Gas Exchange in Humans
Every cell needs oxygen for respiration and must get rid of carbon dioxide. The breathing system delivers air to a surface where those gases can be exchanged with the blood.
The first thing to fix: gas exchange is not respiration, and neither is breathing. Breathing moves air; gas exchange moves gases between air and blood; respiration is the chemical reaction inside cells.
1. Features of a gas exchange surface
Every efficient gas exchange surface in biology shares the same four features, and the alveoli are the model example:
| Feature | Why it helps |
|---|---|
| Large surface area | More gas can diffuse at once |
| Thin surface (one cell thick) | Short diffusion distance |
| Good blood supply | Maintains a steep concentration gradient by removing oxygen and bringing carbon dioxide |
| Good ventilation with air | Keeps the concentration gradient steep on the air side |
Every one of these works by either increasing the area, shortening the distance, or maintaining the gradient — the three variables in the rate of diffusion. If you can name which one an adaptation serves, you can explain any exchange surface in the syllabus.
2. The breathing system
The pathway of air:
nose / mouth → trachea → bronchi → bronchioles → alveoli
| Structure | Function |
|---|---|
| Nasal cavity | Warms, moistens and filters the air |
| Trachea | Windpipe; held open by rings of cartilage |
| Bronchi | Two tubes, one to each lung; also contain cartilage |
| Bronchioles | Narrow branching tubes; no cartilage |
| Alveoli | Tiny air sacs where gas exchange occurs |
| Ribs | Protect the lungs and heart; move during breathing |
| Intercostal muscles | Between the ribs; move the ribcage |
| Diaphragm | Sheet of muscle below the lungs; changes the volume of the chest |
Cartilage is in the trachea and bronchi, but not the bronchioles. Questions that ask you to identify a structure from a table of features often hinge on exactly this — cartilage present, plus distance from the larynx.
Alveoli
There are hundreds of millions of alveoli, which together give an enormous surface area. Each has a wall one cell thick, is surrounded by a dense network of capillaries (also one cell thick), and is moist, so gases dissolve before diffusing.
Gas exchange at the alveolus:
- Oxygen diffuses from the alveolus into the blood — its concentration is higher in the alveolar air
- Carbon dioxide diffuses from the blood into the alveolus — its concentration is higher in the blood
Both move down their concentration gradients, by diffusion. No energy is required.
Protecting the lungs
| Cell type | Role |
|---|---|
| Goblet cells | Produce mucus, which traps dust and bacteria |
| Ciliated cells | Cilia beat to sweep the mucus up and away from the lungs, to be swallowed |
This is why smoking is so damaging: it paralyses and destroys the cilia, so mucus is not cleared. It collects in the lungs, causing “smoker’s cough” and increasing the risk of infection.
3. The mechanism of ventilation
Breathing works by changing the volume of the chest, which changes the pressure, which makes air move.
Volume and pressure are inversely related. Increase the volume and the pressure falls; air then moves into the lungs from the higher pressure outside.
Inspiration (breathing in)
- External intercostal muscles contract, pulling the ribcage up and out
- Diaphragm contracts and flattens, moving down
- Volume of the chest increases
- Pressure inside decreases (below atmospheric pressure)
- Air moves into the lungs
Expiration (breathing out)
- External intercostal muscles relax; the ribcage moves down and in
- Diaphragm relaxes and returns to its domed shape
- Volume of the chest decreases
- Pressure inside increases
- Air moves out of the lungs
A reliable memory aid: the diaphragm and the external intercostals do the same thing as each other — both contract for breathing in, both relax for breathing out. The internal intercostals do the opposite. Learn one and derive the rest.
The diaphragm flattens when it contracts. Muscles get shorter when they contract, and a shorter, tenser diaphragm is flatter. When relaxed, it is domed. Students often say the reverse.
4. Inspired vs expired air
| Gas | Inspired air | Expired air |
|---|---|---|
| Oxygen | About 21% | About 16% |
| Carbon dioxide | About 0.04% | About 4% |
| Nitrogen | About 78% | About 78% — unchanged |
| Water vapour | Variable | More |
| Temperature | Variable | Warmer |
Expired air still contains plenty of oxygen — about 16%, not zero. This is exactly why mouth-to-mouth resuscitation works. Saying expired air contains “no oxygen” is a common and costly error.
Nitrogen does not change. The body does not use it.
Testing for the difference: bubble each sample through limewater. The limewater turns milky faster with expired air, because it contains more carbon dioxide. Hydrogen-carbonate indicator turns from red to yellow with expired air.
5. The effect of exercise
During exercise:
- Breathing rate increases — more breaths per minute
- Depth of breathing increases — a larger volume per breath
The reason, as a chain: muscles contract more → they need more energy → the rate of aerobic respiration increases → more oxygen is needed and more carbon dioxide produced → so more air must be moved in and out.
After exercise, breathing stays deep and rapid to supply the extra oxygen needed to break down the lactic acid produced by anaerobic respiration — the oxygen debt.
6. Mistakes that cost marks
Confusing breathing, gas exchange and respiration.
Saying expired air has no oxygen. It has about 16%.
Saying the diaphragm domes when it contracts. It flattens.
Saying the nitrogen percentage changes. It does not.
Saying air is “sucked in.” Air moves because of a pressure difference created by a change in volume.
Saying bronchioles contain cartilage. Trachea and bronchi do; bronchioles do not.
Listing alveoli adaptations without reasons. Each needs its “so that”.
Saying gas exchange requires energy. It is diffusion — passive.
Frequently asked questions
What is the difference between breathing, gas exchange and respiration? Breathing moves air in and out. Gas exchange moves gases between air and blood by diffusion. Respiration is the chemical reaction in cells that releases energy.
How are alveoli adapted for gas exchange? Large surface area, walls one cell thick, rich blood supply, and a moist surface — giving a short diffusion distance and a steep concentration gradient.
What happens to the diaphragm when you breathe in? It contracts and flattens, increasing chest volume and lowering pressure.
Why does air move into the lungs? Because the pressure inside falls below atmospheric pressure when the chest volume increases.
How much oxygen is in expired air? About 16% — considerably less than inspired air’s 21%, but far from zero.
Why does smoking cause a cough? Smoke damages the cilia, so mucus is not swept away and collects in the airways.
How can you show expired air has more carbon dioxide? Bubble both through limewater — it turns milky faster with expired air.
Quick revision checklist
- I can distinguish breathing, gas exchange and respiration
- I can name the four features of a gas exchange surface and what each achieves
- I can name the pathway of air in order
- I know cartilage is in the trachea and bronchi but not bronchioles
- I can give the alveoli adaptations with reasons
- I can state which way each gas diffuses and why
- I can explain the role of goblet cells and ciliated cells
- I can describe inspiration and expiration as full sequences
- I know the diaphragm flattens when it contracts
- I can explain breathing using volume and pressure
- I know the figures for inspired and expired air, including 16% oxygen
- I can explain why breathing rate and depth increase during exercise
These notes cover topic 11 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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