# During vigorous exercise, muscles require large amounts of energy for contraction. Explain how the body responds to meet the increased energy demands of contracting muscles during vigorous exercise.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.2 Muscle contraction · Explain · 5 marks

> A student runs a 400-metre race at full speed. Within seconds, their breathing rate increases, their heart rate rises, and their muscles begin to fatigue towards the end of the race.

## Mark scheme (5 marks)

1. Muscles need energy in the form of ATP for contraction
2. Aerobic respiration in muscle cells produces ATP using glucose and oxygen
3. Heart rate increases to deliver more oxygenated blood / glucose to the muscles more quickly
4. Breathing rate / depth increases to take in more oxygen and remove carbon dioxide
5. When oxygen supply is insufficient, anaerobic respiration occurs producing lactic acid, which causes muscle fatigue

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