Cambridge O Level Biology 5090: Human gas exchange (5090)

This study guide teaches Topic 9, Human gas exchange, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, and covers every learning outcome in section 9.1 (9.1.1 to 9.1.9), including the two investigations the section lists. The 5090 syllabus is not tiered, so every candidate studies all of it.

Use it with the revision notes for this topic and the practice questions. The course hub lists every topic, and the printable checklist lets you tick off each outcome. Gas exchange is diffusion, so revise Movement into and out of cells first if the idea of a concentration gradient is shaky. To find your weak spots quickly, try one of the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
9.1.1	Describe the features of human gas exchange surfaces: large surface area, thin surface, good blood supply, good air supply
9.1.2	State the percentages of the gases in atmospheric air
9.1.3	Investigate and explain the differences between inspired and expired air
9.1.4	Identify the larynx, trachea, lungs, bronchi, bronchioles, alveoli and associated capillaries
9.1.5	State the characteristics of the alveolar exchange surface and describe its role
9.1.6	Identify the ribs, internal and external intercostal muscles and diaphragm
9.1.7	Explain how these structures change thorax volume and pressure to move air in and out
9.1.8	Investigate and explain the effect of physical activity on the rate and depth of breathing
9.1.9	Explain how goblet cells, ciliated cells and mucus protect the system from pathogens and particles

Three words get mixed up all the time:

• Gas exchange: oxygen diffuses into the blood and carbon dioxide diffuses out, across the alveolar surface.
• Breathing (ventilation): the muscle movements that move air into and out of the lungs.
• Respiration: chemical reactions in cells that release energy from glucose (Topic 10). Breathing is not respiration.

9.1.4 The structures of the gas exchange system

Air takes one route to the exchange surface:

nose or mouth → larynx → trachea → bronchi → bronchioles → alveoli

Structure	How to recognise it on a diagram or image
Larynx	Wider box-like region at the top of the trachea, in the neck (the voice box)
Trachea	Single tube down the neck, with rings of cartilage
Bronchi (singular bronchus)	The two branches of the trachea, one entering each lung
Bronchioles	Many narrow, branching tubes inside each lung
Alveoli (singular alveolus)	Tiny air sacs in clusters at the ends of bronchioles
Associated capillaries	Fine blood vessels forming a network over the outside of each alveolus
Lungs	The two spongy organs filling most of the thorax

On a photomicrograph of lung tissue, alveoli look like empty spaces with very thin walls, and red blood cells can be seen inside the capillaries in those walls.

9.1.1 Features of gas exchange surfaces

The syllabus limits you to four features. For each, know why it speeds up diffusion.

Feature	Why it increases the rate of diffusion
Large surface area	More molecules can cross at the same time
Thin surface	Short diffusion distance
Good blood supply	Blood carries oxygen away and brings carbon dioxide, so a steep concentration gradient is maintained
Good air supply	Breathing replaces alveolar air, keeping oxygen high and carbon dioxide low on the air side

Blood flow keeps the gradient steep on one side; ventilation keeps it steep on the other. Write “maintains a concentration gradient”, not just “lots of blood”.

9.1.5 The alveoli as the exchange surface

Characteristics of the alveolar surface:

• Very many alveoli, giving a very large total surface area.
• Thin walls: the alveolus wall and the capillary wall are each one cell thick.
• Moist lining: gases dissolve in this film of water before they diffuse across.
• Dense capillary network around each alveolus.
• Well ventilated by breathing.

Role. Air in the alveolus has a higher oxygen concentration than the blood arriving in the capillaries, so oxygen dissolves in the moist lining and diffuses into the blood, where red blood cells carry it away. Blood arriving at the lungs has a higher carbon dioxide concentration than alveolar air, so carbon dioxide diffuses out of the blood into the alveolus and is breathed out. Both gases move passively, down their own concentration gradients.

Worked example 1 (using an image). A photomicrograph at ×3000 shows the barrier between the air in an alveolus and the blood in a capillary. The barrier measures 3 mm on the image. Calculate its actual thickness in µm.

• Actual size = image size ÷ magnification.
• Convert first: 3 mm = 3 × 1000 = 3000 µm.
• Actual = 3000 ÷ 3000 = 1 µm.

Oxygen crosses only two very thin cells.

9.1.2 The composition of atmospheric air

Learn these approximate percentages by volume:

Gas	Approximate % in atmospheric air
Nitrogen	78
Oxygen	21
Argon and other noble gases	about 0.9 (round to 1)
Carbon dioxide	0.04
Water vapour	variable

9.1.3 Inspired and expired air

The differences

	Inspired air	Expired air
Oxygen	about 21%	about 16%
Carbon dioxide	about 0.04%	about 4%
Nitrogen	about 78%	about 78% (no real change)
Water vapour	variable	high (saturated)
Temperature	that of the surroundings	warmer, close to body temperature

The investigation

Carbon dioxide. Two tubes hold equal volumes of limewater or hydrogencarbonate indicator. A mouthpiece and valves (or a tube arrangement) let inspired air bubble through one tube and expired air through the other. Limewater turns milky in carbon dioxide. Hydrogencarbonate indicator turns from red/orange to yellow as carbon dioxide rises. Record the time for each tube to change.

Water vapour. Breathe onto a cold, dry mirror or tile: condensation shows expired air carries water vapour.

Temperature. Breathe gently over the bulb of a thermometer and compare the reading with room temperature.

Control the volume and concentration of indicator, the starting temperature and the breathing pattern. Limewater is a moderate hazard: wear eye protection, and never suck liquid up the tube. Each person uses their own clean mouthpiece.

