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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
HL

Exam analysis

Chance of appearing10%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–4

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

May 2025Paper2 ["HL"] · TZ34(d)[ 2 ]B3.1.5—Lung ventilation
May 2025Paper2 ["HL"] · TZ110(b)[ 4 ]B3.1.5—Lung ventilation
May 2023Paper1 ["HL"] · TZ219[ 1 ]B3.1.5—Lung ventilation
May 2022Paper1 ["HL"] · TZ221[ 1 ]B3.1.5—Lung ventilation
November 2021Paper2 ["HL"] · TZ04(c)[ 1 ]B3.1.5—Lung ventilation
Practice this objective

Coverage 2013–2025 · Updated 15 Jul 2026

Ventilation Replaces Air at the Gas-Exchange Surface

Lung ventilation results from muscles changing thoracic volume, which changes pressure relative to the atmosphere and causes air to flow.

During inspiration, the diaphragm contracts and flattens while external intercostal muscles contract, moving the ribs up and out. Thoracic volume increases, pressure falls below atmospheric pressure and air enters.

During quiet expiration these muscles relax and elastic recoil lowers thoracic volume, raising pressure so air leaves. During forced expiration, internal intercostals pull the ribs down and in while abdominal muscles push the diaphragm upward.

Sequence for inhalation: diaphragm contracts + ribs move up/out → thoracic volume increases → intrapulmonary pressure decreases → air flows into the lungs down the pressure gradient.

The diaphragm does not pull air directly. It changes thoracic volume; the resulting pressure difference moves air. Abdominal muscles are especially important in forced, not quiet, expiration.

Lung ventilation

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Outline / Identify.

Command terms

Explain / Outline / Identify / Describe / State

What earns marks

Build the answer around this relationship: Inhalation occurs when diaphragm and external intercostal contraction increases thoracic volume.

Watch for

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

Representative question

Question 1

[Maximum number: 9]

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

Gas Exchange Across Animals And Leaves

Core gas-exchange answers link exchange surfaces to diffusion gradients. For animals, exchange surfaces are explained by diffusion properties, ventilation, and blood flow. For plants, leaves allow carbon dioxide entry and oxygen/water vapour exit while controlling water loss through stomata. Spirometry, transpiration, and stomatal density data provide evidence of gradient and surface-area effects.

  • Core animal answer: large, thin, moist, permeable surface plus ventilation and blood flow.
  • Core plant answer: stomata, guard cells, mesophyll air spaces, cuticle, and transpiration factors.
  • Data questions usually test rate, gradient, volume, or density per area.

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.