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B3.1.5—Lung ventilation

Ventilation moves air by coordinated diaphragm and intercostal muscle actions that change thoracic volume, pressure and airflow direction during breathing.

Syllabus
First assessment 2025
Objective
B3.1.5
Level
SL

Exam analysis

Chance of appearing16%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1–3

Common command terms

  • Explain
  • Outline
  • Identify
  • Describe
  • State

Scoring notes

Common mistake
Reversing the pressure change during inhalation by saying contraction raises thoracic pressure.

Recent exam appearances

November 2025Paper2 ["SL"] · TZ34(b)[ 1 ]B3.1.5—Lung ventilation
November 2025Paper2 ["SL"] · TZ34(a)[ 2 ]B3.1.5—Lung ventilation
May 2025Paper1A ["SL"] · TZ316[ 1 ]B3.1.5—Lung ventilation
November 2024Paper2 ["SL"] · TZ15(a)(iii)[ 3 ]B3.1.5—Lung ventilation
May 2024Paper3 ["SL"] · TZ23(b)[ 2 ]B3.1.5—Lung ventilation

Coverage 2010–2025 · Updated 15 Jul 2026

Muscle Contraction Creates the Pressure Difference for Airflow

Air moves because thoracic volume changes alter lung pressure. Inspiration is active: diaphragm and external intercostals contract, increasing volume and lowering pressure below atmospheric.

Show thorax-ventilation-sequence so the learner can trace Inspiration and Expiration.

inspiration: muscles contract → thorax expands → pressure falls → air enters
quiet expiration: muscles relax + elastic recoil → volume falls → pressure rises → air leaves

SL Summary: One Diffusion Model

Feature Lung Leaf
area many alveoli broad blade + mesophyll walls
path thin barrier thin leaf + air spaces
gradient air + blood flow CO₂ use + stomata

Spirometry measures ventilation; a potometer estimates water uptake; a leaf cast measures stomatal density. Match each method to its direct measurement.

Lung ventilation

[ 9 ]

Explain the mechanism of ventilation in the lungs in order to promote gas exchange for cell respiration.

Concept essentials

  • Inhalation occurs when diaphragm and external intercostal contraction increases thoracic volume.
  • Lower pressure in the thorax causes air to enter the lungs from higher external pressure.
  • Expiration during quiet breathing mainly follows muscle relaxation and elastic recoil.
  • Internal intercostal and abdominal muscles contribute to forced expiration.
  • Exercise increases ventilation by raising breath depth, breath frequency or both.