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How does the heart cycle through filling and pumping, and how do heart rate, stroke volume and cardiac output change with exercise?

Describe the cardiac cycle and define heart rate, stroke volume and cardiac output, calculating target heart-rate training zones

A focused answer to the O-Level ESS outcome on the cardiac cycle. Diastole and systole, heart rate, stroke volume and cardiac output, and how to calculate target heart-rate training zones.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to describe the cardiac cycle (how the heart fills and empties on each beat) and to define and use heart rate, stroke volume and cardiac output, including calculating target heart-rate training zones. The central idea is that the heart's output adjusts to the body's demand, and you can use simple numbers to set the right training intensity.

The answer

The cardiac cycle

The cardiac cycle is one complete heartbeat, made of two phases.

  • Diastole is the relaxation phase. The heart muscle relaxes and the chambers fill with blood returning to the heart.
  • Systole is the contraction phase. The heart muscle contracts and pushes blood out: the atria contract first to top up the ventricles, then the ventricles contract to eject blood into the pulmonary artery and aorta.

The valves open and close in step with these phases to keep blood moving one way. A resting cycle takes a little under a second and repeats continuously.

Heart rate

Heart rate is the number of times the heart beats in one minute, measured in beats per minute (bpm). A typical resting heart rate is around 60 to 80 bpm, and it is lower in trained endurance athletes because their hearts are stronger and more efficient.

Stroke volume

Stroke volume is the volume of blood pumped out of the left ventricle in a single beat, measured in millilitres. Endurance training increases stroke volume, so a trained heart pumps more blood per beat.

Cardiac output

Cardiac output is the volume of blood pumped out of the left ventricle in one minute. It is the product of heart rate and stroke volume:

Q=HR×SVQ = \text{HR} \times \text{SV}

During exercise both heart rate and stroke volume rise, so cardiac output increases sharply to deliver more oxygen to the working muscles.

Examples in context

Example 1. A trained rower at rest. The rower has a low resting heart rate of about 45 bpm but a large stroke volume. Because Q=HR×SVQ = \text{HR} \times \text{SV}, the resting cardiac output still meets the body's needs even though the heart beats far less often than an untrained person's, showing the efficiency that endurance training builds.

Example 2. A games player sprinting for a loose ball. Heart rate jumps and stroke volume rises, multiplying together to raise cardiac output several times above resting. This surge delivers the extra oxygen the leg muscles need for the sprint, and the values fall back during the recovery jog.

Try this

  • Cue. State which phase of the cardiac cycle, diastole or systole, the chambers fill in, and which they empty in. (Fill in diastole; empty in systole.)

  • Cue. Calculate the cardiac output of an athlete with a heart rate of 70 bpm and a stroke volume of 90 ml. (Q=70×90=6300 ml min−1=6.3Q = 70 \times 90 = 6300\ \text{ml min}^{-1} = 6.3 litres per minute.)

  • Cue. Work out the 70 percent target heart rate for a 40-year-old. (Maximum is 220−40=180220 - 40 = 180; 0.70×180=126 bpm0.70 \times 180 = 126\ \text{bpm}.)

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksDefine stroke volume and cardiac output, state the equation that links them to heart rate, and calculate the cardiac output of an athlete with a heart rate of 60 bpm60\ \text{bpm} and a stroke volume of 80 ml80\ \text{ml}.
Show worked answer →

Stroke volume: the volume of blood pumped out of the left ventricle in one beat. Cardiac output: the volume of blood pumped out of the left ventricle in one minute.

The link is: cardiac output=heart rate×stroke volume\text{cardiac output} = \text{heart rate} \times \text{stroke volume}.

Calculation: Q=60×80=4800 ml min−1=4.8 litres per minuteQ = 60 \times 80 = 4800\ \text{ml min}^{-1} = 4.8\ \text{litres per minute}.

What markers reward: correct definitions of stroke volume (per beat) and cardiac output (per minute), the equation, and the correct calculation with sensible units. Converting to litres earns the final touch.

Original4 marksA 16-year-old wants to train for aerobic fitness. Estimate their maximum heart rate, then find the heart-rate range for the aerobic training zone of 60 to 80 percent of maximum.
Show worked answer →

Estimate maximum heart rate as 220−age=220−16=204 bpm220 - \text{age} = 220 - 16 = 204\ \text{bpm}.

Lower bound: 0.60×204=122 bpm0.60 \times 204 = 122\ \text{bpm} (to the nearest whole beat).

Upper bound: 0.80×204=163 bpm0.80 \times 204 = 163\ \text{bpm}.

So the aerobic training zone is roughly 122122 to 163 bpm163\ \text{bpm}.

What markers reward: the correct use of 220−age220 - \text{age} for maximum heart rate, multiplying by each percentage, and giving the zone as a range in beats per minute.

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