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Response to exercise

Bioenergetics · Respiration · note 2 of 3

Spec 4.4.2.2Download all Respiration notes (PDF)Download all notes (PDF)6 pages

Response to exerciseSpec 4.4.2.2

In short

During exercise, heart rate, breathing rate and breath volume increase to supply the muscles with more oxygenated blood for respiration. If the oxygen supply is insufficient, muscles respire anaerobically. The incomplete oxidation of glucose causes a build-up of lactic acid and creates an oxygen debt. After long periods of vigorous activity, muscles become fatigued and stop contracting efficiently.

During exercise the human body reacts to the increased demand for energy. The heart rate, breathing rate and breath volume increase during exercise to supply the muscles with more oxygenated blood. This brings more oxygen and glucose to the muscles for respiration.

If insufficient oxygen is supplied, anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build-up of lactic acid and creates an oxygen debt. During long periods of vigorous activity, muscles become fatigued and stop contracting efficiently.

  1. Exercise increases the demand for energy in the muscles.
  2. Heart rate, breathing rate and breath volume increase, so more oxygenated blood reaches the muscles.
  3. If the oxygen supply is still not enough, muscles respire anaerobically.
  4. Glucose is only partly oxidised, so lactic acid builds up and an oxygen debt is created.
  5. After long vigorous activity, muscles become fatigued and stop contracting efficiently.
Higher tier (what this means)Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels mean

Removing lactic acid and the oxygen debt

Higher tier (what this means)Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels mean

Blood flowing through the muscles transports the lactic acid to the liver, where it is converted back into glucose.

Higher tier (what this means)Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels mean

Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells. This is why you keep breathing hard, and your heart rate stays high, for a while after you stop.

Exam tip:

To investigate exercise, measure pulse or breathing rate at rest, straight after exercise and at regular intervals during recovery. Keep the type and duration of exercise the same, and repeat to find a mean.

Exam tip:

Say that anaerobic respiration happens because the oxygen supply is not enough, that lactic acid builds up, and that the muscles become fatigued. Do not say that muscles run out of energy completely.

Graph of heart rate against time: a flat resting level, a steep rise when exercise starts to a high level during exercise, then a gradual fall back to resting after exercise stops, with the recovery period labelled paying back the oxygen debt. (opens full size in a new tab)
Heart rate stays high after exercise while the oxygen debt is paid back.

Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026

Frequently asked questions

Why is respiration an exothermic reaction?

Respiration is exothermic because energy is transferred from the reaction to the surroundings. It happens continuously in living cells, and the energy transferred supplies all the energy needed for living processes, such as chemical reactions to build larger molecules, movement and keeping warm. Both aerobic and anaerobic respiration transfer energy in this way.

How is respiration different from breathing?

Respiration is a chemical reaction that happens inside living cells to transfer energy from glucose. Breathing moves air into and out of the lungs, which supplies the oxygen used in aerobic respiration. Respiration happens all the time in every living cell, including plant cells, not only at night, and it can also happen anaerobically without oxygen.

What is anaerobic respiration in yeast?

Anaerobic respiration in yeast is respiration without oxygen, in which glucose is converted into ethanol and carbon dioxide. It is called fermentation, and it has economic importance in the manufacture of bread and alcoholic drinks. Much less energy is transferred than in aerobic respiration, because the oxidation of glucose is incomplete.

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