, alveoli and capillary network
9.1.2	Describe where cartilage, ciliated epithelium, goblet cells, squamous epithelium, smooth muscle and capillaries are found
9.1.3	Recognise those tissues in slides, photomicrographs and electron micrographs
9.1.4	Recognise the trachea, bronchi, bronchioles and alveoli, and make plan diagrams of TS trachea and bronchus walls
9.1.5	Describe how ciliated epithelial cells, goblet cells and mucous glands keep the system healthy
9.1.6	Describe the functions of cartilage, smooth muscle, elastic fibres and squamous epithelium
9.1.7	Describe gas exchange between alveolar air and capillary blood

The syllabus limits 9.1.1 to the six structures listed, so the mechanism of breathing is not part of this section.

9.1.1 Structure of the gas exchange system

Air follows one route to the gas exchange surface:

1. Trachea: a wide tube in the neck, held open by C-shaped rings of cartilage.
2. Bronchi: the trachea divides into two bronchi, one to each lung.
3. Bronchioles: each bronchus branches many times into narrower and narrower bronchioles.
4. Alveoli: the smallest bronchioles end in clusters of tiny air sacs, the alveoli. This is where gas exchange happens.
5. Capillary network: each alveolus is wrapped in a dense network of capillaries, branching from the pulmonary artery and draining to the pulmonary vein.

The lungs are the two organs that contain the bronchi, bronchioles, alveoli and their blood supply.

Worked example 1. A student models each alveolus as a sphere of diameter 0.2 mm and assumes a lung pair contains 4.5 × 10⁸ alveoli. Estimate the total alveolar surface area in m². (Surface area of a sphere = 4πr².)

r = 0.2 mm ÷ 2 = 0.1 mm = 1.0 × 10⁻⁴ m
area of one alveolus = 4 × π × (1.0 × 10⁻⁴)² = 1.2566 × 10⁻⁷ m²
total = 1.2566 × 10⁻⁷ × 4.5 × 10⁸ = 56.5 m²   (3 s.f.)

The point of the estimate: tens of square metres of exchange surface fit inside the chest because the surface is divided into hundreds of millions of tiny sacs.

9.1.2 Where each tissue is found

Tissue	Trachea	Bronchus	Bronchiole	Alveolus
Cartilage	C-shaped rings	Irregular plates	None	None
Ciliated epithelium	Yes	Yes	Yes (larger ones)	None
Goblet cells	Yes	Yes	Few or none in the smallest	None
Smooth muscle	Yes	Yes	Yes (a large proportion of the wall)	None
Elastic fibres	Yes	Yes	Yes	Yes
Squamous epithelium	No	No	No	Yes (lines the alveolus)
Capillaries	Yes	Yes	Yes	Dense network around each alveolus

Mucous glands lie in the wall of the trachea and bronchi, beneath the epithelium. Elastic fibres are listed in outcome 9.1.6 for their function, so learn where they sit too.

Two patterns help you remember the table. Moving down the airways, cartilage decreases and disappears before the bronchioles. The proportion of smooth muscle increases from trachea to bronchioles.

9.1.3 and 9.1.4 Recognising tissues and structures

What each tissue looks like

• Cartilage: a pale, glassy matrix with cells sitting in small spaces; large and obvious in TS of the trachea.
• Ciliated epithelium: column-shaped cells with a fringe of cilia on the surface facing the lumen.
• Goblet cells: cup- or goblet-shaped cells scattered among the ciliated cells, often paler because they are full of mucus.
• Squamous epithelium: very thin, flattened cells; in an electron micrograph, the alveolar wall and capillary wall appear as two thin layers side by side.
• Smooth muscle: bands of spindle-shaped cells, usually running around the airway.
• Capillaries: small vessels, often containing red blood cells in single file.

Telling the airways apart

• Trachea: largest diameter; a thick C-shaped ring of cartilage; ciliated epithelium with goblet cells; mucous glands.
• Bronchus: smaller; cartilage in separate irregular plates rather than a C shape.
• Bronchiole: no cartilage; a ring of smooth muscle; the lining often looks folded.
• Alveoli: thin-walled air spaces with no muscle or cartilage, giving a lace-like pattern.

