The structure of the lungs and mechanism of breathing
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of the structure and function of cells and organ systems, including the role of diffusion in the movement of substances.
What is diffusion?
The net movement of particles from an area of higher concentration to an area of lower concentration.
What factors affect the rate of diffusion?
Concentration gradient, temperature, and surface area.
What is the role of the cell membrane in diffusion?
It allows certain substances to pass in and out of the cell by diffusion (as it is partially permeable).
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the structure of the lungs and mechanism of breathing, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
The Lungs and Alveoli
The Structure of the Lungs
The lungs are part of the respiratory system, adapted to carry out efficient gas exchange.
Air Pathway through the Lungs:
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Trachea – windpipe carrying air into the chest.
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Bronchi – two tubes branching off the trachea into each lung.
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Bronchioles – smaller branches inside each lung.
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Alveoli – tiny air sacs at the end of bronchioles where gas exchange occurs.
The lungs are surrounded by the ribcage and the diaphragm, which help with breathing.
Mechanism of Breathing
Breathing (also called ventilation) is the mechanical process of moving air in and out of the lungs. It ensures that oxygen continuously enters the alveoli and carbon dioxide is removed. Breathing is an active process that requires muscle contraction so requires energy. Breathing replaces the air in the lungs and maintains the concentration gradient, increasing the rate of diffusion of oxygen into the blood.
Inhalation (Breathing In):
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Intercostal muscles contract – ribcage moves up and out.
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Diaphragm contracts – flattens and moves down.
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The volume of the lungs increases.
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Pressure inside the lungs decreases.
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Air is drawn into the lungs (from higher pressure outside to lower pressure inside).
Exhalation (Breathing Out):
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Diaphragm relaxes – diaphragm moves up into a domed shape.
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Intercostal muscles relax – ribcage moves down and in.
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Volume of the thorax decreases.
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Pressure inside the lungs increases.
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Air is forced out of the lungs (from higher pressure inside to lower pressure outside).
Alveoli – Adaptations for Gas Exchange
Alveoli – Adaptations for Gas Exchange
The alveoli are the site of gas exchange between the air and the blood. Oxygen moves into the blood, and carbon dioxide moves out, by diffusion.
Adaptations of alveoli:
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Large surface area – to maximise rate of diffusion.
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Thin walls of alveoli (one cell thick) – to provide a short diffusion distance.
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Lots of capillaries – maintains a steep concentration gradient for oxygen and carbon dioxide.
Key Terms
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Trachea – windpipe carrying air into the chest.
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Bronchi – two tubes branching off the trachea into each lung.
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Bronchioles – smaller branches inside each lung.
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Alveoli – tiny air sacs at the end of bronchioles where gas exchange occurs.
No answer provided.
Exam Tips
- Don’t say ‘alveoli are thin’ – say ‘alveoli have thin walls to shorten the diffusion distance for gases’.
- Don't mix up breathing and respiration. Breathing is an active process that moves air in and out of the lungs. Respiration is a chemical reaction that happens inside cells to release energy.
No answer provided.
Practice Question
Describe how air is brought into the lungs. (4 marks)
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The diaphragm contracts and flattens / moves down.
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The intercostal muscles contract and lift the ribcage up and out.
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This increases the volume of the lungs.
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The pressure inside the lungs decreases, and air moves in.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!