Explain how gas exchange occurs in fish gills and why the counter-current system makes this process more efficient than if water and blood flowed in the same direction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Fish extract dissolved oxygen from water as it passes over their gills. Blood flowing through the gill filaments picks up oxygen and releases carbon dioxide.
Model answer (5 marks)
Water flows over the gill filaments (lamellae) in one direction, while blood flows in the opposite direction (counter‑current). This arrangement keeps a concentration gradient of oxygen along the entire length of the lamellae, so oxygen always diffuses from the water into the blood. Consequently, a larger amount of oxygen is absorbed compared with a parallel flow system. In parallel flow the concentrations of oxygen in water and blood would equalise along the length of the lamellae, so diffusion would stop before all available oxygen is taken up.
Examiner tips
- Use the terms 'lamellae', 'counter‑current', 'concentration gradient', 'diffusion', and 'parallel flow' as they appear in the mark scheme.
- Show the sequence of water → lamellae → blood and explain the direction of flow and its effect on the gradient.
- Mention the result – more oxygen absorbed and why diffusion would cease in parallel flow.
Common mistakes
- Confusing the direction of flow or saying both water and blood flow in the same direction.
- Failing to mention the concentration gradient or the diffusion of oxygen into blood.
- Not explaining why parallel flow would stop diffusion before all oxygen is absorbed.
Mark scheme (5 marks)
- Water flows over the gill filaments / lamellae in one direction
- Blood flows in the opposite direction to water (counter-current)
- A concentration gradient is maintained along the entire length of the lamella / oxygen always diffuses from water into blood
- More oxygen is absorbed / diffuses into the blood compared with parallel flow
- In parallel flow the concentrations would equalise / reach equilibrium so diffusion would stop (before all available oxygen is absorbed)
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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