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This lesson covers gaseous exchange at the alveoli as required by the OCR GCSE PE specification (J587). You need to understand the process by which oxygen enters the blood and carbon dioxide leaves the blood, the features of the alveoli that make this process efficient, and the role of red blood cells in transporting oxygen.
Gaseous exchange is the process by which oxygen (O₂) passes from the air in the alveoli into the blood, and carbon dioxide (CO₂) passes from the blood into the air in the alveoli. This exchange occurs by diffusion — the movement of molecules from an area of high concentration to an area of low concentration.
graph LR
A["Air in alveolus<br>(high O₂, low CO₂)"] -->|"O₂ diffuses into blood"| B["Blood in capillary"]
B -->|"CO₂ diffuses into alveolus"| A
style A fill:#27ae60,color:#fff
style B fill:#e74c3c,color:#fff
The alveoli have several features that maximise the efficiency of gaseous exchange:
| Feature | How It Helps |
|---|---|
| Enormous surface area | ~300 million alveoli provide a surface area of ~70 m², allowing a huge amount of exchange to occur simultaneously |
| Thin walls (one cell thick) | Short diffusion distance — gases can pass through very quickly |
| Rich blood supply (dense capillary network) | A constant flow of blood maintains the concentration gradient — deoxygenated blood is always arriving, and oxygenated blood is always leaving |
| Moist lining | Gases dissolve in the moisture, making diffusion easier |
| Good ventilation | Continuous breathing brings fresh air (high O₂) and removes stale air (high CO₂), maintaining the concentration gradient |
Exam Tip: When asked to explain why gaseous exchange is efficient, always mention: (1) large surface area, (2) thin walls (short diffusion distance), (3) rich blood supply (maintains concentration gradient), and (4) moist lining. These four features are the key to a full-marks answer.
Red blood cells (erythrocytes) are essential for transporting oxygen from the lungs to the working muscles.
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