Mechanics of Gas and Fluid Transport in the Tracheobronchial Tree

In the human respiratory system, the distribution system in charge of bringing fresh air from the upper airways to the air-blood exchange region (the acinar regions) is a highly branched structure called the tracheobronchial tree. Interestingly, it is possible to show through physical arguments that this complex structure corresponds to a simultaneous optimization of its occupied space, of the hydrodynamic resistance, and of the transit time across the structure of the inhaled gas mixture. However, this optimization disappears when dealing with fluid transport, as it is the case for instance in surfactant replacement therapy. This therapy, which aims at replacing the surfactant missing in the acinar regions of the lung, operates by propagating through the bronchial system a liquid plug initially instilled into the trachea. Two main mechanisms govern this propagation: (i) coating of the liquid on the airway wall when the liquid plug moves along an individual airway, (ii) splitting of the plug at each bifurcation of the tree. They hence determine the homogeneity and the efficiency of the final surfactant distribution. Unfortunately, homogeneity and efficiency depend in contradictory ways on the delivery conditions, especially on the instilled flow rate. Our computations show that, even if this double constraint can be easily fulfilled in the premature neonate, it leads in the adult lung to a considerable narrowing of the window of admissible delivery parameters and can explain several unsuccessful clinical trials in surfactant replacement therapy. The non linearity of the physical phenomena and the sensitivity of the therapy toe the delivery conditions require here a real “engineering” of the delivery and subsequently, a very close dialog between medicine, physics, and fluid mechanics.