<p>An accurate understanding of the respiratory fluid dynamics is instrumental for medical applications, such as drug delivery system and treatment of diseases. Substantial research has been done to study such flow. However, a great number of these studies have the prevailing assumption of having a smooth wall, in despite the human trachea and bronchi is sustain by a series of cartilaginous rings, which creates height differences near the wall. To study the effect of including cartilaginous rings in the respiratory flow we developed two experiments, presenting a comparison between a smooth model and a model with cartilaginous rings. First, we present an experimental observation of a simplified Weibel-based model of the human trachea and bronchi with cartilaginous rings. The experiments were carried out with a flow rate comparable with a resting state (trachea-based Reynolds number of Re_D = 2650). In the second experiment, we developed a similar experiment but in a model with a tracheal stenosis (70% in the middle of the model) and no bronchi. In this case we increase the Reynolds number to Re_D= 3350, still a resting breathing state condition.</p> <p>For both experiments, we used transparent models and refractive index-matching methods were used to observe the flow, particularly near the wall. The flow was seeded with tracers to perform particle image velocimetry and particle tracking velocimetry to quantify the effect the rings have on the flow near the trachea and bronchi walls. From the results, we present a previously unknown phenomenon in the cavities between the cartilaginous rings: a small recirculation is observed in the upstream side of the cavities throughout the trachea. This recirculation is due to the adverse pressure gradient created by the expansion, which traps particles within the ring cavity. In addition, we found that the cartilaginous rings induce velocity fluctuations into the flow, which enhances the near-wall momentum of the flow reducing the separation after the stenosis. Size of the recirculation is reduced by 11% and the maximum upstream velocity is reduced by 38%, resulting in a much weaker recirculation because of the rings. Also noticed a delay in the separation from the trachea to bronchi bifurcation. </p> <p>The detection of recirculation zones in the cartilage ring cavities and the perturbation sheds light on the particle deposition mechanism and helps explain results from previous studies that have observed an enhancement of particle deposition in models with cartilage rings. The results highlight the importance to include the cartilaginous rings in respiratory flow studies. Finally, we compared the results from the stenotic case with Reynolds-averaged Navier-Stokes (RANS) models (k – ε, k – ε RNG, k – ω, k – ω SST, k – ω SST LRN and 4-equation Transition SST). In the results, indicate significant discrepancies between the experimental measurements and the simulations, mainly in the area with flow separation after the contraction. Therefore, RANS algorithms should not be considered reliable for research purposes in respiratory fluid dynamics without experimental validation. </p>