The respiratory time constant is defined as which of the following?

The respiratory time constant is defined as which of the following?

• A. Resistance multiplied by Compliance
• B. Pressure divided by Volume
• C. Velocity times Area
• D. Flow divided by Resistance

Answer: (A)

In respiration, the time constant describes how quickly the lungs respond to a pressure change, and it equals the product of airway resistance and lung/chest wall compliance, tau = R × C. This combination sets the time scale for filling and emptying a breath, with a larger product meaning a slower approach to the new volume. The units come out in seconds because resistance (how hard it is to push air) and compliance (how much volume changes per unit pressure) multiply to give a time dimension. When you apply a step change in pressure, the resulting volume change decays toward its new value over time with this tau; if either resistance or compliance increases, tau increases and filling or emptying becomes slower. The other formulations don’t describe this time-based response: pressure divided by volume reflects elastance, not time; velocity times area relates to flow characteristics rather than the time course; and flow divided by resistance gives conductance, not a time constant.

In respiration, the time constant describes how quickly the lungs respond to a pressure change, and it equals the product of airway resistance and lung/chest wall compliance, tau = R × C. This combination sets the time scale for filling and emptying a breath, with a larger product meaning a slower approach to the new volume. The units come out in seconds because resistance (how hard it is to push air) and compliance (how much volume changes per unit pressure) multiply to give a time dimension. When you apply a step change in pressure, the resulting volume change decays toward its new value over time with this tau; if either resistance or compliance increases, tau increases and filling or emptying becomes slower. The other formulations don’t describe this time-based response: pressure divided by volume reflects elastance, not time; velocity times area relates to flow characteristics rather than the time course; and flow divided by resistance gives conductance, not a time constant.