The Lungs and Gas Exchange

The respiratory system is responsible for getting oxygen into the body and removing carbon dioxide. Gas exchange occurs in the alveoli of the lungs, where oxygen diffuses into the blood and carbon dioxide diffuses out. This lesson covers the structure and function of the lungs and gas exchange surfaces, as required by the AQA GCSE Combined Science Trilogy specification (8464).

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Structure of the Respiratory System

graph TD
    A["Nose and Mouth"] --> B["Trachea (windpipe)"]
    B --> C["Bronchi (singular: bronchus)"]
    C --> D["Bronchioles"]
    D --> E["Alveoli (air sacs)"]
    F["Ribs and intercostal muscles"] -.-> G["Protect lungs and assist breathing"]
    H["Diaphragm"] -.-> I["Sheet of muscle below the lungs that assists breathing"]

Structure	Description	Function
Trachea	A tube reinforced with C-shaped rings of cartilage	Carries air from the mouth/nose to the bronchi; cartilage keeps it open
Bronchi	Two branches of the trachea, one leading to each lung	Carry air into the lungs
Bronchioles	Smaller tubes that branch off from the bronchi	Carry air to the alveoli
Alveoli	Tiny air sacs at the ends of the bronchioles (about 300 million in each lung)	Site of gas exchange
Diaphragm	A sheet of muscle beneath the lungs	Contracts and relaxes to change the volume of the thorax during breathing
Intercostal muscles	Muscles between the ribs	Contract and relax to move the ribs during breathing
Pleural membranes	Double membrane surrounding each lung with pleural fluid between them	Reduce friction during breathing movements

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The Mechanism of Breathing

Inhalation (Breathing In)

1. The intercostal muscles contract, pulling the ribs up and out
2. The diaphragm contracts and flattens (moves down)
3. The volume of the thorax (chest cavity) increases
4. The pressure inside the thorax decreases (below atmospheric pressure)
5. Air is drawn into the lungs down the pressure gradient

Exhalation (Breathing Out)

1. The intercostal muscles relax, and the ribs move down and in
2. The diaphragm relaxes and curves upwards (returns to its dome shape)
3. The volume of the thorax decreases
4. The pressure inside the thorax increases (above atmospheric pressure)
5. Air is pushed out of the lungs

	Inhalation	Exhalation
Intercostal muscles	Contract	Relax
Ribs	Move up and out	Move down and in
Diaphragm	Contracts (flattens)	Relaxes (domes up)
Thorax volume	Increases	Decreases
Thorax pressure	Decreases	Increases
Air flow	Into the lungs	Out of the lungs

Exam Tip: Remember that breathing in is an active process (muscles contract, requiring energy). Normal breathing out is largely passive (muscles relax and the elastic lungs recoil). You can remember this with: "In = intercostals and diaphragm contract."

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Gas Exchange in the Alveoli

Gas exchange is the process by which oxygen moves from the air in the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli.

This occurs by diffusion — the net movement of particles from an area of higher concentration to an area of lower concentration, down the concentration gradient.

graph LR
    subgraph Alveolus
        A1["High O₂ concentration"]
        A2["Low CO₂ concentration"]
    end
    subgraph Capillary Blood
        B1["Low O₂ concentration"]
        B2["High CO₂ concentration"]
    end
    A1 -->|"O₂ diffuses into blood"| B1
    B2 -->|"CO₂ diffuses into alveolus"| A2

Adaptations of Alveoli for Efficient Gas Exchange

Adaptation	How It Increases the Rate of Gas Exchange
Enormous surface area	About 300 million alveoli provide a combined surface area of roughly 70 m² (about the size of a tennis court) — more surface for diffusion
Walls are only one cell thick	Very short diffusion distance — gases can cross quickly
Moist lining	Gases dissolve in the moisture, making diffusion easier
Rich blood supply (dense capillary network)	Blood continuously flows past, maintaining a steep concentration gradient for both O₂ and CO₂
Good ventilation	Breathing constantly refreshes the air in the alveoli, maintaining a high O₂ and low CO₂ concentration

The rate of diffusion can be summarised by Fick's Law:

$$\text{Rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of membrane}}$$Rate of diffusion∝thickness of membranesurface area×concentration difference​

This equation shows that gas exchange is maximised when:

• Surface area is large (many alveoli)
• Concentration difference is large (maintained by blood flow and ventilation)
• Membrane thickness is small (one cell thick)

Exam Tip: When explaining why alveoli are efficient at gas exchange, always refer to at least three adaptations and explain HOW each one increases the rate of diffusion. Link each feature to the relevant factor in Fick's Law.

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Worked Example

Question: Explain how the structure of the alveoli is adapted for efficient gas exchange. [6 marks]

Answer:

1. Alveoli have a very large surface area (about 70 m² total) because there are approximately 300 million of them — this means there is a large area over which diffusion can occur ✓
2. The alveolar walls are one cell thick, providing a very short diffusion distance so that oxygen and carbon dioxide can cross rapidly ✓
3. Each alveolus is surrounded by a dense network of blood capillaries — blood constantly flows past, carrying oxygen away and bringing carbon dioxide — this maintains a steep concentration gradient ✓
4. The alveoli have a moist lining, which allows gases to dissolve before diffusing across the membrane ✓
5. Breathing (ventilation) continuously brings fresh air into the alveoli, maintaining a high oxygen and low carbon dioxide concentration inside them ✓
6. The capillary walls are also only one cell thick, further reducing the overall diffusion distance ✓

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Common Mistakes