Explain why a person's breathing rate and heart rate remain elevated for several minutes after they finish a period of vigorous exercise.

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)

During vigorous exercise the muscles cannot receive enough oxygen, so they use anaerobic respiration and produce lactic acid.
Lactic acid builds up in the muscle fibres, creating an oxygen debt that must be repaid after the activity.
The lactic acid is carried to the liver where it is converted back to pyruvate and then to glucose or CO₂, a process that requires oxygen.
To provide this extra oxygen the breathing rate remains high, and the heart rate stays elevated so that oxygen‑rich blood and lactic acid are transported more quickly.

Examiner tips

  • Use the word ‘anaerobic respiration’ and mention lactic acid explicitly. Explain the oxygen debt and the need for extra oxygen. Show the link between breathing, heart rate and oxygen delivery.
  • common_mistakes
  • :
  • Forgetting that the oxygen debt is repaid by breathing and heart rate. Using vague terms like ‘more oxygen’ without linking to lactic acid removal. Mixing up aerobic and anaerobic respiration in the explanation.

Mark scheme (4 marks)

  1. During vigorous exercise, anaerobic respiration occurs (because oxygen supply to muscles is insufficient), producing lactic acid.
  2. Lactic acid accumulates in muscles, creating an oxygen debt that must be repaid after exercise.
  3. Lactic acid must be transported to the liver where it is broken down / oxidised using oxygen.
  4. Breathing rate remains high to take in extra oxygen to repay the oxygen debt, and heart rate remains high to transport this oxygen (and lactic acid) more quickly.

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