Response to Exercise

During exercise, the body's demand for energy increases dramatically. This lesson explores how the body responds to exercise to meet this increased demand, including changes to heart rate, breathing rate and blood flow, as well as the concept of oxygen debt and the recovery period. This is a key topic for AQA GCSE Biology.

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Why Does the Body Need More Energy During Exercise?

When you exercise, your muscles contract more frequently and more forcefully. This requires more energy, which comes from aerobic respiration:

glucose + oxygen ---> carbon dioxide + water (+ energy)

To increase the rate of aerobic respiration, the muscles need:

• More glucose — to be broken down as fuel
• More oxygen — to react with the glucose
• Faster removal of carbon dioxide — which is a waste product that would lower the pH and inhibit enzymes if it accumulated

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How the Body Responds to Exercise

The body makes several automatic adjustments during exercise to increase the delivery of oxygen and glucose to the muscles and to remove carbon dioxide more quickly.

graph TD
    A[Exercise Begins] --> B[Muscles need more energy]
    B --> C[Brain detects increased CO2 in blood]
    C --> D[Heart rate increases]
    C --> E[Breathing rate increases]
    C --> F[Breathing depth increases]
    D --> G[More blood pumped to muscles per minute]
    E --> H[More air enters lungs per minute]
    F --> H
    G --> I[More O2 and glucose delivered to muscles]
    G --> J[More CO2 removed from muscles]
    H --> K[More O2 absorbed into blood in lungs]
    H --> L[More CO2 removed from blood in lungs]

1. Heart Rate Increases

Change	Effect
Heart rate (beats per minute) increases	More blood is pumped per minute (cardiac output increases)
Blood is redirected to muscles	More oxygen and glucose delivered to working muscles
Blood vessels to muscles dilate (vasodilation)	Increases blood flow to muscles
Blood vessels to non-essential organs constrict	Redirects blood to where it is most needed

2. Breathing Rate Increases

Change	Effect
Breathing rate (breaths per minute) increases	More air moves in and out of the lungs per minute
Depth of breathing increases (tidal volume)	Each breath brings more air into the lungs
Combined effect: ventilation rate increases	Much more gas exchange occurs per minute

Ventilation rate = breathing rate x tidal volume

For example: 20 breaths per minute x 0.8 litres per breath = 16 litres per minute

3. Glycogen Is Broken Down

During exercise, the glucose in the blood is used up quickly. To maintain the supply:

• Glycogen stored in the muscles and liver is broken down into glucose
• Glycogen is a storage polysaccharide — a large molecule made of many glucose units joined together
• The liver releases glucose into the blood, and muscles use their own glycogen stores directly

Storage Molecule	Location	What Happens During Exercise
Glycogen (muscles)	Skeletal muscles	Broken down into glucose for respiration in the muscle cells
Glycogen (liver)	Liver	Broken down into glucose, released into the blood, transported to muscles
Blood glucose	Blood	Used directly by muscle cells for respiration

Exam Tip: Glycogen is NOT the same as glucose. Glycogen is a large, insoluble storage molecule made of many glucose units. During exercise, glycogen is broken down into glucose which is then used in respiration. Make sure you use the correct term.

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What Happens During Intense Exercise?

During very vigorous exercise (e.g. sprinting, high-intensity interval training), the body cannot supply oxygen to the muscles fast enough. Even though the heart and lungs are working at maximum capacity, the oxygen demand exceeds the oxygen supply.

When this happens:

1. The muscles begin to respire anaerobically (without oxygen)
2. Anaerobic respiration produces lactic acid as a waste product
3. Lactic acid accumulates in the muscles

Effects of Lactic Acid Build-Up

Effect	Explanation
Muscle fatigue	Muscles become tired and cannot contract as effectively
Muscle pain	A burning sensation caused by the acidic environment
Reduced performance	The athlete cannot maintain the same intensity
Lower pH in muscles	Lactic acid is acidic, which inhibits enzyme activity

Eventually, the muscles may become so fatigued that the person has to stop or slow down. This is why sprinters cannot maintain their top speed for very long.

Exam Tip: The body does not "switch" entirely to anaerobic respiration during exercise. Both aerobic and anaerobic respiration occur simultaneously — the muscles use aerobic respiration as much as possible and supplement it with anaerobic respiration when oxygen supply is insufficient.

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Oxygen Debt and Recovery

After intense exercise, the body does not immediately return to its resting state. You continue to breathe heavily and your heart rate remains elevated for a period of time. This is because the body needs to repay the oxygen debt.

What Is Oxygen Debt?

Oxygen debt (also called excess post-exercise oxygen consumption, or EPOC) is the amount of extra oxygen the body needs after exercise to:

1. Break down lactic acid — lactic acid is transported in the blood to the liver, where it is converted back into glucose (which can then be respired aerobically or converted back to glycogen)
2. Replenish oxygen stores in the blood and muscles (oxygen is stored by myoglobin in muscles and haemoglobin in blood)
3. Restore glycogen stores in the muscles and liver
4. Return body temperature to normal

graph TD
    A[Intense Exercise Ends] --> B[Lactic acid in muscles and blood]
    B --> C[Blood transports lactic acid to liver]
    C --> D[Liver converts lactic acid back to glucose]
    D --> E[Glucose stored as glycogen or respired aerobically]
    A --> F[Breathing rate stays elevated]
    A --> G[Heart rate stays elevated]
    F --> H[Extra oxygen taken in to repay oxygen debt]
    G --> H
    H --> I[Body returns to resting state]

The Recovery Period

What Happens	Why
Heavy breathing continues	To take in extra O2 to break down lactic acid in the liver
Heart rate remains elevated	To transport lactic acid to the liver and deliver O2 to cells
Sweating continues	Body temperature is still elevated and needs to return to normal
Muscle soreness	Lactic acid and micro-tears in muscle fibres cause delayed soreness

The recovery period depends on the intensity and duration of the exercise. After a short sprint, it may take just a few minutes. After a marathon, it can take hours or even days.

Exam Tip: When explaining oxygen debt, always include the following chain of reasoning: (1) lactic acid builds up during anaerobic respiration, (2) lactic acid is transported to the liver by the blood, (3) in the liver, lactic acid is converted back to glucose, (4) this conversion requires oxygen, (5) the extra oxygen needed is the oxygen debt. This full explanation is needed for full marks.

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Measuring the Response to Exercise

You can investigate the body's response to exercise by measuring: