VCE Physical Education (Year 11) · Unit 1 — The Human Body in Motion · 14m read

Acute Responses to Exercise

The moment you start exercising, your body adjusts — heart rate rises, breathing deepens, blood is redirected. These acute responses are tested in nearly every PE exam.

Learning objective: Explain the acute cardiovascular, respiratory and muscular responses to a single bout of exercise.

By the end of this lesson you can:

• I can list acute responses in each system.
• I can explain why each response occurs.
• I can interpret exercise data.

Acute responses are an emergency team. When an alarm sounds, firefighters, paramedics and police respond immediately. When exercise starts, your cardiovascular, respiratory and muscular systems respond instantly — then go back to normal afterwards.

Acute Responses to Exercise

CARDIOVASCULAR RESPONSES:
• Increased heart rate, stroke volume and cardiac output.
• Increased systolic blood pressure.
• Redistribution of blood flow to working muscles (vasodilation) and away from the gut (vasoconstriction).
• Increased a-VO2 difference (muscles extract more oxygen).

RESPIRATORY RESPONSES:
• Increased respiratory rate, tidal volume and ventilation.
• Increased pulmonary diffusion and oxygen uptake (VO2).

MUSCULAR RESPONSES:
• Increased motor unit recruitment.
• Increased muscle temperature and enzyme activity.
• Increased oxygen use and lactate/H+ production at higher intensities.
• Decreased fuel stores (PC, glycogen).

STEADY STATE:
• When oxygen supply meets demand during submaximal exercise.

| System | Acute responses |
|---|---|
| Cardiovascular | ↑ HR, ↑ SV, ↑ Q, ↑ systolic BP, vascular shunting, ↑ a-vO₂ diff |
| Respiratory | ↑ RR, ↑ TV, ↑ V̇E, ↑ diffusion, ↑ O₂ uptake |
| Muscular | ↑ motor unit recruitment, ↑ muscle temperature, ↑ enzyme activity, ↓ fuel stores (ATP, PC, glycogen), ↑ lactate/H⁺ |

Key ideas at a glance

• Acute = immediate, short-term: Chronic = long-term adaptations.
• Use arrows and direction: ↑ or ↓.
• Explain why: Link to O₂ demand.
• Interpret graphs: Steady state, O₂ deficit.

Worked example (Medium)

Why does a-VO2 difference increase during exercise?

1. Definition. The difference in oxygen content between arterial and venous blood.
2. Reason. Working muscles use more oxygen for aerobic ATP production.
3. Result. More oxygen is extracted, so venous blood returns with less.

Answer: Muscles extract and use more oxygen, so the arterial–venous gap widens.

Worked example (Hard)

Interpret: HR rises steeply then plateaus during a 20-minute submaximal run.

1. Rise. O₂ demand increases; HR rises to deliver O₂.
2. Plateau. Steady state: O₂ supply meets demand.
3. Term. Steady state.

Answer: HR rises to meet demand and plateaus at steady state.

Common mistakes

• Confusing acute responses with chronic adaptations Acute = during one session; chronic = after weeks of training. Fix: Check the time frame in the question.
• Listing without explaining Marks are for "why". Fix: Link each response to oxygen delivery or energy production.
• Calling acute responses "adaptations" Confuses acute and chronic. Fix: Acute = during/just after exercise.

Oxygen deficit and EPOC

At the start of exercise, O₂ supply lags behind demand — the oxygen deficit — so anaerobic pathways supply ATP.

After exercise, O₂ consumption stays elevated (EPOC) to restore PC, remove H⁺, lower temperature and HR, and replenish myoglobin.

Practice questions

1. Which is an ACUTE response to exercise?

   1. Cardiac hypertrophy
   2. Increased heart rate
   3. Lower resting heart rate
   4. More mitochondria

   (1 marks)

2. Explain why blood is redistributed during exercise.

   (2 marks)

3. Which is an acute muscular response?

   1. Increased mitochondria
   2. Increased motor unit recruitment
   3. Hypertrophy
   4. Increased capillaries

   (1 marks)

4. Steady state means:

   1. Max effort
   2. O₂ supply meets O₂ demand
   3. HR is zero
   4. Fatigue

   (1 marks)

5. Explain why systolic blood pressure rises during exercise.

   (2 marks)

Summary

• HR, SV, Q, ventilation and VO2 increase.
• Blood is redistributed to working muscles.
• Steady state when supply meets demand.
• Acute responses meet immediate O₂ demand.