# Poiseuille equation (airway resistance)

## Description

In respiratory physiology, airway resistance is the resistance of the respiratory tract to airflow during inspiration and expiration. In fluid dynamics, the Hagen–Poiseuille equation is a physical law that gives the pressure drop in a fluid flowing through a long cylindrical pipe. While the assumptions of the Hagen–Poiseuille equation are not strictly true of the respiratory tract it serves to show that, because of the fourth power, relatively small changes in the radius of the airways causes large changes in airway resistance. The airway resistance is depended on the length of pipe and the radius of the pipe.

Related formulas## Variables

R | The airway resistance (Pa*s/m^{3}) |

μ | The dynamic viscosity (Pa*s) |

l | The length of the pipe (m) |

π | pi |

r | The radius of the pipe (m) |