Respiratory system represents relatively large system of gradually branching channels which can be hardly solved by numerical simulations. Nowadays, research in this area is focused to solve problems in selected parts of respiratory tract rather than whole system. This simplification comes with problem of accurate assessment of boundary conditions on model geometry. Geometry used on Department of Thermomechanics and Environmental Engineering on Brno University of Technology consists of mouth cavity, larynx, trachea and bronchial tree up to seventh generation of branching. This article is focused on comparison of two different settings of boundary conditions steady inspiration during light activity regime. First set of boundary conditions represents commonly used setting with zero pressure resistance on outlet from the model and second method deals with more realistic assumption, where incomplete 3D geometry is coupled with the rest of bronchial tree described by 1D equations and also correlated by the amount of air, which flows in specific lung lobe. The article observed differences in individual mass flow through the model branches under different conditions and its influence on the flow structures.