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This lesson covers three of the most important measurements in the cardio-respiratory system: cardiac output, stroke volume and heart rate. You will learn the key formula that links them, how to interpret heart rate data, and what happens to these values during exercise. This is a core part of the AQA GCSE PE specification (3.1.1.2) and is frequently tested in examinations.
Before diving into the formula, you must understand each term precisely:
| Term | Definition | Unit |
|---|---|---|
| Heart rate (HR) | The number of times the heart beats per minute | Beats per minute (bpm) |
| Stroke volume (SV) | The volume of blood pumped out of the left ventricle per beat | Millilitres per beat (ml/beat) |
| Cardiac output (Q) | The total volume of blood pumped out of the left ventricle per minute | Litres per minute (l/min) or millilitres per minute (ml/min) |
Exam Tip: Make sure you can define all three terms precisely. AQA awards marks for accurate definitions. Note that cardiac output is sometimes written as CO in some textbooks but represented as Q in the AQA specification.
The three measurements are linked by a simple but crucial formula:
Q = SV × HR
Where:
You can rearrange this formula to find any of the three values:
| To Find | Formula |
|---|---|
| Cardiac output (Q) | Q = SV × HR |
| Stroke volume (SV) | SV = Q ÷ HR |
| Heart rate (HR) | HR = Q ÷ SV |
A resting athlete has a heart rate of 60 bpm and a stroke volume of 85 ml/beat. Calculate their cardiac output.
Q = SV × HR Q = 85 × 60 = 5,100 ml/min = 5.1 l/min
During exercise, an athlete's cardiac output is 25,000 ml/min and their heart rate is 190 bpm. Calculate their stroke volume.
SV = Q ÷ HR SV = 25,000 ÷ 190 = 131.6 ml/beat (to 1 d.p.)
A performer has a cardiac output of 4,900 ml/min and a stroke volume of 70 ml/beat. Calculate their heart rate.
HR = Q ÷ SV HR = 4,900 ÷ 70 = 70 bpm
Exam Tip: Always show your working clearly. Write out the formula, substitute the values, then calculate. Even if your final answer is wrong, you can still pick up marks for correct method. Also check your units — if the question gives cardiac output in litres, you may need to convert to millilitres (multiply by 1,000) before using the formula.
| Measurement | Average Untrained Person | Trained Athlete |
|---|---|---|
| Heart rate (HR) | 72 bpm | 50–60 bpm (or lower) |
| Stroke volume (SV) | 70 ml/beat | 85–110 ml/beat |
| Cardiac output (Q) | ~5 l/min | ~5 l/min |
Notice that at rest, cardiac output is approximately the same (~5 litres per minute) for both trained and untrained individuals. However, a trained athlete achieves this with a lower heart rate and a higher stroke volume. This is because regular training causes the heart muscle (particularly the left ventricle) to become larger and stronger — a phenomenon known as cardiac hypertrophy or "athlete's heart."
Exam Tip: The fact that resting cardiac output is similar for trained and untrained individuals is a common exam question. The key difference is that athletes have a lower resting heart rate (bradycardia) and a higher resting stroke volume.
The average resting heart rate for an adult is approximately 72 bpm. However, trained endurance athletes often have resting heart rates as low as 40–50 bpm. This condition is called bradycardia and is a positive adaptation to regular aerobic training.
Why does training lower resting heart rate?
This is more efficient because the heart does less work to achieve the same output, reducing wear on the heart muscle over time.
Maximum heart rate (MHR) is the highest number of beats per minute your heart can achieve during maximum physical exertion. It is estimated using the formula:
MHR = 220 − age
For example:
This formula is useful for calculating training zones:
| Training Zone | % of MHR | Purpose |
|---|---|---|
| Aerobic zone | 60–80% | Improves cardiovascular fitness, endurance |
| Anaerobic zone | 80–90% | Improves speed, power and anaerobic capacity |
| Maximum zone | 90–100% | Short bursts of maximum effort |
During exercise, the body's demand for oxygen increases dramatically. The cardio-respiratory system must respond to meet this demand:
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