Responding to exercise (AQA GCSE Biology): Revision Notes
Responding to exercise
The human body must respond quickly when we exercise because our muscles need much more energy. This creates several important changes in how our body works.
Understanding how your body responds to exercise helps explain why you feel tired during intense activities and why you keep breathing heavily even after you stop exercising.
Effects of exercise
When you start exercising, your muscles work much harder than usual. They need more energy to keep contracting and moving your body. This means they require more oxygen and glucose to carry out respiration.
Changes that happen during exercise
Your body makes three main changes to meet this increased demand:
- Heart rate increases - your heart beats faster
- Breathing rate increases - you breathe more frequently
- Breath volume increases - each breath takes in more air
These changes work together to help your body cope with exercise. A faster heart rate means blood gets pumped around your body more quickly. This helps transport oxygen and glucose to your muscle cells faster, and removes carbon dioxide more efficiently.
Practical Demonstration: Measuring Exercise Response
Try this simple experiment:
- Count your resting heart rate for 15 seconds and multiply by 4
- Do 30 jumping jacks
- Immediately count your heart rate again for 15 seconds and multiply by 4
- Notice how much it has increased!
You'll also notice you're breathing much faster and taking deeper breaths.
Oxygen debt
Sometimes during intense exercise, your muscles work so hard that they cannot get enough oxygen, even with all these changes happening.
What happens when oxygen runs out
When there is not enough oxygen reaching your muscles, something called anaerobic respiration takes place instead of normal aerobic respiration. This causes several problems:
- Glucose is not fully broken down - this means less energy is released
- Lactic acid builds up in your muscles - this is a waste product
- Your muscles become tired - they cannot contract properly anymore
This is why your muscles start to feel sore and weak during very intense exercise.
After exercise
When you stop exercising, you still breathe heavily for a while. This is because your body has built up an oxygen debt. You need to take in extra oxygen to:
- Break down the lactic acid that has built up
- Replace the oxygen that was used from your blood and tissues
- Return your body to its normal resting state
The heavier your breathing after exercise, the greater your oxygen debt was during the activity. This is why you breathe harder after sprinting compared to gentle jogging.
How these changes are measured
Scientists can measure these changes in different ways:
- Heart rate - count your pulse at your wrist, usually for 15 seconds then multiply by 4
- Breathing rate - count breaths per minute (during exercise, shorter periods like 15-30 seconds work better)
- Breath volume - measured using a special device called a spirometer
Worked Example: Calculating Heart Rate
If you count 22 heartbeats in 15 seconds:
- Heart rate = 22 × 4 = 88 beats per minute
If during exercise you count 35 heartbeats in 15 seconds:
- Heart rate = 35 × 4 = 140 beats per minute
The increase = 140 - 88 = 52 beats per minute increase
Key Points to Remember:
- During exercise, your heart rate, breathing rate and breath volume all increase
- These changes help transport more oxygen and glucose to working muscles
- When there is not enough oxygen, anaerobic respiration occurs and lactic acid builds up
- Oxygen debt means you keep breathing heavily after exercise to clear lactic acid
- Lactic acid buildup causes muscle fatigue and makes muscles stop working efficiently