The emergence of the fractal bronchial tree

The fractal design of the bronchial tree, as described by the Hess-Murray law, facilitates energy-efficient lung ventilation, yet its developmental origins remain unclear. Here, we quantify the rearrangement of the embryonic bronchial tree into its fractal architecture, using SkelePlex, a new neural network-based image processing pipeline, and elucidate the biophysical principles governing its formation. We find that the branch shapes are such that pressure drop, shear, axial and hoop stress are equal across all branches. The seemingly random diameter of sister branches reflects the different number of tips they connect to in the asymmetric lung tree. By analysing lungs after pneumonectomy and those affected by chronic obstructive pulmonary disease (COPD), we show the same principles to persist in adulthood and disease, potentially providing quantitative biomarkers for disease progression and therapeutic guidance.