BreathingOxford AQA IGCSE Biology: Revision notes
Section 1
What is the structure of the human respiratory system?
The respiratory system is a series of organs and tissues through which air travels to enable gas exchange. The pathway of air is:
- Trachea – a single windpipe that carries air from the nose and mouth down into the chest
- Bronchi – the trachea splits into two bronchi, one going to each lung
- Bronchioles – smaller branches that subdivide from the bronchi within the lungs
- Alveoli – tiny air sacs at the end of bronchioles where gas exchange occurs
The respiratory system also includes muscular and skeletal components:
- Diaphragm – a dome-shaped muscle below the lungs that contracts and relaxes to change the volume of the thoracic cavity
- Intercostal muscles – muscles between the ribs; external intercostal muscles pull ribs upward and outward, internal intercostal muscles pull them downward and inward
- Ribs – bony structures that protect the lungs and work with intercostal muscles to change thoracic volume
All these structures work together to draw air into the lungs and facilitate gas exchange.
When describing the respiratory system, examiners expect you to name each structure in order and explain its role. Simply listing structures without explaining their function will not gain full marks.
Section 2
How does the mechanism of breathing work in terms of pressure and volume?
Breathing (ventilation) involves two stages: inhalation (breathing in) and exhalation (breathing out). Both rely on pressure and volume changes in the thoracic cavity.
Inhalation (inspiration):
- The diaphragm contracts and flattens, moving downward
- External intercostal muscles contract, pulling the ribs upward and outward
- The volume of the thoracic cavity increases
- As volume increases, pressure inside the lungs decreases (becomes lower than atmospheric pressure)
- Air flows into the lungs (from high to low pressure)
Exhalation (expiration):
- The diaphragm relaxes and returns to its dome shape, moving upward
- External intercostal muscles relax; internal intercostal muscles may contract (in forced exhalation)
- The volume of the thoracic cavity decreases
- As volume decreases, pressure inside the lungs increases (becomes higher than atmospheric pressure)
- Air flows out of the lungs (from high to low pressure)
Key principle: Gas always flows from an area of high pressure to an area of low pressure. During breathing, pressure changes in the thoracic cavity create the pressure difference that moves air.
Examiners mark answers about breathing based on correct sequencing and linking pressure/volume changes to muscle action. Write 'when the diaphragm contracts, volume increases, so pressure decreases, causing air to flow in' to show understanding of the mechanism.
Think of the lungs like a syringe: pulling the plunger out increases volume and decreases pressure (air flows in); pushing it in decreases volume and increases pressure (air flows out).
Section 3
How are alveoli specially adapted for efficient gas exchange?
The alveoli are perfectly designed to maximise the rate of gas exchange between air and blood. Their adaptations are:
| Adaptation | How it helps gas exchange |
|---|---|
| Large surface area | Thousands of alveoli in each lung provide an enormous total surface area for oxygen to diffuse into capillaries and carbon dioxide to diffuse out |
| Thin walls | The wall of each alveolus is only one cell thick, providing a short diffusion distance for gases |
| Moist surface | Gases dissolve in the thin layer of moisture (fluid) lining the alveolus before diffusing across |
| Good blood supply | Each alveolus is surrounded by a dense network of capillaries; blood constantly removes oxygen and brings carbon dioxide |
These four features work together to ensure gas exchange happens as quickly and efficiently as possible. The combination of large surface area and thin walls minimises diffusion distance and maximises the area available. The moist surface is essential because gases must dissolve before diffusing. The good blood supply maintains the concentration gradient by continuously removing oxygen-rich blood and bringing oxygen-poor blood.
Gas exchange in alveoli:
- Oxygen diffuses from the air in the alveolus across the thin wall into the capillary blood (to be transported to cells)
- Carbon dioxide diffuses from the capillary blood (where it is carried from respiring cells) into the air in the alveolus (to be exhaled)
When explaining alveolar adaptations, always link the structural feature to how it improves gas exchange rate. For example: 'thin walls provide a short diffusion distance, so oxygen diffuses quickly into the blood' is much stronger than 'thin walls allow gas exchange'.
Students often forget to mention the moist surface. Remember: gases must dissolve in fluid before they can diffuse across the alveolar wall. This is a separate, essential adaptation.
Section 4
What is the composition of inhaled and exhaled air?
