A sports scientist is studying the effects of exercise intensity on respiration in athletes. During a high-intensity sprint, an athlete's muscles work so hard that oxygen cannot be delivered quickly enough to meet demand. Explain what happens to respiration in the athlete's muscles during and immediately after the sprint, and why the athlete continues to breathe heavily for several minutes after stopping.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During a 400 m sprint, an athlete's breathing rate and heart rate increase dramatically. Even after crossing the finish line, the athlete remains bent over, breathing deeply for several minutes.
Model answer (5 marks)
During the sprint the muscles cannot receive enough oxygen, so they switch from aerobic to anaerobic respiration.
Anaerobic respiration breaks down glucose to pyruvate without oxygen, producing lactic acid as a by‑product.
The accumulation of lactic acid lowers pH in the muscle, causing fatigue, pain and cramps, which limit performance.
After the sprint the athlete keeps breathing heavily to take in extra oxygen. The extra oxygen is used to oxidise the lactic acid (re‑oxidise it to pyruvate) and to repay the oxygen debt incurred during the sprint.
The lactic acid is carried in the blood to the liver, where it is converted back to glucose via gluconeogenesis.
Anaerobic respiration breaks down glucose to pyruvate without oxygen, producing lactic acid as a by‑product.
The accumulation of lactic acid lowers pH in the muscle, causing fatigue, pain and cramps, which limit performance.
After the sprint the athlete keeps breathing heavily to take in extra oxygen. The extra oxygen is used to oxidise the lactic acid (re‑oxidise it to pyruvate) and to repay the oxygen debt incurred during the sprint.
The lactic acid is carried in the blood to the liver, where it is converted back to glucose via gluconeogenesis.
Examiner tips
- Use the exact terms: anaerobic respiration, lactic acid, oxygen debt, gluconeogenesis.
- Show the cause–effect chain: oxygen shortage → anaerobic → lactic acid → fatigue → heavy breathing to repay debt and remove acid.
- Mention transport to liver for conversion back to glucose.
- Keep answer concise and to the point.
Common mistakes
- Confusing aerobic with anaerobic respiration; not stating that oxygen is insufficient.
- Failing to mention the oxygen debt or the role of the liver in converting lactic acid back to glucose.
- Using vague terms like "fatigue" without linking it to lactic acid accumulation.
Mark scheme (5 marks)
- During the sprint, muscles switch to anaerobic respiration (because oxygen supply is insufficient)
- Anaerobic respiration produces lactic acid (in addition to releasing energy from glucose)
- Lactic acid causes muscle fatigue / pain / cramp (which limits performance)
- After the sprint, the athlete continues to breathe heavily to take in extra oxygen to break down / oxidise the lactic acid (repaying the oxygen debt)
- The lactic acid is transported (in the blood) to the liver where it is converted back to glucose
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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