Figure 8 shows parts of the human breathing system.
Explain how the human breathing system is adapted to maximise the rate of gas exchange.
How to approach this question
This is a 6-mark question, so you need to make several distinct points. Structure your answer around the key features needed for efficient diffusion (based on Fick's Law).
1. **Large Surface Area:** How do the lungs achieve this? Mention the specific structures.
2. **Short Diffusion Pathway:** How are the structures designed to make the distance for gas movement as short as possible? Mention the thickness of the walls.
3. **Steep Concentration Gradient:** How is a high concentration of oxygen and a low concentration of carbon dioxide maintained in the lungs, and vice versa in the blood? Mention the blood supply and ventilation.
Full Answer
The rate of diffusion is maximised when there is a large surface area, a short diffusion pathway, and a steep concentration gradient. The human lungs are adapted to provide all three.
1. **Large Surface Area:** The lungs contain millions of tiny air sacs called alveoli. This structure, resembling a bunch of grapes, creates an enormous total surface area (around 70m² in an adult) for gas exchange to occur simultaneously.
2. **Short Diffusion Pathway:** The walls of the alveoli are extremely thin, only one cell thick. The capillaries that surround the alveoli also have walls that are one cell thick. This means that oxygen and carbon dioxide only have to diffuse across two single layers of cells, creating a very short distance between the air in the lungs and the blood.
3. **Steep Concentration Gradient:**
* **Good Blood Supply:** A dense network of capillaries constantly flows around the alveoli. This blood continuously transports oxygen away from the lungs to the body's cells and brings carbon dioxide from the cells to the lungs. This ensures that the blood arriving at the alveoli is always low in oxygen and high in carbon dioxide, maintaining a steep gradient for diffusion.
* **Ventilation:** The process of breathing in and out constantly replaces the air inside the alveoli. This brings in fresh air with a high concentration of oxygen and removes stale air with a high concentration of carbon dioxide, again ensuring the concentration gradient is as steep as possible.
Common mistakes
✗ Only listing the features (e.g., "large surface area, thin walls") without explaining *how* they help maximise the rate of gas exchange.\n✗ Not making enough distinct points to earn all 6 marks.\n✗ Confusing the direction of gas movement (e.g., saying CO2 moves from lungs to blood).
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