The air we breathe in and out has different compositions. This difference shows which gases are used by the body during respiration.
| Gas | Inhaled air (%) | Exhaled air (%) | Comment |
|---|---|---|---|
| Oxygen (O₂) | 21 | 16 | Some oxygen is taken from the air by the blood in the lungs |
| Carbon dioxide (CO₂) | 0.04 | 4 | Carbon dioxide produced by respiring cells is added to the air in the lungs |
| Nitrogen (N₂) | 78 | 78 | Nitrogen is not used by the body and passes through unchanged |
| Water vapour (H₂O) | Variable | Higher | Water vapour from respiration and the moist lining of the airways is exhaled |
| Argon and other gases | 0.96 | 0.96 | These remain unchanged |
Why these differences occur:
- Oxygen decreases because it diffuses from the alveolar air into the capillary blood, where it binds to haemoglobin in red blood cells
- Carbon dioxide increases because it diffuses out of the capillary blood (carried from respiring cells) into the alveolar air
- Nitrogen remains constant because it is biologically inert and does not participate in respiration
- Water vapour increases because the warm, moist lining of the respiratory tract adds water to exhaled air
These changes show that the lungs are actively exchanging gases with the blood.
If a student breathes in 500 cm³ of air containing 21% O₂, the lungs receive 105 cm³ of oxygen. If exhaled air contains 16% O₂, only 80 cm³ leaves. The difference (25 cm³) is the oxygen transferred to the blood. This quantifies gas exchange.
Section 5
What are the effects of smoking on the respiratory system?
Tobacco smoke contains harmful substances that damage the respiratory system and increase the risk of serious diseases.
Main harmful components of tobacco smoke:
| Component | Effects on the body |
|---|---|
| Tar | Sticky substance that coats the lining of the airways and lungs; accumulates in the alveoli, damaging them and reducing gas exchange surface area; increases risk of lung cancer |
| Nicotine | An addictive stimulant that increases heart rate and blood pressure; damages blood vessel walls; reduces blood flow to tissues |
| Carbon monoxide | A poisonous gas that binds to haemoglobin with higher affinity than oxygen; reduces the oxygen-carrying capacity of blood; starves tissues of oxygen |
Long-term diseases associated with smoking:
-
Lung cancer – Tar is a carcinogen; repeated exposure to tar in the lungs causes cells to become cancerous. Smoking is the leading preventable cause of lung cancer.
-
Bronchitis – Smoking damages the ciliated epithelial cells lining the airways. Cilia normally beat to remove mucus and pathogens. Damaged cilia cannot protect the airways, so the lining becomes inflamed and infected, causing chronic cough and excess mucus production.
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Emphysema – Smoking damages and destroys the alveolar walls. This reduces the surface area available for gas exchange, making breathing difficult and reducing oxygen uptake. The lungs lose elasticity, making it hard to exhale air completely.
Why these diseases develop: Repeated exposure to tar, nicotine, and carbon monoxide causes chronic inflammation, cell damage, and loss of lung tissue. The body cannot repair this damage quickly enough, leading to progressive disease.
To get full marks, explain how each smoking effect causes disease. For example: 'Tar damages cilia, so pathogens are not removed, causing bronchitis' is better than 'tar causes bronchitis'.
Students sometimes confuse emphysema with bronchitis. Remember: emphysema destroys alveolar walls (reducing surface area); bronchitis inflames the airways (causing cough and mucus). Both are caused by smoking but are different diseases.
Must Know
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The respiratory system consists of the trachea, bronchi, bronchioles, and alveoli, with the diaphragm and intercostal muscles controlling breathing by changing thoracic volume
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Inhalation: diaphragm contracts and moves down, external intercostal muscles pull ribs up and out, thoracic volume increases, lung pressure decreases, air flows in
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Exhalation: diaphragm relaxes and moves up, external intercostal muscles relax, thoracic volume decreases, lung pressure increases, air flows out
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Alveoli adaptations: large surface area (many alveoli for gas exchange), thin walls (short diffusion distance), moist surface (gases dissolve before diffusing), good blood supply (constant removal of oxygen, supply of carbon dioxide)
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Air composition: oxygen decreases from 21% to 16% (used in respiration), carbon dioxide increases from 0.04% to 4% (waste product), nitrogen stays at 78% (inert), water vapour increases (from moist airways)
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Smoking damage: tar accumulates in lungs causing lung cancer and bronchitis (inflamed airways); carbon monoxide reduces oxygen transport in blood; emphysema destroys alveolar walls, reducing gas exchange surface area
That's the notes covered.
Carry on to the next subtopic.