Respiratory system: Minute Ventilation.

The idea

Minute ventilation is the total volume of air moved into or out of the lungs in one minute. It is calculated by multiplying tidal volume by breathing frequency, so a resting tidal volume of about 0.5 dm³ at 12 breaths per minute gives a minute ventilation of about 6 dm³ per minute. The value is expressed in dm³ min⁻¹ or litres per minute. During exercise both tidal volume and breathing frequency rise, so minute ventilation increases steeply, and it is measured by combining a spirometer reading of tidal volume with a count of breaths per minute.

What earns the marks

• Minute ventilation is the total volume of air breathed in or out in one minute.
• A typical resting value is about 6 dm³ min⁻¹, for example 0.5 dm³ × 12 breaths per minute.

Common mistakes and how to put them right

• Confusing minute ventilation with tidal volume.

  Put it right: Tidal volume is one breath, while minute ventilation is the volume for a whole minute.

• Dividing instead of multiplying.

  Put it right: Minute ventilation is tidal volume multiplied by breathing frequency, not divided by it.

• Using the wrong time unit.

  Put it right: The frequency must be breaths per minute so the answer is a volume per minute.

• Assuming minute ventilation is fixed at rest.

  Put it right: It increases during exercise as both tidal volume and breathing frequency rise.

Exam tips

• Write the equation as minute ventilation = tidal volume × breathing frequency before substituting values.
• Check that breathing frequency is given per minute; convert if it is stated per other time period.
• Keep units consistent, for example dm³ × min⁻¹ gives dm³ min⁻¹.
• Use a typical resting calculation such as 0.5 dm³ × 12 to sanity-check whether an answer is plausible.