Time constant is used to describe the change in filling time and emptying time of a lung unit. One time constant

is the time it takes to fill 63% of maximal inflation or deflation of the lung unit. 96% of the lung unit is filled after three time constants. The time constant of the lung is a product of the resistance and the compliance.

is the time it takes to fill 63% of maximal inflation or deflation of the lung unit. 96% of the lung unit is filled after three time constants. The time constant of the lung is a product of the resistance and the compliance.

The resistance for normal lung tissue is 2 cmH2O/L and the total compliance is 100 cmH2O/ml, therefore the time constant for the whole lung is 0.2s

- A long
*Time Constant*results in alveoli that fill slowly on inspiration. - A short
*Time Constan*t results in alveoli that fill and empty quickly.

## Time Constant Respiratory Calculation

Static Lung Compliance: cmH2O/L | |

Airway Resistance: cmH2O/mL/s | |

Respiratory Time Constant: Seconds |