Cardiac Output, Stroke Volume and Heart Rate

This lesson covers the key cardiovascular measurements — cardiac output, stroke volume, and heart rate — and the relationship between them, as required by the Edexcel GCSE PE specification (1PE0 — Topic 1). You must know the definitions, the formula that links them, and how they change during exercise.

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Key Definitions

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 (ml)
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)

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The Key Formula

The three measurements are linked by a simple formula that you must memorise:

Cardiac Output = Stroke Volume x Heart Rate

$Q = SV \times HR$Q=SV×HR

This means:

• If you know stroke volume and heart rate, you can calculate cardiac output.
• If you know cardiac output and heart rate, you can calculate stroke volume.
• If you know cardiac output and stroke volume, you can calculate heart rate.

Exam Tip: The Edexcel exam regularly asks you to use this formula. Make sure you can rearrange it: SV = Q ÷ HR, and HR = Q ÷ SV. Practise calculation questions using realistic values.

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Typical Values

Measurement	At Rest (untrained)	At Rest (trained athlete)	During Max Exercise
Heart rate	72 bpm	50-60 bpm (bradycardia)	Up to 200+ bpm
Stroke volume	70 ml	100-110 ml	110-150+ ml
Cardiac output	~5 l/min	~5 l/min	20-35+ l/min

Worked Example 1: At Rest

An untrained person has a heart rate of 72 bpm and a stroke volume of 70 ml. What is their cardiac output?

Q = SV × HR = 70 ml × 72 = 5,040 ml/min (approximately 5 l/min)

Worked Example 2: During Exercise

A person exercises with a heart rate of 160 bpm and a stroke volume of 120 ml. What is their cardiac output?

Q = SV × HR = 120 ml × 160 = 19,200 ml/min (approximately 19.2 l/min)

Worked Example 3: Finding Stroke Volume

An athlete has a cardiac output of 25 l/min (25,000 ml/min) and a heart rate of 180 bpm. What is their stroke volume?

SV = Q ÷ HR = 25,000 ÷ 180 = approximately 139 ml

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Heart Rate in Detail

Resting Heart Rate

The resting heart rate is the number of times the heart beats per minute when the body is at complete rest. For an average untrained adult, this is approximately 72 bpm.

Maximum Heart Rate (MHR)

The maximum heart rate is the fastest rate at which the heart can beat during maximal exercise. It can be estimated using the formula:

MHR = 220 - age

For example, a 16-year-old student's estimated MHR = 220 - 16 = 204 bpm.

This formula is important for calculating training thresholds (covered in a later lesson).

How Heart Rate Changes During Exercise

graph LR
    A["Rest<br>~72 bpm"] --> B["Warm-up<br>Gradual increase"]
    B --> C["Exercise<br>Rapid increase to<br>match demand"]
    C --> D["Cool-down<br>Gradual decrease"]
    D --> E["Recovery<br>Returns to resting"]

    style A fill:#27ae60,color:#fff
    style C fill:#c0392b,color:#fff
    style E fill:#27ae60,color:#fff

• Before exercise: Heart rate may increase slightly due to anticipatory rise (the nervous system prepares the body).
• During warm-up: Heart rate increases gradually.
• During exercise: Heart rate rises rapidly to meet the increased demand for oxygen.
• During cool-down: Heart rate decreases gradually.
• Recovery: Heart rate returns to resting level. The fitter the person, the faster the recovery.

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Stroke Volume in Detail

How Stroke Volume Changes During Exercise

Stroke volume increases during exercise because:

1. Increased venous return — more blood flows back to the heart (due to the muscle pump and deeper breathing), stretching the ventricles more.
2. Starling's Law — the more the ventricle is stretched by incoming blood, the more forcefully it contracts, ejecting a larger volume of blood.
3. Increased force of contraction — the nervous system stimulates the heart to beat more powerfully.

Stroke volume increases during low to moderate exercise but tends to plateau at high intensities — beyond this point, further increases in cardiac output come from increasing heart rate.

Exercise Intensity	Stroke Volume Response
Rest	~70 ml (untrained), ~100-110 ml (trained)
Low–moderate exercise	Increases progressively
High–maximal exercise	Plateaus (further increases come from HR)

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Cardiac Output in Detail

How Cardiac Output Changes During Exercise

Since cardiac output = stroke volume × heart rate, and both increase during exercise, cardiac output can rise dramatically:

• At rest: ~5 l/min (for both trained and untrained individuals — untrained compensate with a higher HR for a lower SV).
• During maximal exercise: up to 20-25 l/min for untrained individuals and 30-40 l/min for elite endurance athletes.

Exam Tip: An interesting point for higher-mark questions: resting cardiac output is approximately the same for trained and untrained individuals (~5 l/min). A trained athlete achieves this with a lower HR and higher SV, while an untrained person has a higher HR and lower SV. The difference becomes apparent during maximal exercise, where the trained athlete can achieve a much higher cardiac output.

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The Effect of Training

Variable	Effect of Regular Aerobic Training
Resting heart rate	Decreases (bradycardia) — the heart becomes more efficient
Resting stroke volume	Increases — the heart is larger and stronger, pumping more blood per beat
Resting cardiac output	Stays approximately the same (~5 l/min)
Maximum heart rate	Stays approximately the same (determined by age)
Maximum stroke volume	Increases significantly
Maximum cardiac output	Increases significantly — more blood can be pumped per minute during maximal exercise

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Bradycardia

Bradycardia is the term for a resting heart rate below 60 bpm. In the context of sport, this is a positive adaptation that occurs in response to regular aerobic training.

• The heart undergoes cardiac hypertrophy (the heart muscle becomes larger and stronger).
• A larger, stronger heart can pump a greater stroke volume with each beat.
• Since the same resting cardiac output (~5 l/min) is needed, the heart can achieve this with fewer beats per minute.
• Elite endurance athletes may have resting heart rates as low as 30-40 bpm.

Example: Mo Farah (long-distance runner) reportedly had a resting heart rate of around 33 bpm — his heart was so strong that each beat pumped a very large volume of blood.

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Common Exam Mistakes