Worked example 2. In a photomicrograph at ×1500, a ciliated epithelial cell from the trachea measures 30 mm from its base to the tips of its cilia. Calculate its actual length in µm.

convert: 30 mm × 1000 = 30 000 µm
actual size = image size ÷ magnification = 30 000 ÷ 1500 = 20 µm

Always convert to µm before dividing, or your answer will be 1000 times too small.

Drawing a plan diagram of the trachea or bronchus wall

• Draw the outlines of each tissue layer only. Do not draw individual cells.
• Use a sharp pencil, single clear continuous lines, and no shading.
• Keep the correct proportions: for the trachea, the cartilage layer is much thicker than the epithelium.
• Include every layer visible, from the lining to the outer edge: ciliated epithelium, the layer containing glands, cartilage and the outer layer.
• Label with ruled lines that touch the layer they name and do not cross each other.

9.1.5 Keeping the system healthy

Air carries dust, pollen, bacteria and viruses. Three structures deal with them.

• Goblet cells in the epithelium secrete mucus, which traps particles and pathogens.
• Mucous glands in the wall of the trachea and bronchi secrete more mucus onto the surface.
• Ciliated epithelial cells beat their cilia in a coordinated way, moving the mucus and trapped particles up the airways towards the throat, where it is swallowed. This keeps particles out of the alveoli, where they could damage the exchange surface or cause infection.

9.1.6 Functions of four tissues

Tissue	Function
Cartilage	Supports the trachea and bronchi and keeps them open, so they do not collapse when air pressure inside falls during breathing in. The C shape of tracheal rings lets the oesophagus behind expand when food is swallowed
Smooth muscle	Contracts to narrow the airways and relaxes to widen them, controlling the diameter of the bronchi and bronchioles and so the airflow
Elastic fibres	Stretch when the alveoli fill with air, then recoil to help push air out. They also return the airways to their resting size
Squamous epithelium	Very thin cells lining the alveoli, giving a short diffusion distance for gases

Why bronchioles have no cartilage: they are narrow enough to stay open, and their diameter can be changed by smooth muscle.

9.1.7 Gas exchange in the alveoli

Gas exchange happens by diffusion down concentration gradients (for gases, gradients in partial pressure).

• Oxygen diffuses from the air in the alveolus, through the alveolar wall and the capillary wall, into the blood plasma and then into the red blood cells, where it combines with haemoglobin.
• Carbon dioxide diffuses the opposite way, from the blood into the alveolar air, and is then breathed out.

Features that make exchange efficient

• Large surface area: millions of alveoli (see worked example 1).
• Short diffusion distance: the alveolar wall is one layer of squamous epithelial cells and the capillary wall is one layer of endothelial cells.
• Narrow capillaries: red blood cells are pressed against the capillary wall, reducing the distance and slowing flow so there is more time for exchange.
• Steep gradients maintained: the capillary network carries oxygenated blood away and brings deoxygenated blood continuously; breathing keeps replacing the alveolar air.
• Moist lining: gases dissolve in the thin film of water lining the alveolus before diffusing through the wall.

Worked example 3. The partial pressure of oxygen in alveolar air is 13.3 kPa. Blood arriving in an alveolar capillary has a pO₂ of 5.3 kPa, and blood leaving has a pO₂ of 13.1 kPa. The pCO₂ of arriving blood is 6.1 kPa and of alveolar air is 5.3 kPa. Calculate the oxygen gradient at the start and end of the capillary, and the carbon dioxide gradient at the start.

O₂ at start:  13.3 − 5.3  = 8.0 kPa   (air → blood)
O₂ at end:    13.3 − 13.1 = 0.2 kPa
CO₂ at start: 6.1 − 5.3   = 0.8 kPa   (blood → air)

The oxygen gradient almost disappears by the end of the capillary: blood has nearly reached equilibrium with the alveolar air. Constant blood flow brings new deoxygenated blood, restoring the steep gradient.

Common errors

• Saying bronchioles contain cartilage.
• Writing that cilia “trap” dust: mucus traps; cilia move the mucus.
• Saying mucus is moved “down” to the lungs.
• Stating that elastic fibres “contract”: they recoil passively; only muscle contracts.
• Describing the alveolar wall as “one cell thick” without saying the capillary wall is also one cell thick.
• Drawing cells or shading in a plan diagram.

Next steps

• Recall drills and a self-test: Gas exchange revision notes
• Original exam-style questions: Gas exchange practice
• How the blood carries these gases: Transport in mammals study guide
• Check your AS topics: free 9700 AS diagnostic

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).