The flow energy loss (head loss) through a cast of canine central airways is found to be nearly independent of flow direction. By contrast, head loss in geometrically-simpler branching sections at comparable flow conditions is highly irreversible, with inspiratory loss being greater by nearly two units of dynamic pressure (2•1/2ρV2). In these branching sections head loss appears to be independent of important geometric parameters such as the branch length/diameter ratio and the exit/inlet flow-area ratio. An analysis of these observations suggests that kinetic energy factors, not shear stresses, account for most of the energy dissipated in central airways and in simple bifurcating sections. Inspiratory loss in bifurcations is greatly increased by the onset of flow separation: irreversibility is minimal in central airways, where separation either is absent or else is much less pronounced.