Exercise Response

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. Your muscles contract far more, so they **respire faster** to release more energy: 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

Follow a sprinter through a hard race, one decision at a time.

• They start sprinting and their muscles suddenly demand far more energy. What happens to their heart rate?
• The sprint is so intense that oxygen cannot reach the muscles fast enough. How do the muscles keep contracting?
• The sprinter crosses the line and stops, but keeps breathing hard for a while. Why?

The body responds

To meet the demand, your body makes three key changes:

Why each change?

Explain the response

Explain why both your heart rate and your breathing rate increase during exercise. Take the reasoning all the way from the muscles to the change you feel.

• Start with what the muscles are doing and what respiring faster needs
• Say what a faster heart rate delivers, and what it removes
• Say what deeper, faster breathing changes about the blood

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".

Build the chain

Put the stages of intense exercise and recovery into the order they happen.

• Oxygen cannot be supplied to the muscles fast enough
• The muscles switch to anaerobic respiration
• Lactic acid builds up in the muscles
• Muscle fatigue sets in and contraction becomes inefficient
• After stopping, extra oxygen is taken in to repay the oxygen debt
• Blood carries the lactic acid to the liver, where it becomes glucose

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**. The two kinds of respiration behind that are worth separating:

Plot the recovery

A fit person's heart rate was measured after stopping exercise. The x-axis is time in minutes; the y-axis is heart rate in units of 10 bpm (so y = 12 means 120 bpm). Plot the five points: 0 min → 160, 1 min → 120, 2 min → 90, 3 min → 70, 4 min → 60.

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