Human lung consists of the bronchi in which the trachea continues to bifurcate and finally reaches 300-600 millions alveoli having a total surface area of approximately 100 m² at the end of the bronchi. Quantitative evaluation of respiratory air flow in the bronchus is capable of providing further information about pulmonary function and specifying respiratory disease. The objective of the present study is to reveal respiratory air flow velocity and pressure drop by a three-dimensional asymmetric human bronchial tree model. This model consists of three simplified elements and expresses a structure of five lobes of right upper lobe (RUL), right middle lobe (RML), right lower lobe (RLL), left upper lobe (LUL) and left lower lobe (LLL).

We analyzed the respiratory air flow by the three-dimensional asymmetric human bronchial tree model using the finite element method (ANSYS, PA). It was difficult to model all of the human bronchial air way so that we devised three simplified models such as asymmetric Y-bifurcation model, symmetric Y-bifurcation model and T-bifurcation model. Human bronchi were made up by these three models. The asymmetric Y-bifurcation model was used for the first bronchus that consisted of right lung at 30° and left lung at 45° connected to the mother bronchus based on anatomical data. We used the T-bifurcation model for the second and third bronchi in the right lung. The RUL was located perpendicularly to the second bronchus. The RML was located perpendicularly to the third bronchus and further the RLL was located axially to the third bronchus.

Maximum average air flow velocity was calculated to be 1140 mm/s in RUL, 1490 mm/s in RML, 2650 mm/s in RLL, 1330 mm/s in LUL, 1200 mm/s in LLL. When we assumed all pressure drops to be 100%, the pressure drop until the seventh bronchus was 80 % in RUL, 85 % in RML, 84 % in RLL, 79 % in LUL, 81 % in LLL respectively.