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Inspiratory reserve volume

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Inspiratory Reserve Volume (IRV)

Definition: The inspiratory reserve volume is the extra volume of air that can be forcefully inspired above and beyond a normal tidal volume breath - i.e., the additional air you can pull in when you inhale as hard as you can after a normal, resting inspiration.
Typical value: About 3000 mL in an average healthy young man (approximately 1900 mL in women).
Where it fits in lung volumes/capacities:
  • Lungs are divided into four basic volumes: tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV).
  • Inspiratory capacity (IC) = TV + IRV - this is the total amount of air (~3500 mL) a person can inhale starting from the normal resting expiratory level and filling the lungs maximally.
  • Vital capacity (VC) = TV + IRV + ERV
  • Total lung capacity (TLC) = TV + IRV + ERV + RV
Measurement: IRV, along with tidal volume and expiratory reserve volume, can be measured directly with a simple spirometer (unlike residual volume, which requires more sophisticated methods like body plethysmography or gas dilution techniques since it can't be exhaled).
Clinical relevance: In diseases that increase lung volumes at baseline (e.g., COPD with hyperinflation), the resting lung volume shifts closer to total lung capacity, which compresses the available IRV — this contributes to the sensation of dyspnea and limits the ability to further expand tidal volume during exercise, since the lungs are already operating closer to their maximum distensible capacity.
Source: Guyton and Hall Textbook of Medical Physiology, "Pulmonary Volumes and Capacities," p. 38 (Ch. 38); also corroborated in Goldman-Cecil Medicine and Murray & Nadel's Textbook of Respiratory Medicine.
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