Exercise Response
What really happens when you sprint: why your heart pounds, why muscles burn and fatigue, and how you repay the oxygen debt.
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Exercise Response 🏃
Sprint up a flight of stairs and your body transforms in seconds — heart pounding, chest heaving, legs burning. Every one of those changes is your body scrambling to keep the muscles supplied with energy. Let's see exactly why.
Muscles get greedy 💪
When you exercise, your muscles contract far more, so they **respire faster** to release more energy. That means they need much more **oxygen** and **glucose** delivered, and produce much more **carbon dioxide** that has to be removed. Your body has to respond fast.
The sprinter
An interactive activity.
The body responds
To meet the demand, your body makes three key changes: • **Heart rate increases** — pumping blood to the muscles faster. • **Breathing rate and depth increase** — taking in more oxygen and removing more CO₂. • Stored **glycogen** in the muscles is converted back to **glucose** for respiration.
Why each change?
- Heart rate increases
- Breathing rate and depth increase
- Glycogen is converted to glucose
- Delivers oxygen and glucose to muscles faster
- Takes in more oxygen and removes more CO₂
- Provides extra fuel for respiration
The pounding heart
Why does your heart rate increase during exercise?
- To deliver oxygen and glucose to the muscles faster and remove CO₂
- To warm up the blood
- To make the blood thicker
- To make the lungs bigger
The burn: lactic acid 🥛
*(Higher tier.)* If exercise is so vigorous that oxygen can't be supplied fast enough, the muscles switch to **anaerobic respiration**. This produces **lactic acid**. As lactic acid builds up, it causes muscle **fatigue** — the muscles stop contracting efficiently, and you feel the "burn".
Feeling the burn
During very intense exercise, what causes muscle fatigue?
- The build-up of lactic acid from anaerobic respiration
- Too much oxygen reaching the muscles
- The muscles filling with water
- A shortage of carbon dioxide
Repaying the debt 😮💨
*(Higher tier.)* After exercise, that lactic acid has to go. The **oxygen debt** is the extra oxygen your body needs to react with (oxidise) the built-up lactic acid — which is why you keep panting after you stop. The blood carries the lactic acid to the **liver**, where it is converted back into **glucose**.
Reading recovery
On a graph of heart rate against time, a fitter person's heart rate returns to its resting level **faster** after exercise. What does a shorter recovery time show?
- A higher level of fitness
- That the person is unwell
- That they did not really exercise
- Nothing about fitness at all
How the body responds
Pick the THREE ways your body responds to exercise.
- Heart rate increases
- Breathing rate and depth increase
- Glycogen is converted to glucose for fuel
- Body temperature drops sharply
- Less carbon dioxide is produced