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06102026–2028 syllabus

BIOLOGY · CHAPTER 11

Gas exchange in humans

How ventilation maintains steep gradients at specialised alveoli.

Core + Supplement3 connected sectionsSyllabus-aligned guide

LEARNING OBJECTIVES

What you will be able to do

  • identify breathing structures
  • explain alveolar adaptation
  • compare inspired and expired air
  • explain ventilation and exercise responses

AT A GLANCE

Syllabus0610Coverage2026–2028Sections3LevelCore + Supplement

INTRODUCTION · THE BIG IDEA

How ventilation maintains steep gradients at specialised alveoli.

Gas exchange and ventilation are related but different. Diffusion swaps gases at alveoli, while breathing movements continually refresh the air and maintain steep gradients.

Link muscle contraction to thoracic volume, pressure and airflow, then connect alveolar adaptations to diffusion rate.

01

SECTION 01

Airways and alveoli

Core concept

Air passes through larynx, trachea, bronchi and bronchioles to alveoli. Cartilage keeps the trachea open; goblet-cell mucus traps particles and cilia move it towards the throat.

DETAILED EXPLANATION

  • Oxygen diffuses into capillaries while carbon dioxide diffuses into alveoli.
ORIGINAL STUDY DIAGRAMAir pathway
1trachea
2bronchus
3bronchiole
4alveolus ⇄ capillary
02

SECTION 02

Ventilation

Core concept

Thoracic volume changes alter pressure, moving air down a pressure gradient.

DETAILED EXPLANATION

  • Exhale: muscles relax, ribs lower, diaphragm domes, volume falls and pressure rises.
  • Internal intercostals assist forced exhalation.
ORIGINAL STUDY DIAGRAMPressure mechanism
1volume ↑ → pressure ↓ → air in
2volume ↓ → pressure ↑ → air out
03

SECTION 03

Air composition and activity

Core concept

Expired air has less oxygen, more carbon dioxide and water vapour, while nitrogen changes little. Limewater tests increased carbon dioxide.

DETAILED EXPLANATION

  • The brain detects its rise and increases breathing rate and depth.
  • A fair investigation standardises exercise duration and intensity.

STEP-BY-STEP EXAM EXAMPLE

Original worked example

Explaining inhalation

  1. External intercostal muscles contract, moving the ribs up and out.
  2. The diaphragm contracts and flattens.
  3. Thoracic volume increases, so pressure inside falls below atmospheric pressure.
  4. Air moves down the pressure gradient into the lungs.

Answer: Muscle contraction increases volume, lowers pressure and draws air in.

QUICK CHAPTER SUMMARY

The ideas to carry forward

  • Alveoli are large, thin, supplied and ventilated.
  • Pressure changes drive breathing.
  • Expired air differs because of respiration.
  • Exercise increases ventilation.

QUICK REVISION CHECKLIST

Can you do each of these without your notes?

  • identify breathing structures
  • explain alveolar adaptation
  • compare inspired and expired air
  • explain ventilation and exercise responses
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