Cambridge O Level Biology 5090: Human gas exchange (5090) – Revision Notes

For full explanations and worked examples, use the study guide for this topic. These notes condense Topic 9, Human gas exchange, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028: every learning outcome in section 9.1 (9.1.1 to 9.1.9). The syllabus is not tiered, so all of it applies to every candidate.

When you are ready, move on to the practice questions. The course hub and the printable checklist cover the rest of the course, and the free 10-minute diagnostics show where to focus. Diffusion itself is revised in Movement into and out of cells.

Key definitions

Term	Meaning
Gas exchange	Diffusion of oxygen into the blood and carbon dioxide out of it across the alveolar surface
Breathing (ventilation)	Muscle movements that move air into and out of the lungs
Respiration	Chemical reactions in all living cells that release energy from glucose (not breathing)
Inspired air	Air breathed in
Expired air	Air breathed out
Rate of breathing	Number of breaths per minute
Depth of breathing	Volume of air taken in per breath

9.1.4 and 9.1.6 Structures to identify

Airway, in order:

larynx → trachea → bronchi → bronchioles → alveoli (with associated capillaries)

• Larynx: voice box at the top of the trachea.
• Trachea: windpipe, with rings of cartilage.
• Bronchi: two branches, one to each lung.
• Bronchioles: many small branching tubes.
• Alveoli: air sacs in clusters, wrapped in capillaries.
• Lungs: two spongy organs in the thorax.

Thorax wall and floor:

• Ribs: bony cage around the lungs.
• External intercostal muscles: outer layer between the ribs.
• Internal intercostal muscles: inner layer, nearer the lungs.
• Diaphragm: dome-shaped muscular sheet below the lungs.

9.1.1 Four features of gas exchange surfaces

Feature	Effect
Large surface area	More gas diffuses at once
Thin surface	Short diffusion distance
Good blood supply	Removes oxygen, brings carbon dioxide: keeps the gradient steep
Good air supply	Ventilation refreshes alveolar air: keeps the gradient steep

The syllabus list stops at these four for gas exchange surfaces in general.

9.1.5 Alveoli: characteristics and role

Characteristics: very many alveoli (large total area); walls one cell thick; capillary walls one cell thick; moist lining; dense capillary network; well ventilated.

Role, in steps:

1. Oxygen dissolves in the moist lining.
2. Oxygen diffuses from alveolar air (high concentration) into the blood (low concentration).
3. Red blood cells carry the oxygen away.
4. Carbon dioxide diffuses from the blood (high concentration) into alveolar air (low concentration).
5. Carbon dioxide is breathed out.

9.1.2 Atmospheric air

Gas	% by volume (approx.)
Nitrogen	78
Oxygen	21
Argon and other noble gases	about 0.9
Carbon dioxide	0.04
Water vapour	variable

9.1.3 Inspired versus expired air

	Inspired	Expired	Reason for the change
Oxygen	~21%	~16%	Diffuses into blood; used in aerobic respiration
Carbon dioxide	~0.04%	~4%	Made in respiration; diffuses from blood into alveoli
Nitrogen	~78%	~78%	Not used by the body
Water vapour	variable	saturated	Evaporates from moist alveolar lining
Temperature	surroundings	near body temperature	Warmed by the body

Method in steps (carbon dioxide):

1. Set up two tubes with equal volumes of limewater or hydrogencarbonate indicator.
2. Draw inspired air through one tube; breathe expired air through the other.
3. Time how long each takes to change: limewater goes milky, hydrogencarbonate indicator goes from red/orange to yellow.
4. The expired-air tube changes much sooner: more carbon dioxide.

Worked reminder. Limewater went milky after 20 s with expired air and 300 s with inspired air. Rate = 1 ÷ time, so the expired-air rate is 300 ÷ 20 = 15 times faster. Faster change means more carbon dioxide. Always compare times from tubes with equal volumes of the same solution.

Water vapour: breathe on a cold mirror; condensation forms. Temperature: compare a thermometer in breathed-out air with room temperature.

Safety: limewater is a moderate hazard; wear eye protection and never suck back. Use your own clean mouthpiece.

9.1.7 Breathing mechanism

	Breathing in	Breathing out (at rest)	Forced breathing out
External intercostals	contract	relax	relax
Internal intercostals	relax	relax	contract
Ribs	up and out	down and in	pulled down and in further
Diaphragm	contracts, flattens	relaxes, domes	relaxes, domes
Thorax volume	increases	decreases	decreases further
Thorax pressure	falls below atmospheric	rises above atmospheric	rises further
Air flow	in	out	more air out

Method in steps (any breathing explanation): muscle → ribs or diaphragm → volume → pressure → direction of air flow. Always pair volume with pressure, in that order.

