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3.2.6Exchange and Transport

Ventilation and Gas Exchange — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 15 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.2.6 Ventilation and Gas Exchange — AQA A Level Biology 7402 notes: Describe the mechanism of breathing in and out in humans and explain how ventilation maintains gas exchange.Gas exchange itself relies on diffusion, but diffusion would stop quickly if the air in the alveoli never changed.

  • internal intercostal muscles
  • antagonistic pair
  • volume changes, so pressure changes, so air flows down a pressure gradient
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Section 1 of 9Why ventilation is needed

Gas exchange itself relies on diffusion, but diffusion would stop quickly if the air in the alveoli never changed. Ventilation (breathing) moves air in and out of the lungs, keeping a high concentration of oxygen and low concentration of carbon dioxide in alveoli. Breathing is a mechanical process driven by changes in volume and pressure of the thorax.

A frequent exam trap is to say that air moves because the lungs "pull" it in.

Why ventilation is needed
State instead that the muscles change the volume of the thorax and that the pressure difference moves the air.
Another trap is to mix up the changes in the ribs: during inspiration the ribcage moves upwards and outwards,
  • and during expiration it moves downwards and inwards.

Section 2 of 9Inspiration

Inspiration is an active process:

1
External intercostal muscles contract, raising the ribs up and out.
2
The diaphragm contracts and flattens, moving downward.
3
Thorax volume increases.
4
Pressure in the lungs falls below atmospheric pressure.
5
Air flows in down the pressure gradient.

Section 3 of 9Expiration

Normal quiet expiration is passive:

1
External intercostal muscles relax, ribs move down and in under gravity.
2
The diaphragm relaxes and rises to a dome shape.
3
Thorax volume decreases and the lungs recoil due to elastic fibres.
4
Pressure in the lungs rises above atmospheric pressure and air is forced out.

In forced expiration (exercise) the internal intercostal muscles contract, pulling the ribs down and in, and the abdominal muscles push the diaphragm up. The external and internal intercostals act as an antagonistic pair.

Section 4 of 9Comparison

FeatureInspirationExpiration (quiet)
External intercostalsContractRelax
DiaphragmContracts, flattensRelaxes, domed
Thorax volumeIncreasesDecreases
Lung pressureBelow atmosphericAbove atmospheric
EnergyNeeds ATPMostly passive, uses elastic recoil

Section 5 of 9Ventilation and the gradient

Fresh air has about 21% oxygen and 0.04% CO2, whereas alveolar air contains about 14% oxygen and 5% CO2 because of gas exchange.

Ventilation and the gradient
Ventilation prevents these concentrations from reaching equilibrium with the blood.
Not all air is changed: the residual volume stays, so alveolar air is always partly stale.
Deeper, faster breathing changes more of the air each minute and so raises the rate of gas exchange.

Pulmonary ventilation = tidal volume × breathing rate. During exercise both rise, so ventilation may increase over tenfold.

Ventilation is regulated by the medulla oblongata, which receives impulses from chemoreceptors that detect the pH of the blood. A rise in CO2 concentration lowers pH, and the medulla increases the rate of impulses sent to the intercostal muscles and diaphragm. Stretch receptors in the lungs send impulses back to prevent over-inflation, which is a negative feedback mechanism.

Section 6 of 9Pressure and volume: explaining it fully

Gas pressure is inversely proportional to volume at constant temperature (Boyle's law).

1
When the thorax expands, the same quantity of gas in the lungs occupies a larger volume and so exerts a smaller pressure.
2
Because the pressure inside is now less than the atmospheric pressure outside, air moves into the lungs.
3
When the thorax volume decreases the reverse happens.
4
This is the reasoning behind each mark in an examination answer: always include volume changes, so pressure changes, so air flows down a pressure gradient.

The lungs have no muscle of their own.

Pressure and volume: explaining it fully
They are held against the thoracic wall by the pleural membranes and a thin film of pleural fluid between them,
  • so when the ribs move outwards and the diaphragm flattens the lungs are pulled to expand.
If the pleural cavity is punctured (a pneumothorax) air enters between the membranes and the lung collapses,
  • because the pressure difference that holds it open is lost.

Ventilation in other animals is different

  • insects move air through tracheae using abdominal pumping, and bony fish use a flow of water over gills in the opposite direction to the blood (countercurrent).
  • Compare these with mammals in terms of how a gradient is maintained.

Pulmonary ventilation at rest is typically about 6–8 dm3 min−1 and can rise to more than 100 dm3 min−1 in elite athletes during maximal exercise.

Section 7 of 9Exam-style questions

  1. Describe how air is drawn into the lungs during inspiration. [4]4 marks

  2. Explain why expiration at rest is described as passive but forced expiration is not. [2]2 marks

  3. The tidal volume of a person is 0.45 dm3 and the ventilation is 6.3 dm3 min−1. Calculate the breathing rate. [2]2 marks

Section 8 of 9Key terms

TermMeaning
VentilationMovement of air into and out of the lungs.
DiaphragmDome-shaped muscle beneath the lungs that flattens to inspire.
External intercostal musclesMuscles that raise the ribs during inspiration.
Internal intercostal musclesMuscles that pull ribs down in forced expiration.
Antagonistic musclesPair of muscles whose actions oppose each other.
Elastic recoilReturn of stretched lung tissue to its original size.

Section 9 of 9Exam technique

1
Saying air is sucked in by the lungs; the lungs are passive and pressure change in the thorax causes airflow.
2
Stating that the diaphragm relaxes during inspiration.
3
Forgetting that lung pressure must be lower than atmospheric for inspiration.
4
Describing the pressure and volume relationship the wrong way round.
5
Ignoring the internal intercostals in forced expiration.

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