⚡ Enough oxygen
Aerobic respiration. Glucose is completely oxidised. Carbon dioxide and water are made.
Respiration » Response to Exercise
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.4.2.2
- How the body reacts to the increased demand for energy during exercise
- Why heart rate, breathing rate and breath volume increase
- What happens when muscles do not get enough oxygen: lactic acid, oxygen debt and fatigue
- Higher tier: how lactic acid is removed and what oxygen debt means
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Unlock This CourseThe body reacts to exercise
When you run for a bus or play a fast game of football, your muscles contract much more often than when you sit still. That needs a lot more energy. The energy comes from respiration, so the muscle cells need to respire faster.
Respiration needs glucose and oxygen, and the blood brings both to the muscles. So during exercise the body has to deliver more oxygenated blood to the muscles. It does this in three ways:
❤ Heart rate
The heart beats faster, so blood is pumped round the body more quickly.
☁ Breathing rate
You take more breaths every minute.
⚖ Breath volume
Each breath is deeper, so more air moves in and out of the lungs each time.
All three increase during exercise to supply the muscles with more oxygenated blood. A faster heart rate carries more oxygen and glucose to the muscles each minute. Faster, deeper breathing brings more oxygen into the blood.
Key terms:
- Heart rate: the number of times the heart beats each minute.
- Breathing rate: the number of breaths taken each minute.
- Breath volume: the amount of air taken in with each breath.
You can feel this happen. Put two fingers on your wrist and count your pulse at rest. Then do a minute of star jumps and count again. Your pulse will be higher, and you will be breathing faster and more deeply too. Your body is responding to the increased demand for energy.
When there is not enough oxygen
Even with a faster heart and deeper breaths, hard exercise can use oxygen faster than the blood can deliver it. When insufficient oxygen is supplied, the muscles switch to anaerobic respiration.
The glucose is only partly broken down. This is called incomplete oxidation of glucose. It transfers less energy than aerobic respiration, and it makes lactic acid, which builds up in the muscles.
⚠ Not enough oxygen
Anaerobic respiration in the muscles. Glucose is incompletely oxidised. Lactic acid builds up.
Key term:
- Incomplete oxidation: glucose being only partly broken down, as in anaerobic respiration in muscles.
Oxygen debt
The build up of lactic acid creates an oxygen debt. You will see exactly what this means in the Higher tier section below.
Key term:
- Oxygen debt: created by the build up of lactic acid in the muscles.
Muscle fatigue
During long periods of vigorous activity, muscles become fatigued. This means they stop contracting efficiently. A runner may feel their legs getting heavy and weak. The muscles cannot work as well, so performance drops.
Key term:
- Muscle fatigue: when muscles stop contracting efficiently after long periods of vigorous activity.
Fatigue is the reason athletes have to slow down or stop, however hard they try. The muscles simply cannot keep contracting as well as they did at the start.
Removing the lactic acid
Higher tier only
Blood flowing through the muscles transports the lactic acid to the liver. In the liver, the lactic acid is converted back into glucose.
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 for a while after you stop running.
Putting it together
- Exercise increases the demand for energy.
- Heart rate, breathing rate and breath volume increase.
- More oxygenated blood reaches the muscles, so more aerobic respiration can happen.
- If the oxygen supplied is still not enough, anaerobic respiration takes place and lactic acid builds up.
- Higher tier only: after exercise, the lactic acid is removed using extra oxygen, which pays off the oxygen debt.
Worked example
A student's heart rate is 70 beats per minute at rest and 150 beats per minute while running. The increase is 150 - 70 = 80 beats per minute. The heart is pumping blood much faster, so more oxygenated blood reaches the muscles. If the student's breath volume rises from 0.5 dm3 to 2.0 dm3 per breath, each breath brings in 2.0 ÷ 0.5 = 4 times as much air.
Common mistakes
Saying that muscles use no oxygen during exercise. They use more, until the supply cannot keep up.
Saying that lactic acid is made all the time. It builds up when insufficient oxygen is supplied.
Higher tier only: writing that oxygen debt means the body has run out of oxygen. It is the extra oxygen needed afterwards.
Mixing up breathing rate (breaths per minute) with breath volume (the size of each breath).
Exam-style question
A student runs as fast as possible for five minutes.
(a) Give two changes in the body that supply the muscles with more oxygenated blood. [2 marks]
(b) After a few minutes the muscles become tired. Explain why lactic acid builds up in the muscles. [2 marks]
(c) Higher tier only: Explain why the student keeps breathing heavily after the run has finished. [3 marks]
Model answer
(a) Heart rate increases (1). Breathing rate or breath volume increases (1).
(b) Insufficient oxygen is supplied to the muscles (1), so anaerobic respiration takes place, which is incomplete oxidation of glucose and makes lactic acid (1).
(c) There is an oxygen debt (1). Extra oxygen is needed to react with the accumulated lactic acid (1) and remove it from the cells (1).
Exam tip
In (b), say 'insufficient oxygen' and link it to anaerobic respiration to earn both marks. Do not just write 'no oxygen'.