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: