Lung ventilation, volumes, and measurement

Overview of ventilation
Ventilation is the exchange of air between the lungs and the surroundings. Several terms are used to describe ventilation that are based on the root pnoea (breathing) and various prefixes:

Spirometrical Parameters
A spirometer measures pressure differences, and uses this to calculate inspired and expired volumes of air. These are used to find several important parameters.

Static volumes
Static volumes depend on alveolar space and can inform about possible restrictive diseases. Tidal volume: amount of air that flows into the lungs during inspiration or out of them during expiration ~500 mL

Inspiratory reserve volume (IRV): volume that is additionally inspired during maximal effort, ~ 3 L.

Expiratory reserve volume (ERV): volume that is additionally expired after maximal effort, ~ 1.7 L.

Residual volume (RV): Air that remains in the lungs after maximal expiratory effort, ~1.2 L. Useful to keep airways inflated, so that gas exchange proceeds continuously. Must be found via methods other than spirometry:

Volume of distribution (ex. helium dilution): ask subject to exhale completely (only RV will be left), breathe in a known mass of non-toxic gas, then compute volume from measured concentration.

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Boyle's law: a plethysmograph is used to measure the change in the subject's body volume during inspiration, which, together with measured pressures, are used to calculate RV.

Static capacities
Static capacities are sums of above volumes

Inspiratory capacity (IC): Maximum inhaled air after normal expiration; IC = TV+ IRV

Vital capacity (VC): Maximum inspiration and after maximum expiration, ~ 5L.


 * VC = TV + IRV + ERV

Expiratory capacity: EC = TV+ERV

Functional residual capacity (FRC): The amount of air that remains in the lungs at the end of normal expiration. FRC = ERV + RV

Total lung capacity: The maximum volume the lungs can contain, ~ 6L.


 * TLC = VC + RV

Dynamic volumes
Dynamic volumes depend on airflow in the airways and can inform about obstructive or restrictive diseases.

One-second forced expiratory volume (FEV1): maximum volume forcibly expired after max. possible inspiration over one second. Decreases in obstructive diseases.

Forced Vital Capacity (FVC): maximum volume that can be exhaled after max. inhale. Does not usually change in obstructive diseases, but can decrease in restrictive diseases.

Tiffeneau index: FEV1/FVC ~ 0.7-1.0