During a sprint race, a runner's muscles respire anaerobically. Explain what happens to the lactic acid that accumulates in the muscles during the race.

Cambridge International IGCSE Biology (0610) — 12.3 Anaerobic respiration · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Lactic acid produced in the muscles is transported via the bloodstream to the liver.
In the liver it is oxidised (the Cori cycle) using oxygen.
This oxidation requires extra oxygen, creating an oxygen debt.
After the sprint the breathing and heart rates stay high to supply the extra oxygen needed to repay the debt.

Examiner tips

  • Use the phrase "Cori cycle" to show knowledge of the pathway. Show the sequence: muscle → blood → liver → oxidation. Mention the oxygen debt and post‑exercise elevated rates. Keep the answer concise and use correct UK spelling.
  • common_mistakes
  • :
  • Confusing lactic acid with lactate; students sometimes write "lactate is transported". Forgetting to mention the oxygen debt or the elevated breathing/heart rates. Using vague terms like "more oxygen" instead of "extra oxygen".

Mark scheme (4 marks)

  1. Lactic acid is transported to the liver
  2. Lactic acid is oxidised / broken down using oxygen
  3. This requires extra oxygen, creating an oxygen debt
  4. Breathing rate / heart rate remains elevated after exercise to supply the extra oxygen needed

Key terms in this question

lactic acid

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