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2.10 Heart rate and cardiac output

Edexcel: calculating cardiac output from stroke volume and heart rate, rearranging the equation, measuring heart rate from the pulse, and why cardiac output rises during exercise.

Edexcel 2 min read

Key terms

Key terms

  • Heart rate: the number of times the heart beats in one minute (beats per minute, bpm).
  • Stroke volume: the volume of blood pumped out of the left ventricle in one heartbeat (cm3).
  • Cardiac output: the volume of blood pumped by the heart in one minute (cm3 per minute).

The equation

cardiac output = stroke volume × heart rate
Cardiac output = stroke volume x heart rate, with its units, and a bar chart showing cardiac output rising from 4900 cm3 per minute at rest to 14 400 cm3 per minute during exercise
Cardiac output = stroke volume x heart rate, with its units, and a bar chart showing cardiac output rising from 4900 cm3 per minute at rest to 14 400 cm3 per minute during exercise

Example: at rest, a person's heart rate is 70 beats per minute and their stroke volume is 70 cm3. Cardiac output = 70 × 70 = 4900 cm3 per minute (4.9 dm3 per minute).

You may need to rearrange the equation:

stroke volume = cardiac output / heart rate
heart rate = cardiac output / stroke volume

Example: cardiac output is 6000 cm3 per minute and heart rate is 80 bpm. Stroke volume = 6000 ÷ 80 = 75 cm3.

Measuring heart rate

Each heartbeat sends a surge of blood through the arteries, which you can feel as a pulse in the wrist or neck. Count the beats in 15 seconds and multiply by 4 to get the beats per minute. A heart rate monitor gives a more precise value.

During exercise

Muscles need more energy, so they respire faster. They need more oxygen and glucose, and they make more carbon dioxide that must be removed. So during exercise:

  • the heart rate increases
  • the stroke volume increases, because the heart contracts more strongly
  • so the cardiac output increases, and blood flows to the muscles faster.

People who exercise regularly often have a larger stroke volume, so their heart can supply the same cardiac output at rest with a lower resting heart rate.

Test yourself on this lesson

6 exam questions · 14 marks · with mark schemes and model answers

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