Explain how the countercurrent flow of water and blood in fish gills maintains a concentration gradient for oxygen along the entire length of a gill lamella.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In counter‑current flow, water and blood move in opposite directions across the gill lamella.
The blood that has already been partially oxygenated meets water that is still richer in O₂, so a diffusion gradient is maintained along the entire length of the lamella.
In a parallel or concurrent arrangement the two fluids would reach equilibrium part‑way along, stopping net diffusion before the blood leaves the lamella.
Thus, up to ~80 % of dissolved O₂ can be extracted from the water, making gas exchange highly efficient.
The blood that has already been partially oxygenated meets water that is still richer in O₂, so a diffusion gradient is maintained along the entire length of the lamella.
In a parallel or concurrent arrangement the two fluids would reach equilibrium part‑way along, stopping net diffusion before the blood leaves the lamella.
Thus, up to ~80 % of dissolved O₂ can be extracted from the water, making gas exchange highly efficient.
Examiner tips
- Use the term "counter‑current flow" early. Show the direction of flow for both water and blood. Explain why the gradient is maintained. Mention the efficiency figure (~80 %).
Common mistakes
- Confusing counter‑current with parallel flow. Forgetting to state that the gradient is maintained along the whole lamella. Omitting the efficiency percentage.
Mark scheme (4 marks)
- In countercurrent flow, water and blood move in opposite directions across the gill lamella.
- Blood that is already partially oxygenated meets water that is even more oxygen-rich, so a diffusion gradient is maintained along the entire length of the lamella.
- In a parallel/concurrent arrangement the two fluids would reach equilibrium part-way along, stopping net diffusion before the blood leaves the lamella.
- As a result, up to ~80% of dissolved oxygen can be extracted from the water, making gas exchange highly efficient.
Key terms in this question
countercurrent flow · gill lamella · concentration gradient
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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