B9.2 The Response to Exercise

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how heart rate, breathing rate and breath volume change during exercise, and why; measuring and analysing the response. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how heart rate, breathing rate and breath volume change during exercise
• Explain how these changes supply the muscles with more oxygenated blood and remove carbon dioxide
• Describe the role of muscle glycogen during exercise
• Measure pulse and breathing rate, and calculate rates and percentage changes from data

Key words

• Heart rate: The number of times the heart beats per minute (bpm), measured as the pulse.
• Breathing rate: The number of breaths taken per minute.
• Breath volume: The volume of air breathed in or out in one breath.
• Glycogen: A storage carbohydrate made from glucose. Stored in muscles and the liver.
• Recovery time: The time it takes for heart rate (or breathing rate) to return to its resting value after exercise.

Why muscles need more during exercise

During exercise, muscles contract more often and more strongly. They need more energy, so the rate of respiration in muscle cells increases. They use oxygen and glucose faster and produce more carbon dioxide, which must be removed.

Key fact: All the changes during exercise increase the supply of oxygenated blood and glucose to the muscles and the removal of carbon dioxide.

The body's response

• Heart rate increases; How it helps Blood is pumped faster, so more oxygen and glucose reach the muscles each minute and more carbon dioxide is carried away.
• Breathing rate and breath volume increase; How it helps More air enters the lungs, so more oxygen diffuses into the blood and more carbon dioxide diffuses out.
• Arteries supplying the muscles dilate; How it helps More blood flows to the working muscles.
• Glycogen in muscles is converted to glucose; How it helps Extra glucose is available in the muscle cells for respiration.

A fitter person usually has a lower resting heart rate (the heart pumps more blood per beat), a smaller rise during exercise and a shorter recovery time.

If the muscles still do not get enough oxygen, they also respire anaerobically, making lactic acid (see B9.3). Heart rate and breathing stay high for a while after exercise to repay this.

Measuring and analysing the response

• Pulse rate: place two fingers (not the thumb) on the wrist or neck. Count the beats in 15 seconds and multiply by 4, or count for 60 seconds.
• Breathing rate: count breaths in a set time and convert to breaths per minute.
• Repeat each measurement and calculate a mean. Rest until the pulse returns to normal between tests.

Worked example: percentage increase

Person A's heart rate rises from 58 bpm at rest to 150 bpm during exercise.
Increase = 150 − 58 = 92 bpm. Percentage increase = 92 ÷ 58 × 100 = 159% (3 s.f.).

Answer: 159% increase

Worked example: volume of air per minute

At rest Sam takes 4 breaths in 15 s, each 0.6 dm³. During exercise he takes 9 breaths in 15 s, each 2.2 dm³.
Rest: 4 × 4 = 16 breaths/min; 16 × 0.6 = 9.6 dm³/min. Exercise: 36 breaths/min; 36 × 2.2 = 79.2 dm³/min.
79.2 ÷ 9.6 = 8.25 times more air each minute.

Answer: 9.6 dm³/min at rest, 79.2 dm³/min during exercise

Common mistakes

• Saying the heart "beats faster to make more energy". It pumps blood faster to deliver oxygen and glucose for respiration.
• Saying glycogen travels in the blood to the muscles. Muscle glycogen is stored in the muscle cells and converted to glucose there.
• Saying arteries constrict during exercise. Arteries to the working muscles dilate.
• Forgetting to convert a count in 15 seconds into a rate per minute (× 4).

Revision questions

Try these, then check your answers below each one.

Question 1. What happens to breathing during exercise? (Choose from: Breathing rate and breath volume both increase, Breathing rate increases but breath volume falls, Only breath volume increases, Breathing slows to save oxygen.)

Answer: Breathing rate and breath volume both increase

Working: You breathe faster and more deeply, so more oxygen enters the blood and more carbon dioxide is removed.

Question 2. True or false? During exercise the arteries supplying the working muscles dilate (widen).

Answer: True

Working: True. The arteries dilate, so more blood carries oxygen and glucose to the muscles.

Question 3. Name the storage carbohydrate in muscles that is converted to glucose during exercise.

Answer: glycogen

Working: Glycogen is made from glucose and stored in muscles and the liver. During exercise it is converted back to glucose for respiration.

Related lessons

• Previous lesson: B9.1 Aerobic Respiration
• Next lesson: B9.3 Anaerobic Respiration
• In this chapter: B9.4 Metabolism and the Liver

Continue in the full course

This lesson is section B9.2 (AGCSC0902) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.