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This lesson covers gas exchange in the lungs — specifically the structure and adaptations of the alveoli — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to explain how oxygen and carbon dioxide are exchanged between the air and the blood, describe how alveoli are adapted for efficient gas exchange and explain the role of ventilation.
The respiratory system brings air into and out of the body so that gas exchange can take place in the lungs.
| Structure | Function |
|---|---|
| Trachea (windpipe) | Carries air from the throat to the bronchi; lined with cilia and mucus to trap particles |
| Bronchi (singular: bronchus) | Two branches from the trachea; one leading to each lung |
| Bronchioles | Smaller branches of the bronchi; carry air deeper into the lungs |
| Alveoli (singular: alveolus) | Tiny air sacs at the end of bronchioles; the site of gas exchange |
| Diaphragm | Dome-shaped muscle beneath the lungs; contracts and flattens during inhalation |
| Intercostal muscles | Muscles between the ribs; contract to lift the rib cage during inhalation |
| Pleural membranes | Double membrane surrounding each lung; reduces friction during breathing |
graph TD
A["Air inhaled through nose/mouth"] --> B["Trachea"]
B --> C["Bronchi (left and right)"]
C --> D["Bronchioles"]
D --> E["Alveoli — gas exchange occurs here"]
E --> F["O₂ diffuses into blood"]
E --> G["CO₂ diffuses out of blood into 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 air in the alveoli. Both gases move by diffusion — from a region of higher concentration to a region of lower concentration.
| Gas | Direction | Concentration gradient |
|---|---|---|
| Oxygen | Alveolar air → blood | Higher in alveoli, lower in blood |
| Carbon dioxide | Blood → alveolar air | Higher in blood, lower in alveoli |
The lungs contain approximately 300 million alveoli, giving an enormous total surface area of about 70 m² (roughly the area of a tennis court). This huge surface area is essential for efficient gas exchange.
| Adaptation | How it increases the rate of gas exchange |
|---|---|
| Enormous surface area (millions of alveoli) | More area for diffusion to occur across |
| Walls just one cell thick | Very short diffusion distance (about 1 μm) |
| Rich blood supply from capillaries | Maintains a steep concentration gradient: oxygen is constantly carried away and CO₂ is constantly delivered |
| Moist lining | Gases dissolve in the moisture, which allows them to diffuse across the cell membranes |
| Good ventilation (breathing in and out) | Constantly refreshes the air in the alveoli, maintaining the concentration gradient |
graph LR
A["Alveolar air (high O₂, low CO₂)"]
B["Alveolar wall (one cell thick)"]
C["Capillary wall (one cell thick)"]
D["Blood in capillary (low O₂, high CO₂)"]
A -->|"O₂ diffuses across"| B
B --> C
C --> D
D -->|"CO₂ diffuses across"| C
C --> B
B --> A
Exam Tip: When explaining how alveoli are adapted for gas exchange, always link each adaptation to Fick's law factors: large surface area, thin walls (short diffusion distance) and steep concentration gradient (maintained by blood flow and ventilation).
Fick's law describes the rate of diffusion:
Rate of diffusion∝thickness of membranesurface area×concentration difference
The alveoli are adapted to maximise all three factors:
| Factor | How alveoli maximise it |
|---|---|
| Surface area | Millions of alveoli; total area ~70 m² |
| Concentration difference | Blood flow and ventilation maintain steep gradient |
| Membrane thickness | Alveolar and capillary walls each one cell thick (~1 μm total) |
Ventilation is the mechanical process of moving air into and out of the lungs. It maintains the concentration gradient for gas exchange by bringing fresh air (high O₂, low CO₂) into the alveoli and removing stale air (lower O₂, higher CO₂).
| Phase | Diaphragm | Intercostals | Thorax volume | Lung pressure | Air movement |
|---|---|---|---|---|---|
| Inhalation | Contracts (flattens) | External contract (rib cage up and out) | Increases | Decreases | Air in |
| Exhalation | Relaxes (domes up) | Internal contract (rib cage down and in) | Decreases | Increases | Air out |
Exam Tip: Remember: ventilation is a pressure change process. The lungs themselves do not contain muscle — they are inflated and deflated by changes in the volume (and therefore pressure) of the thorax caused by the diaphragm and intercostal muscles.
| Gas | Inhaled air (%) | Exhaled air (%) |
|---|---|---|
| Nitrogen | ~78 | ~78 |
| Oxygen | ~21 | ~16 |
| Carbon dioxide | ~0.04 | ~4 |
| Water vapour | Variable | Saturated (high) |
The exhaled air contains less oxygen (some has been absorbed into the blood) and more carbon dioxide (released from the blood into the alveoli).
| Condition | Effect on gas exchange |
|---|---|
| Asthma | Bronchioles become inflamed and constricted; less air reaches alveoli |
| Chronic obstructive pulmonary disease (COPD) | Alveolar walls break down, reducing surface area; bronchioles produce excess mucus |
| Lung cancer | Tumour growth reduces the surface area available for gas exchange |
| Smoking | Tar damages cilia, increases mucus; destroys alveoli over time (emphysema) |
Exam Tip: Smoking-related questions are common. Link the damage to reduced surface area (alveoli destroyed) and reduced concentration gradient (mucus and inflammation block gas diffusion).
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