Edexcel IGCSE Biology · Spec 2.50
Exercise and Breathing
Required practical: how exercise affects breathing rate, why muscles need more oxygen, and the oxygen debt.
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Exercise and Breathing
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Explained
Why breathing changes during exercise, and how to measure it
The experiment itself is simple. Count your breaths for a minute while sitting still, exercise for a set time, then count again immediately afterwards, and keep counting each minute until the rate returns to its resting value.
Everything worth marks is in the detail around that: how you make it fair, how you present it, and why the change happens.
Doing it properly
Take the resting rate after sitting quietly for a few minutes, not straight after walking into the room. Use the same exercise, at the same intensity, for the same time for every person tested. Count for a full minute rather than for fifteen seconds and multiplying, because breaths are not evenly spaced.
Repeat and take a mean. Breathing rate varies from one minute to the next even at rest, so a single reading is unreliable.
Record the recovery time as well, meaning how long the rate takes to return to normal. A fitter person recovers faster, and comparing recovery times is often more revealing than comparing peak rates.
Why the rate rises
Muscles contract more during exercise, so they need more energy, so they respire faster. Faster aerobic respiration needs more oxygen delivered and produces more carbon dioxide to remove.
Breathing rate and depth both increase to supply that oxygen and remove that carbon dioxide, and heart rate increases to carry them between the lungs and the muscles more quickly.
The trigger is the carbon dioxide rather than the oxygen. Receptors detect the rising level in the blood, and the brain increases the rate in response.
Anaerobic respiration and oxygen debt
During hard exercise the blood cannot deliver oxygen fast enough. Muscles then also respire anaerobically, breaking glucose down to lactic acid without oxygen.
This releases much less energy per glucose molecule, and the lactic acid builds up, causing muscle fatigue and pain.
Afterwards, breathing stays fast even though you have stopped moving. The extra oxygen is being used to break the lactic acid down, and that requirement is the oxygen debt. It explains the one thing about the results that looks odd, which is why the rate does not drop the instant the exercise ends.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology gives advice that applies directly to writing up this practical. Discussing a question on heart rate, it says it is important for learners always to give a direction of a change where possible, and notes that some learners simply stated that a substance would change the heart rate, which was given in the question stem.
So write that the breathing rate increases, not that it changes. And if the question already told you it changed, saying so again earns nothing.
The same reports repeatedly credit the chain rather than the observation. On a related question the mark scheme wants the hormone named, the fact that it travels in the blood, and the effect it has, as three separate points. Applied here, that means naming the increased respiration, the increased oxygen demand and the increased carbon dioxide removal, rather than saying the body needs more oxygen and stopping.
Presenting the results
Time goes on the horizontal axis and breathing rate on the vertical, because time is the independent variable. Choose a scale that fills at least half the grid and that goes up in steps you can read between, such as twos or fives rather than threes.
Plot the points accurately and draw a smooth curve or a line of best fit rather than joining the dots with straight segments. Then describe the shape in words: a sharp rise during exercise, a peak just after it stops, and a gradual fall back to the resting rate.
Spec 2.50
What you need to know
- Measure breathing rate before and after exercise
- Present the results clearly
- Explain why the breathing rate changes
Active recall
Quick check
Answer each question before opening the answer.
How and why do breathing rate and depth change during exercise?
They increase, to supply more oxygen to muscles and remove the extra carbon dioxide produced by faster respiration.
Why does an oxygen debt build up during hard exercise?
Muscles respire anaerobically, producing lactic acid; oxygen is later needed to break it down.
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