Edexcel IGCSE Biology · Spec 2.46-2.48
Breathing and the Lungs
The breathing system, how we breathe in and out, and how the alveoli are adapted for gas exchange.
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Breathing and the Lungs
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Explained
Breathing, and the pressure change that drives it
Breathing and gas exchange are two different things, and questions expect you to keep them apart. Breathing is the movement of air in and out of the lungs, caused by pressure differences. Gas exchange is diffusion between the alveoli and the blood. One is mechanical, the other is not.
The route air takes
Air passes through the trachea, which is held open by rings of cartilage, then into two bronchi, then into smaller bronchioles, and finally into the alveoli, the tiny air sacs where exchange happens.
The lungs are surrounded by pleural membranes with fluid between them, which reduces friction as the lungs move against the chest wall.
Breathing in
The diaphragm contracts and flattens, moving downwards. The external intercostal muscles contract, pulling the ribs up and out.
Both movements increase the volume of the thorax. A larger volume means a lower pressure inside the thorax, and air flows in from the higher pressure outside.
Note the direction of that chain. Volume increases, so pressure decreases. Getting it the wrong way round is one of the recorded errors on this topic.
Breathing out
The diaphragm relaxes and returns to its domed shape. The ribs move down and in. The volume of the thorax decreases, the pressure rises, and air is pushed out.
At rest, breathing out is largely passive. The muscles relax rather than actively pushing, which is a point worth making if a question asks you to compare the two.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology are precise here. Candidates should refer to the increase in volume of the thorax rather than the volume of the lungs. The thorax is the cavity whose volume the muscles change; the lungs follow.
The reports also note candidates describing exhalation when asked about inhalation, and a few who incorrectly said pressure increases when thorax volume increases.
A credited answer is described as stating that the diaphragm moves down, the volume of the thorax increases, and the pressure decreases. Three steps, in that order.
How the alveoli are adapted
- There are millions of them, giving a very large surface area.
- The walls are one cell thick, so the diffusion distance is short.
- They have a rich blood supply, which keeps the concentration gradient steep.
- They are moist, so gases dissolve before diffusing.
- Ventilation constantly replaces the air, maintaining the gradient.
Each of those is an application of the factors affecting diffusion, so an answer that names the adaptation and then says which factor it improves is doing exactly what the mark scheme asks.
Spec 2.46-2.48
What you need to know
- Name the parts of the breathing system
- Describe how we breathe in and out
- Explain how alveoli are adapted for gas exchange
Active recall
Quick check
Answer each question before opening the answer.
Describe what happens during inhalation.
The diaphragm contracts and flattens, external intercostal muscles contract pulling ribs up and out, chest volume increases and pressure falls, so air moves in.
Give three adaptations of alveoli for gas exchange.
Any three of: large surface area, thin (one-cell) walls, moist surface, good blood supply maintaining a steep concentration gradient.
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