Worked example 2. Hydrogencarbonate indicator turned yellow after 15 s with expired air and after 240 s with inspired air. Compare the rates of colour change.

• Rate = 1 ÷ time.
• Expired air: 1 ÷ 15 = 0.067 s⁻¹.
• Inspired air: 1 ÷ 240 = 0.0042 s⁻¹.
• Ratio: 240 ÷ 15 = 16, so the colour changed 16 times faster with expired air, showing it holds much more carbon dioxide.

The explanation

• Less oxygen: oxygen diffuses from alveolar air into the blood and is used by cells in aerobic respiration.
• More carbon dioxide: cells produce it in respiration; the blood carries it to the lungs, where it diffuses into the alveoli.
• More water vapour: water evaporates from the moist alveolar lining into the air.
• Warmer: air is warmed by the body as it passes through the airways and lungs.
• Nitrogen unchanged: the body does not use it, so it passes in and out.

Each explanation needs a process (diffusion, respiration, evaporation) and a direction.

9.1.6 and 9.1.7 Breathing: volume and pressure

The structures

• Ribs: bones around the thorax, joined to the sternum and backbone.
• External intercostal muscles: the outer layer of muscle between the ribs.
• Internal intercostal muscles: the inner layer, nearer the lungs.
• Diaphragm: a dome-shaped sheet of muscle and fibrous tissue under the lungs, separating the thorax from the abdomen.

The mechanism

Air always moves from higher pressure to lower pressure. Muscles change the volume of the thorax; volume changes pressure; pressure difference moves air.

Breathing in (inspiration)

1. External intercostal muscles contract; internal intercostals relax.
2. Ribs move up and out.
3. Diaphragm contracts and flattens (moves down).
4. Thorax volume increases, so pressure inside decreases below atmospheric pressure.
5. Air moves into the lungs.

Breathing out (expiration)

1. External intercostals relax; ribs move down and in.
2. Diaphragm relaxes and returns to its dome shape (moves up).
3. Thorax volume decreases, so pressure inside increases above atmospheric pressure.
4. Air moves out of the lungs.

In forced breathing out (exercise, coughing), the internal intercostal muscles contract to pull the ribs down and in further, pushing more air out. External and internal intercostals are antagonistic: when one set contracts, the other relaxes.

Worked example 3 (a model). In a bell-jar model, balloons hang from a Y-tube inside a jar, and a rubber sheet closes the base. Explain why the balloons inflate when the sheet is pulled down.

• The sheet represents the diaphragm; the jar represents the thorax; the balloons represent the lungs.
• Pulling the sheet down increases the volume inside the jar.
• Pressure inside the jar falls below atmospheric pressure.
• Air moves in through the Y-tube (the trachea and bronchi), so the balloons inflate.

A limitation: the jar’s walls are rigid, so the model cannot show the ribs and intercostal muscles moving.

9.1.8 Physical activity and breathing

Investigating it

• Rate: count breaths for a fixed time (for example 30 s) and convert to breaths per minute.
• Depth: the volume of air per breath. Measure with a spirometer if you have one, or compare chest expansion with a tape measure.
• Method: rest, record; do a fixed exercise for a fixed time; record straight away and then every minute until values return to resting. Keep the person, exercise and duration the same, and repeat.

Worked example 4. At rest, a student counts 9 breaths in 30 s. After three minutes of step-ups, she counts 13 breaths in 20 s. Her depth rises from 0.45 dm³ to 1.8 dm³ per breath.

• Resting rate = 9 × (60 ÷ 30) = 18 breaths per minute.
• Rate after exercise = 13 × (60 ÷ 20) = 39 breaths per minute.
• Percentage increase = (39 − 18) ÷ 18 × 100 = 116.7%.
• Volume per minute at rest = 18 × 0.45 = 8.1 dm³.
• Volume per minute after exercise = 39 × 1.8 = 70.2 dm³, about 8.7 times the resting value.

Explaining it

1. Contracting muscles need more energy, so they respire faster.
2. More oxygen is needed, and more carbon dioxide is produced.
3. Carbon dioxide concentration in the blood rises; the brain detects this.
4. The brain makes the breathing muscles contract faster and more strongly, so rate and depth increase.
5. More oxygen reaches the alveoli and diffuses into the blood; more carbon dioxide is removed.

Breathing stays fast and deep for a while after exercise to repay the oxygen debt; that belongs to Topic 10 (anaerobic respiration).

9.1.9 Protecting the gas exchange system

Inhaled air carries pathogens (such as bacteria and viruses) and particles (such as dust and pollen). The lining of the trachea and bronchi contains two cell types:

• Goblet cells secrete mucus.
• Mucus is sticky and traps pathogens and particles.
• Ciliated cells have cilia that beat in waves, moving the mucus up to the throat, where it is swallowed.

This keeps pathogens and particles away from the alveoli, where they could cause infection or damage the thin exchange surface. Ciliated cells are a specialised cell you also meet in Cell structure and organisation.

Common errors

• Writing “alveoli are thin” without “so the diffusion distance is short”.
• Saying the diaphragm moves up when breathing in. It contracts, flattens and moves down.
• Reversing volume and pressure. Volume up, pressure down, air in.
• Writing that the lungs “suck in” air. Air is pushed in by higher atmospheric pressure.
• Saying inspired air has “no carbon dioxide”. It has about 0.04%.
• Explaining faster breathing only as “needing more oxygen” without the carbon dioxide and brain link.
• Saying cilia trap dust. Mucus traps; cilia move the mucus.
• Calling breathing “respiration”.

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.