A spirometer turns breathing into a wiggly line, and almost every mark in this topic comes from reading that line correctly. The four measurements you need are all just vertical distances on the same graph — once you know where to put the ruler, they become straightforward.
📚 What you need to know
A spirometer has a chamber of air or oxygen sitting over water, closed by a hinged lid that rises and falls as the person breathes through a mouthpiece.
Filled with air it measures lung capacity. Filled with oxygen plus soda lime (which absorbs carbon dioxide) it measures oxygen consumption.
The trace is drawn on a revolving drum or by a computer.
Tidal volume (TV) is the volume breathed in or out during normal quiet breathing.
Inspiratory reserve volume (IRV) = maximum inspiratory level − the top of a normal breath. Expiratory reserve volume (ERV) = bottom of a normal breath − maximum expiratory level.
Vital capacity: VC = TV + IRV + ERV.
Ventilation rate = number of breaths per minute. Exercise increases both tidal volume and ventilation rate.
The apparatus
A chamber filled with water, covered by a hinged plastic lid. The lid rises when the person breathes out and falls when they breathe in.
A mouthpiece and tubing connecting the person to the chamber, plus a nose clip so all the air goes through the apparatus.
A soda lime canister in the circuit when oxygen consumption is being measured, to absorb the carbon dioxide the person breathes out.
A recording system: a pen drawing on a revolving drum, or a computer producing a graph.
🤔 Why soda lime changes what you are measuring
Without soda lime, every breath returns roughly the same total volume of gas to the chamber — oxygen is removed by the body but carbon dioxide is added back, so the overall volume barely drops. Put soda lime in the circuit and the carbon dioxide is absorbed instead of returning. Now the total gas in the chamber falls steadily, and the rate of that fall is the rate of oxygen consumption. Same machine, one chemical, completely different measurement.
Reading a spirometer trace
The y-axis is the volume of air in the lungs, not the volume breathed. That is why the line never reaches zero — you cannot empty your lungs completely.
The four measurements
Measurement
How to read it off the trace
On this trace
Tidal volume (TV)
The height of one normal quiet breath, from trough to peak
3.0 − 2.5 = 0.5 dm³
Inspiratory reserve volume (IRV)
From the top of a normal breath up to the maximum inspiratory level
5.5 − 3.0 = 2.5 dm³
Expiratory reserve volume (ERV)
From the bottom of a normal breath down to the maximum expiratory level
2.5 − 1.2 = 1.3 dm³
Vital capacity (VC)
Maximum inspiratory level down to maximum expiratory level, in one breath
5.5 − 1.2 = 4.3 dm³
Ventilation rate
Count the peaks in a known time and scale to one minute
Peaks every 4 s, so 15 breaths per minute
The equation to memorise
VC = TV + IRV + ERV
Use that equation as a check, not just as a formula. If a question gives you three of the four values, the fourth is free marks. If it gives you all four and they do not add up, you have misread the graph.
What exercise does to the trace
Tidal volume increases — the peaks and troughs get further apart, because more air is moved in and out with each breath.
Ventilation rate increases — the peaks get closer together, because more breaths are taken per minute.
Vital capacity does not change. It is a fixed property of the person’s lungs, not something exercise alters in the short term.
A useful extra number: total ventilation per minute is tidal volume × ventilation rate. For the trace above that is 0.5 dm³ × 15 breaths = 7.5 dm³ of air per minute. Under hard exercise both factors rise, so the total can climb many times higher.
Worked examples
WORKED EXAMPLE
A person’s trace shows a tidal volume of 0.45 dm³, an inspiratory reserve volume of 2.80 dm³ and an expiratory reserve volume of 1.15 dm³. Calculate the vital capacity. [2]
Step 1: recall the equationVC = TV + IRV + ERVStep 2: substituteVC = 0.45 + 2.80 + 1.15VC = 4.40 dm³Keep the units. A number with no units usually drops a mark.
WORKED EXAMPLE
On a spirometer trace, 9 complete breaths occur in 36 seconds and the tidal volume is 0.55 dm³. Calculate the ventilation rate and the volume of air breathed per minute. [3]
Step 1: breaths per second9 ÷ 36 = 0.25 breaths per secondStep 2: scale to one minute0.25 × 60 = 15 breaths per minuteVentilation rate = 15 breaths per minuteStep 3: total volume per minute0.55 × 15 = 8.25 dm³8.25 dm³ of air per minute
WORKED EXAMPLE
Describe two changes you would expect to see in a spirometer trace taken during exercise compared with one taken at rest. [2]
Change 1The peaks and troughs are further apart vertically, showing an increased tidal volume.Change 2The peaks are closer together horizontally, showing an increased ventilation rateDo not write “vital capacity increases” — it does not.
💡 Exam tip
Read the y-axis carefully: it is the volume of air in the lungs, so the trace starts partway up.
Always check your values against VC = TV + IRV + ERV before writing the final answer.
For ventilation rate, count complete breaths and always convert to per minute.
IRV is measured from the top of a normal breath; ERV from the bottom. Mixing these up is the most common error.
Remember what soda lime does: it absorbs carbon dioxide, which lets you measure oxygen consumption.
A nose clip is a control, not decoration — without it some air bypasses the apparatus and the readings are invalid.
⚠ Common mix-up
Reading tidal volume from the axis instead of the peak-to-trough distance. It is a difference, not a value.
Thinking the trace should reach zero. Residual air always stays in the lungs.
Swapping IRV and ERV. Inspiratory is above the normal breath; expiratory is below it.
Saying exercise increases vital capacity. It increases tidal volume and ventilation rate.
Forgetting to convert to per minute. A rate needs a time unit.
Assuming soda lime is always present. With air and no soda lime, the spirometer measures lung volumes instead.
Up next: Gas Exchange in Plants — the same physics, but with a problem animals do not have: every gas that goes in lets water out.
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