BIOLOGY · Stage 8 · Chapter 5: Respiration and the Respiratory System

Biology 5.6 Breathing — Ventilation Mechanics and Intercostal Muscles

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Biology 5.6 Breathing — Ventilation Mechanics and Intercostal Muscles — Alhussan International Academy — Biology (Stages 7–9) notes: Explain how the diaphragm and intercostal muscles change the volume and pressure of the chest to cause inhalation and exhalation.Gas exchange — it …

Concentration gradients
Inhaled air
Exhaled air
Changing the air
Chest cavity
Aerobic respiration

Section 1 of 7Why we breathe

Gas exchange
- it takes place in the alveoli and supplies oxygen for respiration and removes waste carbon dioxide.
Concentration gradients
- steep gradients between alveolar air and the blood are needed.
Inhaled air
- relatively high in oxygen and low in carbon dioxide.
Exhaled air
- relatively high in carbon dioxide and lower in oxygen.
  - (oxygen about 21% to 16%, carbon dioxide about 0.04% to 4%, nitrogen 79% unchanged)
Changing the air
- it keeps the concentration gradients steep, so gas exchange is fast.

Section 2 of 7The structures involved

Lungs
- they have no muscle of their own and cannot move by themselves.
Intercostal muscles
- the muscles between the ribs, which contract or relax to change the shape of the chest cavity.
Diaphragm
- a dome-shaped sheet of muscle below the lungs that contracts and relaxes.
Movements
- the intercostal muscles and diaphragm force air into and out of the lungs.
Chest cavity
- the lungs are enclosed in an airtight chest cavity and covered by a moist pleural membrane that lets them slide.

Section 3 of 7Inhaling (breathing in)

The intercostal muscles contract, pulling the ribcage up and out.
The diaphragm contracts, moving downwards and flattening.
The volume inside the chest increases.
The pressure inside the chest falls below atmospheric pressure.
Air moves into the lungs from outside.

Energy
- inhalation is active and uses energy for muscle contraction.

Section 4 of 7Exhaling (breathing out)

The intercostal muscles relax, so the ribcage moves down and in.
The diaphragm relaxes and moves upwards.
The volume inside the chest decreases.
The pressure inside the chest increases above atmospheric pressure.
Air is forced out of the lungs and out of the body.

At rest
- exhalation is passive, as the elastic lungs recoil.
Exercise
- the internal intercostal and abdominal muscles contract to force air out faster.

Feature	Inhaling	Exhaling
Ribs	Move up and out	Move down and in
Diaphragm	Contracts, flattens	Relaxes, domed
Chest volume	Increases	Decreases
Chest pressure	Decreases	Increases

Section 5 of 7Breathing and respiration

Aerobic respiration
- it uses oxygen to break down glucose into carbon dioxide and water, releasing energy in a controlled way for cells.
Breathing
- it moves air into and out of the lungs, bringing the oxygen needed for respiration into the body and removing waste carbon dioxide.

Section 6 of 7The bell jar model

Parts
- the jar represents the chest cavity, the rubber sheet the diaphragm and the balloons the lungs.
Action
- pulling the sheet down increases the volume and lowers the pressure, so the balloons fill with air, and pushing it up forces the air out.
Use
- it explains some of the changes seen in breathing.
Limitations
- it has no ribs or intercostal muscles, the balloons are not lung-shaped and have no alveoli, there is no mucus or branching airway,
  - and air enters through a single tube.

Section 7 of 7Measuring breathing

Tidal volume
- the volume of a normal breath, about 500 cm³.
Vital capacity
- the largest volume that can be breathed out after a deep breath, about 4000 to 5000 cm³.
Residual volume
- air that remains after breathing out as hard as possible, which prevents the lungs collapsing.
Spirometer trace
- peaks show inhalation and troughs exhalation, and taller, closer peaks occur during exercise.

Last updated: 14 October 2026