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Learn4BI1Paper 2B · Paper 2 (all content)Structure and functions in living organismsBreathing and gas exchange
Structure and functions in living organisms · Subtopic 11 of 21

Breathing and Gas Exchange

Air passes through the trachea, bronchi and bronchioles to millions of alveoli, where oxygen diffuses into the blood and CO₂ diffuses out. Alveoli have a large surface area, thin walls (one cell thick), a rich capillary network and are ventilated, keeping concentration gradients steep. To inhale, the external intercostal muscles contract (ribs up and out) and the diaphragm contracts and flattens: volume increases, pressure decreases, air flows in. Exhaling reverses this. Expired air has less oxygen, more CO₂ and more water vapour. Smoking damages cilia and alveoli and causes cancer.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe the structure of the respiratory system.
  • Explain how alveoli are adapted for gas exchange.
  • Explain the mechanism of breathing.
  • Describe the effects of exercise and smoking.
Pause and think

If your lungs are spongy bags with no muscles, how do they inflate?

Section 2 of 14

Core idea

Start here20,000 breaths a day

You breathe about 20,000 times a day without thinking. Each breath swaps oxygen for carbon dioxide across a surface thinner than a soap bubble.

The big idea

Muscles change the volume of the chest, changing the pressure, so air moves in and out. Alveoli are adapted to make diffusion fast.

1. Breathing in and out

Ventilation

Volume and pressure change in opposite directions.

Worked exampleAlveoli adaptations

Give four features of alveoli and explain each.

Goal
Link to diffusion.
Strategy
Think area, distance, gradient.
  1. many alveoli\text{many alveoli}
    Huge surface area.
  2. one-cell-thick walls\text{one-cell-thick walls}
    Short diffusion distance.
  3. capillary network\text{capillary network}
    Blood carries O₂ away, keeping the gradient steep.
  4. ventilation; moist lining\text{ventilation; moist lining}
    Fresh air keeps the gradient; gases dissolve.
AnswerArea, thinness, blood supply, ventilation.
✓ CheckAll increase rate of diffusion ✓.
Key takeawaySmoking Tar: cancer and damaged cilia. Nicotine: addiction, raises heart rate. CO: reduces oxygen carried. Emphysema: fewer, larger alveoli, less surface area.
Pause and think

Why does a smoker cough more in the morning?

Summary

  • Volume ↑ → pressure ↓ → air in.
  • Alveoli: area, thin, blood, ventilation.
  • Smoking damages lungs.
Key words
Alveoli
Tiny air sacs where gas exchange happens.
Ventilation
Moving air into and out of the lungs.
Diaphragm
Sheet of muscle below the lungs.
HistoryDoll and Hill. In 1950 Richard Doll and Austin Bradford Hill published evidence linking smoking to lung cancer. Doll stopped smoking during the study.
Fun factA tennis court of lungs. Your lungs contain about 300 million alveoli with a total surface area of around 70 m², about a third of a tennis court.
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