How is the human breathing system built, and how are the alveoli adapted for gas exchange?
The structure of the human breathing system, the mechanism of ventilation, and how the alveoli are adapted for efficient gas exchange by diffusion.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on breathing and gas exchange, covering the structure of the human breathing system, how ventilation works, and how the alveoli are adapted for efficient diffusion of gases.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to describe the structure of the human breathing system, explain the mechanism of ventilation (breathing in and out), and describe how the alveoli are adapted for efficient gas exchange.
The structure of the breathing system
Air enters through the nose and mouth and passes through:
- Trachea (windpipe): carries air to the lungs; held open by rings of cartilage.
- Bronchi: two tubes, one to each lung.
- Bronchioles: smaller branching tubes inside each lung.
- Alveoli: tiny air sacs at the end of the bronchioles where gas exchange happens.
The ribs form a protective cage around the lungs and heart, the intercostal muscles lie between the ribs, and the diaphragm is a sheet of muscle below the lungs.
How ventilation works
Air always moves from high pressure to low pressure, so changing the volume of the chest changes the pressure and makes air flow in or out.
How the alveoli are adapted for gas exchange
The alveoli are adapted so that diffusion is as fast as possible:
- Large surface area: millions of alveoli give a huge area for diffusion.
- Thin walls (one cell thick): the distance gases must diffuse is very short.
- Moist lining: gases dissolve in the moisture before diffusing across.
- Good blood supply (dense capillaries): blood constantly carries oxygen away and brings carbon dioxide, keeping a steep concentration gradient.
These adaptations match the factors that speed up diffusion: a large surface area, a short distance and a steep concentration gradient.
Comparing inhaled and exhaled air
Exam questions often compare the air breathed in with the air breathed out. Inhaled air contains about 21% oxygen and very little carbon dioxide. Exhaled air contains less oxygen (about 16%) and much more carbon dioxide (about 4%), and is warmer and more saturated with water vapour. The change happens because the body has used oxygen in respiration and produced carbon dioxide that diffuses out at the alveoli. The simple test for the extra carbon dioxide is that exhaled air turns limewater cloudy faster than inhaled air.
The direction of gas exchange
At the alveoli, blood arriving from the body is low in oxygen and high in carbon dioxide. This sets up the concentration gradients: oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli. Constant blood flow and constant breathing keep both gradients steep, which is why a good blood supply and regular ventilation are important for efficient gas exchange.
Try this
Q1. Name the tiny air sacs where gas exchange happens. [1 mark]
- Cue. The alveoli.
Q2. State how exhaled air differs from inhaled air in terms of carbon dioxide. [1 mark]
- Cue. Exhaled air contains much more carbon dioxide than inhaled air.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe two ways the alveoli are adapted for efficient gas exchange, and explain how each helps.Show worked answer →
A Unit 1 adaptation question worth 4 marks. Reward: a large surface area (many alveoli) so more gas can diffuse at once; thin walls (one cell thick) so the diffusion distance is short; a moist lining so gases dissolve before diffusing; and a rich blood supply (many capillaries) to maintain a steep concentration gradient. Markers credit any two adaptations linked to faster diffusion. A common error is to list the features without saying they speed up diffusion.
WJEC style3 marksDescribe what happens to the diaphragm and ribs during breathing in (inhalation).Show worked answer →
A Unit 1 ventilation question. During inhalation the diaphragm contracts and flattens (1 mark), the intercostal muscles contract so the ribs move up and out (1 mark), which increases the volume of the chest and lowers the pressure, so air is drawn into the lungs (1 mark). Markers reward the diaphragm movement, the rib movement and the pressure change. A common error is to say the diaphragm "moves up" during inhalation, which describes breathing out.
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