Explain how countercurrent flow in the gills of bony fish increases the efficiency of oxygen uptake compared with concurrent flow.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In counter‑current flow the water and blood move in opposite directions across the gill lamellae. This keeps a concentration gradient for O₂ along the whole length of the lamella, so O₂ continually diffuses from the water into the blood. In concurrent flow the water and blood move in the same direction; the blood equilibrates with the water partway along the lamella, after which no further diffusion can occur. Therefore counter‑current flow allows a much larger proportion of the O₂ in the water (≈80 %) to be absorbed, making gas exchange far more efficient than concurrent flow.
Examiner tips
- Mention the opposite directions of flow and the maintained gradient
- Explain that equilibrium is reached earlier in concurrent flow
- Quantify the efficiency advantage (~80 %)
Common mistakes
- Confusing counter‑current with concurrent flow
- Failing to state that the gradient is maintained along the entire lamella
- Omitting the efficiency percentage or the reason it matters
Mark scheme (4 marks)
- In countercurrent flow, water and blood flow in opposite directions across the gill lamellae.
- A concentration gradient favouring diffusion of oxygen into the blood is maintained along the entire length of the lamella.
- In concurrent flow, blood and water equilibrate (reach the same oxygen concentration) partway along the lamella, so no further diffusion occurs.
- Countercurrent flow therefore allows a greater proportion of the oxygen in the water to be absorbed (up to ~80%), making gas exchange more efficient than concurrent flow.
Key terms in this question
countercurrent flow · concurrent flow
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Gas exchange questions
- Explain how the ventilation of the lungs maintains the concentration gradients n…
- Explain how the tracheal system of insects allows gas exchange to occur without …
- Explain how the structure of a gill lamella is adapted to facilitate efficient g…
- Explain how the structure of the human trachea and bronchioles enables efficient…
- Explain how the structure of an alveolus is adapted to facilitate efficient gas …
- Explain how the skin of a frog functions as a gas exchange surface.