Heart Rate & Exercise

Your heart is a pump made of muscle. Each beat squeezes blood out into your arteries and pushes it around your body.

Heart rate is the number of times the heart beats each minute (beats per minute, bpm).

• A typical resting heart rate for a teenager is around 60–80 bpm.
• You can measure heart rate by counting your pulse — the surge of blood you can feel in an artery (for example at the wrist or neck) each time the heart beats.
• During exercise, heart rate can rise to 150 bpm or more.

The job of the beating heart is to keep blood circulating. Blood carries:

• oxygen and glucose to cells (the raw materials for respiration), and
• carbon dioxide away from cells (a waste product of respiration).

So whenever the body needs to move materials around faster, one simple solution is to make the heart beat faster.

Exam tip. Be precise: heart rate is "beats per minute". An answer that just says "how fast the heart beats" without the per minute idea is weaker. The pulse is how we measure heart rate.

When you exercise, your muscles contract harder and more often. To do this they need a lot more energy, which they release by respiration:

glucose + oxygen → carbon dioxide + water (+ energy released)

This is aerobic respiration, and during exercise the muscles respire much more than at rest. As a result the muscles:

• need more oxygen delivered to them,
• need more glucose delivered to them, and
• produce more carbon dioxide, which must be removed.

The body meets all three needs by increasing the heart rate:

1. A faster heart rate means blood is pumped around the body more quickly.
2. So oxygen and glucose are delivered to the muscle cells faster.
3. And the extra carbon dioxide is carried away from the muscles faster (to the lungs to be breathed out).

In short: more respiration in the muscles → greater demand for oxygen/glucose and for removing CO₂ → heart rate increases to move blood faster.

When you stop exercising, the muscles respire less, demand falls, and the heart rate gradually returns to its resting value.

Exam tip. A common "explain" question asks why heart rate rises in exercise. Score the marks by linking the chain: muscles respire more → need more oxygen and glucose / must remove more CO₂ → heart beats faster to deliver/transport these faster. Always say why the faster blood flow helps.

It helps to picture how heart rate changes over a whole exercise session. The graph below shows a typical pattern.

Reading the graph gives you the four key stages:

1. At rest (before exercise). Heart rate is steady and low (the resting rate).
2. Start of exercise. Heart rate rises sharply as muscles begin to demand more oxygen and glucose.
3. During exercise. Heart rate stays high, keeping blood moving fast to match the high rate of respiration.
4. Recovery (after exercise). Demand falls, so heart rate decreases gradually back to the resting rate.

Fitness link. A fitter person tends to have a lower resting heart rate and their heart rate returns to normal more quickly after exercise — their recovery time is shorter. Recovery time is a useful, simple measure of fitness.

Exam tip. When you describe a heart-rate graph, use data: quote values and times from the axes (e.g. "rose from 70 bpm to 150 bpm" or "took 4 minutes to recover"). "Goes up then down" alone rarely scores full marks.

Heart rate does not only change because of exercise. It also responds to a hormone called adrenaline.

• Adrenaline is released by the adrenal glands (small glands just above the kidneys).
• It is released when you are frightened, stressed or excited — and also during vigorous exercise.
• Once in the blood, adrenaline travels to organs including the heart.

The main effect you need is that adrenaline increases the heart rate. This:

• pumps blood faster around the body,
• delivers more oxygen and glucose to the muscles, and
• so prepares the body for action — sometimes called the "fight or flight" response.

This makes biological sense: if an animal is threatened, raising the heart rate quickly readies the muscles to run away or fight.

How is this different from the exercise response? Both raise heart rate, but for slightly different reasons:

• In exercise, the rise is mainly a response to the muscles respiring more and needing materials delivered faster.
• With adrenaline, the rise is a hormonal response to fear/stress/excitement, preparing the body before or during action — this is why your heart can race even before you start moving.

Exam tip. Two facts score reliably: (1) adrenaline is a hormone released in fear/stress/exercise, and (2) it increases heart rate to prepare the body for action. You do not need detailed hormone mechanisms for this Double Award point — keep it to those ideas.

For this subtopic, remember that heart rate can increase for two linked reasons, and both end with blood being pumped faster:

1. The demand of exercise

• Muscles respire more to release energy for movement.
• They need more oxygen and glucose and make more carbon dioxide.
• Heart rate rises so these substances are transported faster — delivering oxygen/glucose and removing CO₂.

2. The hormone adrenaline

• Released in fear, stress, excitement or exercise.
• Travels in the blood to the heart and increases heart rate.
• This prepares the body for action (fight or flight).

These often happen together: during hard exercise the muscles demand more and adrenaline is released, so heart rate climbs and stays high until you stop.

Why faster blood flow is the answer to all of this: the heart cannot make more oxygen or glucose, but by beating faster it can move the same blood around more often, so cells receive their supplies more frequently and waste is cleared more quickly.

Exam tip. If a question simply says "explain how heart rate changes during exercise and under the influence of adrenaline", structure your answer in two clear parts — one for exercise (respiration/oxygen/glucose/CO₂) and one for adrenaline (hormone/fear/stress/prepare for action). Covering both halves is how you secure full marks.