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2.6.4Additional lessons

Measuring Lung Volumes: Skills — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 30 of 142

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Part 1 of 5Using a Spirometer

2.6.4 Measuring Lung Volumes: Skills
1
The subject breathes through a mouthpiece connected to a chamber whose lid moves up and down with each breath.
2
The movement of the lid is recorded by a pen on a revolving drum or by a computer that draws a trace.
3
Soda lime in the circuit absorbs the carbon dioxide breathed out, so the fall in trace level shows oxygen used.
4
The trace is calibrated so that vertical distance represents volume, usually in dm³ or cm³.
5
Each breath produces one peak and trough, so counting peaks over time gives the ventilation rate.

Part 2 of 5Lung Volumes

Tidal volume
  • The volume of air breathed in or out in one normal breath at rest.
Inspiratory reserve volume
  • The extra air that can be breathed in after a normal inspiration.
Expiratory reserve volume
  • The extra air that can be breathed out after a normal expiration.
Vital capacity
  • The largest volume that can be exhaled after the deepest possible inhalation, VC = TV + IRV + ERV.
Residual volume
  • The air left in the lungs after the strongest exhalation, which cannot be measured by a spirometer.
QuantityTypical useWhere read on trace
Tidal volumeResting breath sizeHeight between peak and trough of a normal breath
Vital capacityMaximum breath sizeHeight from deepest inhalation to deepest exhalation
Ventilation rateBreaths per minuteCount peaks per minute

Part 3 of 5Interpreting Results

Interpreting Results
Exercise
  • During exercise, both tidal volume and ventilation rate increase, so peaks are taller and closer together.
Fitness and size
  • Vital capacity varies with age, sex, height and fitness, so comparisons need the same type of subject.
Reliability
  • Repeat measurements and take a mean to reduce the effect of anomalies.
Safety
  • Use a clean mouthpiece, sterilise the equipment between users and avoid the subject feeling dizzy.

Part 4 of 5Worked Example

Question

  • A spirometer trace shows tidal volume 0.5 dm³, inspiratory reserve volume 3.0 dm³ and expiratory reserve volume 1.1 dm³.
  • The subject takes 14 breaths per minute.
  • Calculate vital capacity and minute ventilation.

  1. Vital capacity = TV + IRV + ERV = 0.5 + 3.0 + 1.1 = 4.6 dm³.
  2. Minute ventilation = ventilation rate × tidal volume = 14 × 0.5.
  3. 14 × 0.5 = 7.0 dm³ per minute.

Answer Vital capacity is 4.6 dm³ and minute ventilation is 7.0 dm³ per minute.

Part 5 of 5Exam Checklist & Tips

2.6.4 – Key Points for the Exam

  • Spirometer A device used to record the volume and rate of breathing.
  • Tidal volume The volume of one resting breath.
  • Vital capacity The maximum volume of air that can be breathed out after a maximal inhalation.
  • Residual volume The air that remains in the lungs after maximum exhalation.

Common Mistakes

  • Including residual volume in the vital capacity calculation, when it is not part of it.
  • Reading ventilation rate from the height of the trace rather than the number of peaks.
  • Forgetting to state the units, such as dm³ or cm³.
  • Saying that a spirometer measures the air in the lungs completely, when it cannot measure residual volume.

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