IB Biology HLGas Exchange SystemsPaper 1 & 2~12 min read
Measuring Lung Volumes
A spirometer turns breathing into a graph. Once you can read that graph, four different lung volumes are sitting there in front of you – and exam questions about them are usually just careful reading plus one subtraction.
📚 What you need to know
A spirometer has a chamber of gas floating over water with a hinged lid; the lid rises and falls as you breathe through a mouthpiece.
Filled with air it measures lung volumes; filled with oxygen and soda lime it can measure oxygen consumption.
The lid movement is recorded by a pen on a revolving drum, or by a computer that draws the trace.
Tidal volume (TV) is the volume of air moved in and out in one normal breath.
Ventilation rate is the number of breaths per minute. Exercise increases both tidal volume and ventilation rate.
What the machine actually does
The classic spirometer is a sealed chamber of gas sitting over a tank of water, with a lid that floats on top. You breathe through a mouthpiece connected to that chamber, wearing a nose clip so all your air goes through the machine and none escapes.
When you breathe in, you take gas out of the chamber, so the lid sinks.
When you breathe out, you put gas back in, so the lid rises.
A pen attached to the lid draws a line on a slowly turning drum, or a sensor sends the data to a computer.
If the chamber is filled with pure oxygen, a canister of soda lime in the circuit absorbs the carbon dioxide you breathe out. Because the carbon dioxide is removed but the oxygen is not replaced, the total volume in the machine slowly falls – and the rate at which the whole trace drifts downwards tells you the person’s oxygen consumption.
Modern electronic spirometers do the same job with a sensor and a screen, but the readings you take off the trace are identical.
The four volumes, in plain words
Measurement
What it means
How you get it from a trace
Tidal volume (TV)
Air moved in and out in one normal, relaxed breath
Height of one small peak, from trough to peak
Inspiratory reserve volume (IRV)
The extra air you could still breathe in on top of a normal breath
Maximum inspiratory level minus the top of a normal breath
Expiratory reserve volume (ERV)
The extra air you could still force out after a normal breath out
Bottom of a normal breath minus the maximum expiratory level
Vital capacity (VC)
The most air you can move in one breath, from fullest to emptiest
Maximum inspiratory level minus maximum expiratory level
Ventilation rate
Number of breaths per minute
Count the peaks in a known time and scale up to 60 s
Learn this one
VC = TV + IRV + ERV
Read the values off the y-axis, then subtract. Here TV = 3.0 − 2.5 = 0.5, IRV = 5.5 − 3.0 = 2.5, ERV = 2.5 − 1.2 = 1.3 and VC = 5.5 − 1.2 = 4.3 dm³.
Always take your readings from a trough to a peak, never from the axis to a peak. The trace shows the volume of air in the lungs, and your lungs are never empty – there is always residual air left, which a spirometer cannot measure.
What exercise does to the trace
Ask someone to exercise and then breathe into the spirometer, and two things change on the graph.
The peaks get taller: tidal volume rises, because more air is moved in and out per breath.
The peaks get closer together: ventilation rate rises, because you are taking more breaths per minute.
Both changes deliver more oxygen to the blood and remove carbon dioxide faster, which is what the working muscles need. Multiply the two together and you get the total volume of air moved per minute:
Air moved per minute
ventilation per minute = tidal volume × ventilation rate
Practical points. Use a fresh, sterilised mouthpiece for each person, check the person has no respiratory condition before they take part, and never let anyone rebreathe from a chamber where the soda lime has been used up – the carbon dioxide would build up.
Worked examples
WE 1
Calculating vital capacity
A student has a tidal volume of 0.5 dm³, an inspiratory reserve volume of 3.0 dm³ and an expiratory reserve volume of 1.1 dm³. Calculate their vital capacity. (2 marks)
Step 1: write the equationVC = TV + IRV + ERVStep 2: put the numbers in
VC = 0.5 + 3.0 + 1.1
VC = 4.6 dm³always carry the unit through; a number with no unit usually loses the second mark
WE 2
Reading a trace and scaling up
On a trace, 12 complete breaths are recorded in 30 seconds and each peak rises from 2.5 dm³ to 3.0 dm³. Calculate the ventilation rate and the volume of air breathed per minute. (3 marks)
Step 1: ventilation rate
12 breaths in 30 s, so in 60 s there are 12 × 2 = 24 breaths per minuteStep 2: tidal volume
TV = 3.0 − 2.5 = 0.5 dm³
Step 3: multiply
0.5 × 24 = 12
24 breaths per minute, 12 dm³ of air per minutecheck what one “breath” is on the trace: one peak plus one trough, not two peaks
WE 3
Comparing rest and exercise
At rest a person has a tidal volume of 0.5 dm³ and takes 14 breaths per minute. During hard exercise their tidal volume rises to 2.4 dm³ and their rate rises to 32 breaths per minute. Calculate the increase in air breathed per minute and explain why it happens. (4 marks)
Step 1: at rest
0.5 × 14 = 7.0 dm³ per minuteStep 2: during exercise
2.4 × 32 = 76.8 dm³ per minuteStep 3: the increase
76.8 − 7.0 = 69.8 dm³ per minute, which is about 11 times more air
Step 4: the explanation
Muscles are respiring faster, so they need more oxygen and produce more carbon dioxide; deeper and faster breathing keeps the concentration gradients in the alveoli steep so gas exchange stays fast.
Increase of 69.8 dm³ per minutesay both parts – deeper breaths AND more of them – and finish on the gradient
💡 Exam tips
Label the axes before you read anything. Volume is usually in dm³ and time in seconds.
Take readings trough to peak, and use the gridlines rather than guessing.
Learn VC = TV + IRV + ERV; many calculation questions are just this rearranged.
To find a rate per minute from a shorter recording, scale up: values per 30 s are doubled, per 15 s are multiplied by four.
If asked to describe the effect of exercise, mention both tidal volume and ventilation rate.
Remember what a spirometer cannot measure: residual volume, the air that always stays in the lungs.
⚠ Common mistakes
Reading tidal volume from zero. It is the height of one breath, not the height of the whole trace.
Counting peaks and troughs as separate breaths. One full cycle is one breath.
Forgetting to convert to per minute. Check the time axis before you answer.
Saying vital capacity is all the air in the lungs. Residual volume is not included.
Mixing up IRV and ERV. Inspiratory is the extra you can take in; expiratory is the extra you can push out.
Leaving units off calculations. dm³ and dm³ per minute are different things.
Up next: Gas Exchange in Plants – no lungs, no muscles, no blood, and yet a leaf solves exactly the same four problems.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