Model reminder. In a bell-jar model the rubber sheet is the diaphragm, the jar is the thorax, the balloons are the lungs and the Y-tube is the trachea and bronchi. Pull the sheet down: volume inside rises, pressure falls, air enters and the balloons inflate. Push it up: the reverse. The model’s rigid jar cannot show the ribs or intercostal muscles moving, so it only models the diaphragm.

9.1.8 Physical activity and breathing

Measuring: rate = breaths counted × (60 ÷ seconds counted). Depth by spirometer, or chest expansion with a tape measure.

Fair test: same person, same exercise, same duration; record at rest, straight after, then each minute until back to resting; repeat.

Explanation chain:

1. Muscles contract more → respire faster.
2. More carbon dioxide produced; more oxygen needed.
3. Blood carbon dioxide concentration rises → detected by the brain.
4. Brain increases rate and depth of breathing.
5. More oxygen in, more carbon dioxide out.

Small worked reminder: 11 breaths in 15 s → 11 × 4 = 44 breaths per minute.

9.1.9 Protection

Part	Job
Goblet cells	Secrete mucus
Mucus	Sticky; traps pathogens and particles
Ciliated cells	Cilia beat, moving mucus up to the throat to be swallowed

Result: pathogens and particles are kept away from the alveoli. If the cilia stop beating, mucus collects in the airways and is harder to clear, so pathogens and particles are more likely to reach the lungs.

Must-know distinctions

• Breathing vs respiration: a physical process moving air vs chemical reactions in cells.
• Gas exchange vs breathing: diffusion across the alveoli vs movement of air by muscles.
• External vs internal intercostals: external raise the ribs (in); internal lower them in forced breathing out.
• Rate vs depth: breaths per minute vs volume per breath.
• Mucus vs cilia: mucus traps; cilia move.
• Limewater vs hydrogencarbonate indicator: limewater shows carbon dioxide only by turning milky; the indicator changes colour (red/orange to yellow) as carbon dioxide rises.

Quick self-test

1. State the percentage of oxygen in atmospheric air.
2. Name the structure between the trachea and the bronchioles.
3. Give two characteristics of the alveoli that make diffusion fast.
4. Why does expired air contain more water vapour than inspired air?
5. What happens to the diaphragm when you breathe in?
6. Which muscles contract during forced breathing out?
7. Expired air contains about 4% carbon dioxide and inspired air about 0.04%. How many times more is that?
8. A student counts 12 breaths in 20 s. Calculate the breathing rate.
9. What does the brain detect that causes faster breathing during exercise?
10. A pie chart shows atmospheric air. What angle represents oxygen at 21%?
11. A red blood cell measures 10.5 mm on a photomicrograph at ×1500. Calculate its actual diameter in µm.
12. State the role of goblet cells.

Answers

1. About 21%.
2. Bronchi (one bronchus to each lung).
3. Any two: large total surface area; thin walls (one cell thick), so short diffusion distance; moist lining; dense capillary network; well ventilated.
4. Water evaporates from the moist lining of the alveoli into the air.
5. It contracts and flattens (moves down).
6. The internal intercostal muscles.
7. 4 ÷ 0.04 = 100 times.
8. 12 × (60 ÷ 20) = 36 breaths per minute.
9. A rise in the carbon dioxide concentration of the blood.
10. 21 ÷ 100 × 360 = 75.6°.
11. 10.5 mm = 10 500 µm; 10 500 ÷ 1500 = 7 µm.
12. They secrete mucus, which traps pathogens and particles.

Where marks are usually lost

• Listing “thin” or “large surface area” with no effect on diffusion attached.
• Writing that the lungs pull or suck air in, instead of linking volume and pressure.
• Saying “pressure increases” when breathing in, by reversing the volume-pressure link.
• Saying the diaphragm moves up on breathing in.
• Giving the internal intercostals a role in quiet breathing in.
• Stating that inspired air contains no carbon dioxide.
• Explaining exercise breathing only as “more oxygen needed”, missing carbon dioxide detected by the brain.
• Giving a breathing rate per 15 s or 30 s when the question asks for per minute.
• Writing that cilia trap dust, or that cilia “sweep mucus down” to the lungs.
• Confusing breathing with respiration in an explanation.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 9: Human gas exchange, published by Cambridge Assessment International Education.