Abstract: A structural and functional model for a lung or similar organ is virtually defined by encoding aspects of branching passageways. Larger passageways that are visible in medical images are surface mesh fitted to the anatomical surface geometry. Smaller distal passageways, beyond a given number of branch generations, are modeled by inference as linear passages with nominal diameters and branching characteristics, virtually filling the space within the outer envelope of the organ. The model encodes finite volumetric elements for elasticity and compliance in passageway walls, and for local pressure and flow conditions in passageway lumens during respiration. The modeling can assess organ performance, help to plan surgery or therapy, determine likely particle deposition, assess respiratory pharmaceutical dosing, and otherwise represent structural and functional organ parameters.