How does the respiratory system get oxygen into the blood, and how does breathing work?
The structure of the human respiratory system, the mechanism of breathing in and out, gas exchange in the alveoli and their adaptations, the difference between breathing and respiration, and the effects of smoking.
A focused CCEA GCSE Biology answer on the respiratory system, covering its structure, the mechanism of breathing in and out, gas exchange in the alveoli and their adaptations, breathing versus respiration, and the effects of smoking.
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What this dot point is asking
CCEA wants you to label the parts of the respiratory system, describe how breathing in and out happens in terms of muscles, volume and pressure, explain how the alveoli are adapted for gas exchange, distinguish breathing from respiration, and describe the effects of smoking.
Structure of the respiratory system
The mechanism of breathing
Breathing works by changing the volume and therefore the pressure inside the chest (thorax).
- Breathing in (inhalation): the intercostal muscles contract and pull the ribs up and out; the diaphragm contracts and flattens. The chest volume increases, the pressure inside falls below the outside pressure, so air flows in.
- Breathing out (exhalation): the muscles relax; the ribs move down and in and the diaphragm domes up. The chest volume decreases, the pressure rises above the outside, so air is forced out.
Gas exchange in the alveoli
Alveoli are adapted by a large surface area (millions of them), thin walls (one cell thick), a good blood supply and moist surfaces, all of which speed up diffusion.
Breathing versus respiration
Examples in context
Example 1. Why exercise makes you breathe harder. During exercise the muscles respire faster, using more oxygen and making more carbon dioxide. The body responds by increasing the breathing rate and depth, so more air is moved in and out each minute. This brings in more oxygen for aerobic respiration and removes the extra carbon dioxide quickly. It is a clear demonstration that breathing exists to serve respiration in the cells.
Example 2. The effects of smoking. Tobacco smoke damages the respiratory system in several ways. Tar coats the airways and destroys the cilia that sweep mucus out, so mucus and bacteria build up, causing smoker's cough and bronchitis. Tar and other chemicals can cause lung cancer, and damage to the alveoli walls reduces the surface area for gas exchange, causing emphysema and breathlessness. Carbon monoxide in the smoke binds to haemoglobin, reducing the oxygen the blood can carry. These linked effects are a common CCEA extended-answer topic.
Try this
Q1. Name the small air sacs where gas exchange takes place. [1 mark]
- Cue. The alveoli.
Q2. Explain what happens to the diaphragm and chest volume when you breathe in. [2 marks]
- Cue. The diaphragm contracts and flattens (moves down); the chest volume increases.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20195 marksDescribe how air is breathed into the lungs (inhalation).Show worked answer →
Five marks for the muscles, the volume change and the pressure change in order.
The intercostal muscles between the ribs contract, pulling the ribcage up and out.
The diaphragm contracts and flattens, moving down.
Together these increase the volume of the thorax (chest cavity).
Increasing the volume decreases the air pressure inside the lungs, so it is now lower than the air pressure outside.
Air therefore moves down the pressure gradient, from high pressure outside to low pressure in the lungs, so air flows in.
Markers reward the muscles named, ribs up and out, diaphragm down and flat, volume up, pressure down, air in. Exhalation is the reverse.
CCEA 20214 marksExplain how an alveolus is adapted for efficient gas exchange.Show worked answer →
Four marks for four adaptations linked to diffusion.
There are millions of alveoli, giving a very large total surface area for gases to diffuse across.
Each alveolus has a wall only one cell thick, so the diffusion distance for oxygen and carbon dioxide is very short.
The alveoli have a rich blood supply (a network of capillaries), which keeps a steep concentration gradient by removing oxygen and bringing carbon dioxide.
The surfaces are moist, so gases dissolve before they diffuse.
Markers reward large surface area, thin wall, good blood supply and moist surface, each linked to faster diffusion.
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)