: Dynamic compression of the airways results when

intrapleural pressure In physiology, intrapleural pressure refers to the pressure within the pleural cavity. Normally, the pressure within the pleural cavity is slightly less than the atmospheric pressure, which is known as ''negative pressure''.Khanorkar, p. 205 When th ...

equals or exceeds

alveolar pressure Alveolar pressure (Palv) is the pressure of air inside the lung Pulmonary alveolus, alveoli. When the glottis is opened and no air is flowing into or out of the lungs, alveolar pressure is equal to the atmospheric pressure, that is, zero cmH2O, cmH2 ...

, which causes dynamic collapsing of the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

airways. It is termed ''dynamic'' given the

transpulmonary pressure Transpulmonary pressure is the difference between the alveolar pressure and the intrapleural pressure in the pleural cavity. During human ventilation, air flows because of pressure gradients. Ptp = Palv – Pip. Where Ptp is transpulmonary pressu ...

(alveolar pressure − intrapleural pressure) varies based on factors including

lung volume Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle. The average total lung capacity of an adult human male is about 6 litres of air. Tidal breathing is normal, resting breathin ...

, compliance, resistance, existing pathologies, etc. It occurs during forced

expiration Expiration or expiration date may refer to: Expiration Expiration may refer to: *Death *Exhalation of breath, breathing out *Expiration (options), the legal termination of an option to take an action *Shelf life, or the time after which a product ...

when intrapleural pressure is greater than

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...

(positive

barometric A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...

values), and not during passive expiration when intrapleural pressure remains at subatmospheric pressures (negative barometric values). Clinically, dynamic compression is most commonly associated to the wheezing sound during forced expiration such as in individuals with chronic obstructive pulmonary disorder (COPD).

References

Further reading