Explain why the gills of a fish are an effective exchange surface for gas exchange between the water and the blood.

OCR A-Level Biology A (H420) — 3.1 Exchange surfaces · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Fish obtain oxygen from water passing over their gills. The gill filaments are lined with thin, flattened epithelial cells and are supplied with an extensive network of blood capillaries. Water flows over the gills in a direction opposite to the flow of blood within the capillaries.

Model answer (4 marks)

1. The gill filaments are very thin and flattened, giving a large surface area for diffusion.
2. The epithelial cells are only a few micrometres thick, so the diffusion distance for O₂ and CO₂ is very short.
3. The counter‑current arrangement of water over the gills and blood in the capillaries keeps a steep O₂ gradient along the whole length, allowing continuous diffusion.
4. A rich capillary network and active ventilation keep the blood O₂ concentration low, maintaining the gradient and ensuring efficient gas exchange.

Examiner tips

  • Use the exact terms: surface area, thin, counter‑current, steep gradient.
  • Show the four points in separate sentences or numbered points.
  • Mention both O₂ uptake and CO₂ release if space allows.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Confusing counter‑current with co‑current flow.
  • Lumping surface area and thickness together instead of separating them.
  • Forgetting to mention the blood supply or ventilation.

Mark scheme (4 marks)

  1. Large surface area increases the rate of diffusion
  2. Short diffusion pathway / thin exchange surface
  3. Countercurrent flow maintains a concentration gradient along the entire length of the gill
  4. Good blood supply / ventilation maintains steep concentration gradient

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