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A spirometer trace records a volume-related signal against time. Establish what rising and falling mean for the apparatus rather than assuming every trace uses the same direction. Tidal volume is the air volume moved in one ordinary breath, estimated from the appropriate local excursion. Breathing frequency counts complete cycles per unit time; do not count both a peak and a trough as two breaths.
Video by The Physiological Society. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Determine tidal volume and breathing frequency from suitable traces
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does tidal volume × breathing frequency give?
Respiratory minute ventilation. Use compatible volume and per-minute units.
After watching 2: Which points are appropriate for a drift-based oxygen-use estimate?
Corresponding phases, such as successive peaks. Breathing excursions must be separated from the baseline trend.
What you'll be able to do
A spirometer trace records a volume-related signal against time. Establish what rising and falling mean for the apparatus rather than assuming every trace uses the same direction. Tidal volume is the air volume moved in one ordinary breath, estimated from the appropriate local excursion. Breathing frequency counts complete cycles per unit time; do not count both a peak and a trough as two breaths.
Respiratory minute ventilation = tidal volume × breathing frequency when units are compatible. It describes air moved through the respiratory system per minute, not the amount of oxygen consumed. Exercise can change both excursion and frequency, so compare the same defined quantities and appropriate intervals on resting and exercise traces.
In a suitable closed-circuit trace with CO₂ absorbed, a downward drift in the chamber’s gas volume can support an oxygen-consumption estimate after appropriate calibration and controls. Compare corresponding points in successive breaths, such as peaks, or a justified baseline trend. A peak-to-trough change mostly includes ventilation movement and must not be mistaken for metabolic oxygen uptake.
The interpretation assumes appropriate system calibration, CO₂ removal, gas conditions and absence of significant leaks. Not every modern spirometer measures oxygen consumption from a drifting volume trace. Identify the instrument and data model before making that calculation, and account for any small baseline change when measuring a local tidal excursion.
Analyse the supplied traces to practise the calculations. Any live respiratory or exercise measurement requires the centre’s approved apparatus, hygiene, participant suitability, consent and supervision arrangements. These notes do not require an unsupervised home breathing or exercise test.
For a physiological comparison, define exercise level, measurement timing, recovery and participant design. Repeated breaths from one participant are not independent participants. Individual variation, apparatus calibration and recovery can affect a result, and a classroom trace is not a diagnosis. Distinguish observed ventilation from inferred gas exchange and oxygen use.
| Time / s | Illustrative chamber volume / L |
|---|---|
| 0 | 3.50 |
| 1 | 3.00 |
| 2 | 3.48 |
| 3 | 2.98 |
| 4 | 3.46 |
| 5 | 2.96 |
| 6 | 3.44 |
| 7 | 2.94 |
| 8 | 3.42 |
Compare matching peaks before interpreting oxygen use. What goes wrong if you select a peak and a trough instead?
Blue: data with straight joining guides. Gold: selected interval. Scroll the plot horizontally on a small screen.
Try Δy ÷ Δx on paper first. Keep the sign and use the units on the axes.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: The illustrative trace repeats its peaks every 2.0 s. Find its breathing frequency.
30 breaths min⁻¹ in this separate schematic trace.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.