Human respiration system is complicated, and the relation between pressure and flow in a respiratory tree structure consisting of a large number of tube segments and bifurcations is difficult. To facilitate investigation of ventilation distribution in the lungs, efforts have been directed towards development of some analog models, in which the resistive, inertial and elastic properties of the respiratory system have been simulated by similar method in electrical system ie. the RLC electrical circuits for each of the main compartments. The purpose of this study is to solve flow in different bifurcations in the respiration system and to determin the lumped model parameters. Viscous resistance is determined by solving steady flow in rigid bifurcations. The effect of inertia which is called inductance is determined by solving unsteady flow for inviscid fluid in rigid bifurcations. To determine the effect of wall elasticity which is called capacitance, the unsteady flow is solved. To determine capacitance parameter, dynamic mesh model in the Fluent software is used. The result of the present study shows that viscous resistance is different in different bifurcations. Simulations show that viscous resistance in bronchi within lobe and bronchiole models is almost constant and is near to Poiseuille flow resistance in a tube. However analytical equation in a tube is accurate but it is not exactly accurate in bifurcations in the respiration system. Solving the governing equations in different frequencies and with various inlet air volumes show that by increasing the frequency and entering air volume to the model inlet pressure and entering air volume to the daughter branche increase. Also by increasing frequency and entering air volume to the model the pendelluft phenomena in each respiration cycle will increase. Key Words Lung, Lump model, Resistance, Inductance, Capacitance, Bifurcation, Electric circuit
