4.6.3

Measuring Gas Exchange

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Measuring Lung Function

The function of the lungs can be measured using a spirometer. Measuring gas exchange allows doctors to identify problems in the lungs.

Spirometer

Spirometer

  • A spirometer is a piece of apparatus that measures the gas exchange in the lungs.
  • A spirometer measures the volume of air that is inspired and expired by an individual.
  • An individual breathes into and out of a spirometer to measure the lung function.
Tidal volume & vital capacity

Tidal volume & vital capacity

  • A spirometer can measure the tidal volume.
    • Tidal volume is the volume of air in a normal breath (at rest).
  • The average tidal volume is about 0.4 dm3 − 0.5 dm3.
  • A spirometer can measure vital capacity.
    • Vital capacity is the maximum volume of air that can be breathed in and out of the lungs.
Ventilation rate

Ventilation rate

  • A spirometer can measure the ventilation rate.
  • The ventilation rate is the number of breaths a person takes per minute (at rest).
  • The average ventilation rate is about 15 breaths per minute.
Forced expiratory volume

Forced expiratory volume

  • A spirometer can measure forced expiratory volume.
  • Forced expiratory volume is the maximum volume an individual can expire in one second.
  • The forced expiratory volume cannot be more than the total volume of gas in the lungs.
  • This is because there is always a small amount of air that cannot be expired (the residual air). The residual air ensures the alveoli do not close.
Pulmonary ventilation rate (PVR)

Pulmonary ventilation rate (PVR)

  • Pulmonary ventilation rate can be calculated using the equation:
    • PVR = tidal volume × breathing rate
  • If you are given two of these values, the equation can be rearranged to calculate the third value.
Exercise

Exercise

  • During exercise, the following can be seen on the spirometer trace:
    • Increased tidal volume.
    • Increased ventilation rate.
    • Constant vital capacity.
  • The tidal volume and ventilation rate increase during exercise as more oxygen is required for respiration.
    • Respiration provides the energy required for exercise (e.g. for increased muscle contraction).
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