Core practical 17: spirometer traces and exercise

The spirometer

A spirometer has a chamber of oxygen floating over water. The person breathes in and out through a mouthpiece, and the chamber lid moves up and down, drawing a trace on a datalogger or kymograph.

Soda lime absorbs the CO2 breathed out, so the total volume of gas in the chamber falls steadily as oxygen is used. The trace slopes downwards.

Safety: use a healthy volunteer, sterilise the mouthpiece, use a nose clip, fill with medical-grade oxygen, check soda lime is fresh, and stop if the person feels dizzy.

Reading the trace

• Tidal volume: volume breathed in or out in one normal breath (height of one normal peak), about 0.5 dm³ at rest.
• Breathing rate: number of peaks per minute.
• Vital capacity: maximum volume breathed out after a maximum breath in.
• Oxygen consumption: the fall in the trace over a set time (drop in volume / time).
• Minute ventilation = tidal volume x breathing rate.

Effect of exercise

Record a trace at rest, then after a period of exercise (for example 2 minutes of step-ups). After exercise, tidal volume, breathing rate and oxygen consumption all increase, because muscles respire faster, CO2 rises, and the ventilation centre increases rate and depth of breathing.

Control: same person, same type and duration of exercise, same time before recording. Repeat for reliability.

Worked example

A spirometer trace falls by 0.6 dm³ over 2 minutes. Calculate the rate of oxygen consumption.

1. Rate = 0.6 / 2
2. = 0.3 dm³ min-1

Answer: 0.3 dm³ min-1

Check yourself

1. A trace falls by 1.5 dm³ over 3 minutes after exercise. Calculate the rate of oxygen consumption in dm³ min-1.
   Show answer

   0.5 dm³ min-1. 1.5 / 3 = 0.5 dm³ min-1.

2. A trace shows 6 breaths in 20 seconds. Calculate the breathing rate in breaths per minute.
   Show answer

   18 breaths per minute. 6 / 20 x 60 = 18.

3. After exercise, tidal volume is 1.2 dm³ and breathing rate is 25 breaths per minute. Calculate the minute ventilation in dm³ min-1.
   Show answer

   30 dm³ min-1. 1.2 x 25 = 30 dm³ min-1.

4. Why does a spirometer trace slope downwards over time?
   • The person breathes out more than they breathe in
   • Water leaks into the chamber
   • Oxygen is produced in the chamber
   • Soda lime absorbs exhaled CO2, so the gas volume falls as oxygen is used
   Show answer

   Soda lime absorbs exhaled CO2, so the gas volume falls as oxygen is used. Removing CO2 means the volume falls by the oxygen used.

5. What is tidal volume?
   • The maximum volume of air the lungs can hold
   • The volume of air breathed in or out in a normal breath
   • The number of breaths per minute
   • The volume of oxygen used per minute
   Show answer

   The volume of air breathed in or out in a normal breath. One normal breath.

6. What happens to tidal volume and breathing rate after exercise?
   • Tidal volume falls and breathing rate rises
   • Tidal volume and breathing rate stay the same
   • Tidal volume and breathing rate decrease
   • Tidal volume and breathing rate increase
   Show answer

   Tidal volume and breathing rate increase. Higher CO2 stimulates deeper, faster breathing.

7. Which is a safety precaution when using a spirometer?
   • Use soda lime that has run out
   • Sterilise the mouthpiece and stop if the subject feels dizzy
   • Fill the chamber with carbon dioxide
   • Use a subject with asthma for the exercise
   Show answer

   Sterilise the mouthpiece and stop if the subject feels dizzy. Hygiene and monitoring the subject are essential.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.