Calculate heart rate, stroke volume and cardiac output, using the equation…

The idea

Cardiac output is the volume of blood pumped out by one ventricle in one minute. It depends on two quantities: stroke volume, the volume of blood ejected by a ventricle in a single beat, and heart rate, the number of beats per minute. These are linked by the equation cardiac output = stroke volume × heart rate. If stroke volume is measured in cm³ and heart rate in beats per minute, cardiac output is in cm³ per minute. You must be able to rearrange the equation to find stroke volume (cardiac output ÷ heart rate) or heart rate (cardiac output ÷ stroke volume), and to substitute values with consistent units. For example, a stroke volume of 70 cm³ and a heart rate of 72 beats per minute give a cardiac output of 70 × 72 = 5040 cm³ min⁻¹. Always check that units match before multiplying or dividing.

What earns the marks

• States the equation cardiac output = stroke volume × heart rate and identifies each quantity correctly.
• Substitutes values correctly, keeping units consistent, for example cm³ for stroke volume and beats per minute for heart rate.
• Calculates cardiac output as the product of stroke volume and heart rate, giving the answer in cm³ min⁻¹ (or equivalent volume per minute).
• Rearranges the equation to find stroke volume (cardiac output ÷ heart rate) or heart rate (cardiac output ÷ stroke volume) when required.
• Interprets the result, for example explaining that a larger stroke volume or higher heart rate increases cardiac output.

Common mistakes and how to put them right

• Multiplying heart rate by stroke volume but leaving the answer without per-minute units.

  Put it right: Cardiac output is a volume per minute, so state cm³ min⁻¹ or the equivalent.

• Rearranging incorrectly, for example dividing stroke volume by heart rate.

  Put it right: Use cardiac output ÷ heart rate to find stroke volume.

• Mixing units, such as stroke volume in cm³ with heart rate per second.

  Put it right: Convert so both quantities use consistent time units before calculating.

Exam tips

• Write the equation first, then substitute numbers, so the working is clear and easy to check.
• Check the units of each value before calculating and convert if needed, then give the answer with correct units.
• If asked for stroke volume or heart rate, rearrange the equation before substituting rather than guessing the operation.