Which statement best describes the factors determining the expiratory time constant?

• A. It is determined by airway resistance and lung compliance
• B. It is independent of airway resistance
• C. It is determined by diffusion capacity
• D. It is equal to tidal volume

Answer: (A)

Expiratory time constant reflects how quickly the lungs empty during passive expiration. It depends on how easily air can flow through the airways (airway resistance) and how the lungs respond to a given pressure change (lung compliance). The relationship is a simple product: tau equals airway resistance times lung compliance. When either resistance or compliance increases, the expiration slows and the time constant grows, meaning it takes longer for the lungs to empty.

Diffusion capacity concerns gas transfer across the alveolar-capillary membrane, not the rate of expulsion of air from the lungs, and tidal volume is a volume moved per breath, not a determinant of how fast expiration occurs. That’s why the statement linking the expiratory time constant to airway resistance and lung compliance is the best choice.